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New influenza A (H1N1) virus: global epidemiological situation, June 2009 (WHO WER, June 19, 2009, edited)
Weekly epidemiological record - 19 JUNE 2009, 84th YEAR - No. 25, 2009, 84, 249?260 (http://www.who.int/wer)
[Original Document: LINK. EDITED.]
New influenza A (H1N1) virus: global epidemiological situation, June 2009
On 11 June 2009, WHO raised the level of pandemic alert from phase 5 to phase 6, indicating that an influenza pandemic is under way,(1) the first in 41 years. Phase 6 is characterized by sustained human-tohuman transmission caused by community-level outbreaks in at least 1 country in ≥2 WHO regions.
Designation of this phase indicates that containment of the virus to a particular geographical area is no longer possible.
During previous pandemics, influenza viruses took >6 months to spread as widely as the new influenza A (H1N1) pandemic virus has taken to spread in <6 weeks since the first cases were detected in California (USA) in 2009.(2)
This report summarizes the global epidemiological situation of new influenza A (H1N1) virus as of 11 June 2009.
The descriptive epidemiology of the pandemic presented in this report includes the distribution by age and sex of laboratory-confirmed cases, the geographical spread of the pandemic virus, global and regional transmission patterns, morbidity and mortality patterns, and information about the co-circulation of new A (H1N1) virus and seasonal influenza viruses in selected countries.
Methods
WHO has compiled data from case-based and aggregated reports on human cases of new influenza A (H1N1) virus infection provided to the Organization by States Parties under the International Health Regulations (2005),(3) published reports and from information posted on the official web sites of national health authorities, including media releases. The case definitions used in this report can be found in the Interim WHO guidance for the surveillance of human infection with new influenza A (H1N1) virus.(4)
The terminology used in this report reflects the indicators of influenza activity recommended by WHO for global surveillance in pandemic phase 6 and during subsequent waves of disease activity. WHO has developed epidemiological and virological indicators of influenza activity to monitor the evolution of the pandemic.(5)
Geographical spread refers to the number and distribution of sites reporting influenza activity. Transmission describes the pattern of spread within countries. Intensity is an estimate of the overall level of respiratory disease activity in the population. Trend refers to the level of respiratory disease activity as a proxy for influenza activity (increasing, unchanged or decreasing) over time. Impact refers to the degree of disruption of health-care services as a result of influenza virus activity.
Geographical spread and transmission dynamics
Map 1 shows the global distribution of cases of new influenza A (H1N1) virus infection. As of 11 June 2009, 28 119 laboratory-confirmed cases, including 144 deaths, had been reported to WHO from Canada (4), Chile (2), Costa Rica (1), the Dominican Republic (1), Mexico (108), Colombia (1) and the United States (27) (Table 1).
Although 74 countries in 5 WHO regions have been affected, approximately 90% of reported cases have occurred in the Americas. Outside of North America, the majority of cases have been reported by Chile (6.1% of total cases), Australia (4.4%), the United Kingdom (2.4%), Japan (1.8%), Spain (1.2%), and Panama (0.8%).
No cases have been reported from any of the 46 countries in the WHO African Region.
International travel has facilitated the geographical spread of new influenza A (H1N1) virus from the initial foci of infection in Mexico and the United States to many countries throughout the world, seeding urban centres(6,7) with a high intensity of transmission before wider geographical spread within countries.
Recent international travel remains an important risk factor for disease. As of 3 June 2009, approximately 50% of the cases (n=415) reported among European Union countries were associated with travel to Mexico (n=133) or the United States (n=90) in the week before the onset of symptoms. Since 1 June 2009, the Health Protection Agency (United Kingdom) has observed a sharp increase in the proportion of locally-acquired infections in the UK.(8) This pattern is being observed in other countries with a rapidly
evolving epidemic. In Canada, among 1630 cases reported as of 4 June 2009, only 182 (10.7%) had a history of travel in the 7 days prior to symptom onset.(9)
Most countries, with the exception of Mexico, are reporting an increasing trend in the number of cases (Table 1).
These countries are describing in-country differences in patterns and intensity of transmission of new influenza A(H1N1) virus at the sub-national level, with reports of zero cases, imported cases only, limited local transmission associated with institutional outbreaks, notably schools (e.g. Australia,(10) Chile,(7) 14 countries within the WHO European Region,(11) Japan,(12) the Philippines(13) and the United States(6)) to widespread and sustained community-level transmission (Canada, Chile, Mexico and the United States). Australia has reported sustained community-level transmission(14) in the state of Victoria while all other jurisdictions are reporting local transmission. Approximately 40% (666/1521) of the laboratory-confirmed cases in Europe for whom an exposure history was obtainable resulted from local transmission.
Estimates of the basic reproduction number
Early estimates of the basic reproduction number (R0) for new influenza A(H1N1) virus are in the range of 1.4?1.6, while a genetic analysis gave a central estimate of 1.2.(15) Transmissibility is considered to be substantially higher than for seasonal influenza and is comparable with lower estimates of R0 calculated from estimates for the 1918 influenza pandemic of 2?3.(16)
A study in Japan estimates the R0 to be as high as 2.3, with a mean generation time of 1.3?4.0 days.(17) As the R0 is higher than previously reported, the authors suggest that high contact rates among adolescents compared with other populations may be one of the main drivers of the 2009 pandemic. Further modelling is under way to better describe transmission dynamics during the explosive outbreaks seen in school settings.
Distribution of cases by age and sex
To date, the vast majority of cases in all countries have occurred among adolescents and young adults. Males and females are similarly affected in all countries. Early cases were associated with students and recent travel to Mexico and the United States during the Spring holidays in the northern hemisphere. Fig. 1 shows the median age and range of cases reported to WHO from selected countries as of 11 June 2009. Data compiled by WHO from Chile, countries of the European Union and the European Free Trade Association, Japan, Panama and Mexico indicate that approximately 25% of cases were aged 0?9 years, 36% were aged 10?19 years, 17% were aged 20?29 years, 9% were aged 30?39 years, 7% were aged 40?49 years and 5% were aged >50 years. In Canada,(9) among cases for whom information was available (n=1695), 5.9% were aged 0?4 years, 10.6% aged 5?9 years, 57.8% aged 10?29 years and 0.7% >65, with a median age of 17 years (range, <1?80 years). In Chile, the median age is 13 years (range 1?65 years). In Japan, approximately 80% of cases are aged between 10 and 19 years. In the Philippines, most of the cases occurred among those aged 5?24 years (range, 1?64 years), and in the UK the median age of cases is 12 years (mean 20 years, range 0?73 years).
As of 21 May 2009, the median age of cases in countries reporting to the WHO European Region(18) was 24 years (range 0?69 years). Overall in the WHO Western Pacific Region, the median age of the approximately 215 cases for whom age is known is 23 years (inter-quartile range 18?30 years).
At this stage of the pandemic, it remains unclear whether younger age groups are over-represented among reported cases given the increased susceptibility in younger, immunologically naive populations, the fact that there has been insufficient time for the virus to fully penetrate beyond the social networks of known cases or that there are biases in disease surveillance.
Global morbidity and mortality
Globally, most cases have been mild, although severe disease has been reported both among known groups at higher risk of adverse outcomes following influenza and in previously healthy young adults, leading WHO to classify the severity of this pandemic to date as ?moderate?. In Kobe City (Japan),(12) of the 49 laboratory-confirmed cases reported to WHO as of 25 May 2009, most patients recovered quickly from their illness.
The duration of onset and resolution of fever was 1?8 days (median, 3 days).
Hospitalization
The United States,(19,20) Mexico,(19) Canada, Chile, Panama, Scotland and Australia have reported cases requiring hospitalization for the medical care of new influenza A (H1N1) virus infection rather than for diagnosis and quarantine. In the United States, of 399 patients with laboratory-confirmed new A (H1N1) influenza infection identified as of 5 May 2009 and for whom hospitalization status was known, 36 (9%) required hospitalization. Of the 22 hospitalized patients for whom data were available, 4 (18%) were aged <5 years, 1 patient (4%) was pregnant and 9 patients (41%) had co-morbidities.(21)
As of 11 June 2009, the New York City Department of Health and Mental Hygiene (USA) had notified 567 hospital admissions and 16 deaths. The hospitalized patients were younger than those who would normally experience severe illness during seasonal influenza outbreaks; approximately 79% of patients were aged <50 years; 46% were aged <18 years and 20% were aged <5; only 5% of hospitalized cases were aged >65 years.(22)
As of 30 May 2009, the Ministry of Health of Canada(9) had notified 84 hospital admissions. The median age of hospitalized patients was 17 years (range, 1?78 years), 13 of whom (15.5%) were admitted to intensive care. Information regarding underlying medical conditions is available for 40 (47.6%) of hospitalized cases, 12 of whom (30.0%) had underlying medical conditions including lung disease, chronic heart disease and other underlying comorbidities.
One hospitalized case was pregnant.
In Chile, 1.7% of total laboratory-confirmed cases (29/1694) required hospitalization.(7)
As of 8 June 2009, of 291 cases reported to WHO from countries of the European Union and the European Free Trade Association, 105 patients (36%) required hospitalization. In several countries (including Austria, Belgium, France and Romania), cases were hospitalized for isolation purposes.(23)
As of 11 June 2009, the WHO Western Pacific Region had notified 116 hospitalizations among 1077 cases (11%) for whom data were available; however, it is unclear what proportion of patients were admitted for severe disease. No hospitalizations have been reported by countries in the WHO South-East Asia Region or the WHO Eastern Mediterranean Region.
Co-morbidities
Although a large proportion of cases have been reported as having >1 underlying health condition (such as asthma, cardiovascular disease and pregnancy),(24) the true extent of co-morbidities on the severity of disease is currently under investigation in several countries. Of the 567 hospitalized patients in New York City,(22) 80% had at least 1 known risk factor for severe illness or complications due to influenza. Asthma is the most common risk factor, present in 41% of all laboratory-confirmed hospitalized cases. Other important risk factors include pregnancy (28% of 142 women of childbearing age hospitalized with confirmed disease), children aged <2 years (12%) and patients with diabetes (11%), immunodeficiency (9%) and cardiovascular disease (9%).
Community-based studies
A community-based telephone survey in New York City (USA) has estimated that 6.9% of the population developed an influenza-like illness during 1?20 May 2009. The city-wide survey found district-level differences in the prevalence of this illness ranging from 9.4% to 3.6% of respondents. While not all those with influenza-like illness had new influenza A (H1N1) virus infection, these results suggest that infections caused by A (H1N1) were common at that time.
Secondary household attack rates
The rates of secondary attack of seasonal influenza range from 5% to 15%. Current estimates of the secondary household attack rates of new influenza A (H1N1) range from 22% to 33%.
Impact on health services of new influenza A (H1N1) virus infection
To date, the overall impact of new influenza A (H1N1) virus infection on health-care services in Canada, Chile, Mexico and the United States is considered to be low. However, the impact on some sub-national health services has been moderate (that is, the demands on health-care services have caused some stress to systems above usual levels, but below maximum capacity [PAHO, unpublished data, 2009].
Co-circulation of new influenza A (H1N1) virus and seasonal influenza viruses
The impact of seasonality on the circulation and severity of new influenza A (H1N1) virus is an important and unanswered question. Although it is still too early to draw conclusions, several influenza surveillance systems have provided evidence of new influenza A (H1N1) virus and seasonal influenza viruses co-circulating in Australia, Canada, Chile, Europe, Mexico and the United States. In Chile, new influenza A (H1N1) virus is replacing the circulating seasonal influenza viruses.(25) By the end of May 2009, 90% of influenza isolates tested were found to be new A (H1N1) influenza virus; that proportion was 65% of isolates in the week ending 2 June 2009. Based on the antigenic and/or genetic characterization of 3548 influenza viruses reported to the European Influenza Surveillance Scheme up to epidemiological week 23, only 18 (0.5%) were characterized as new influenza A (H1N1) virus (see http://www.eiss.org/cgi-fi les/bulletin_v2.cgi). In the United States, new influenza A (H1N1) virus accounted for 77.8% (2071) of the 2663 influenza A viruses characterized during epidemiological week 22 (see http://www.cdc.gov/fl u/weekly/).
Editorial note.
For the 2009 pandemic influenza virus, WHO has observed the following preliminary epidemiological patterns:
As expected, uncertainties remain, including the changes over time of the clinical, virological and epidemiological characteristics of the virus and the impact of seasonality on these parameters. To answer these and other critical questions, epidemiological data collected through pandemic monitoring and special studies are urgently needed and will require:
Limitations of the data
Variations among countries in the case definitions and surveillance methods used to detect cases of new A (H1N1) virus infection suggest that there is significant underreporting, with surveillance systems not designed to detect all cases. In many countries, initial cases have been detected through screening upon entry of airline passengers, targeted testing of travellers developing symptoms of influenza-like illness after returning from affected countries and the investigation of outbreaks of such illnesses in schools. Asymptomatic and mild cases are likely to be missed.
The use of travel to an affected country to identify suspected cases of human infection with influenza A (H1N1) virus infection is limited by the speed of spread of the new virus. The current distribution of affected countries is also likely to reflect variations in surveillance systems and laboratory capacity. Furthermore, many of the intensive case-finding activities have been related to the follow-up of contacts of school outbreaks and therefore reported cases at this stage of the pandemic may be biased towards identifying cases in school-aged children.
Lessons learnt from past pandemics
Findings from an evaluation of past pandemics suggest that influenza viruses are unpredictable and that epidemiological patterns will vary within and among countries and during the different waves of pandemics. (26)
All governments and the international community will need to adapt to the changing dynamics of the situation. The timely sharing of information will be a critical component in supporting global mitigation efforts in the coming months and years.
Table 1 Laboratory-confirmed human cases of new influenza A (H1N1) virus infection, as officially reported to WHO by States Parties under the International Health Regulations (2005),* by WHO region (data as of 11 June 2009)
[WHO region ? n (%) ? No. of cases n (% of total) ? No. of deaths (n) ? Increasing or decreasing trends]
(*) International Health Regulations (2005), 2nd ed. Geneva, World Health Organization, 2005 (available at http://whqlibdoc.who.int/publication...80410_eng.pdf; accessed June 2009).
(a) Data have been updated retrospectively for the European Region (15 June 2009).
(b) With the exception of Mexico, which is reporting a decreasing trend in transmission of infl uenza A (H1N1) virus infection. ? ? l?exception du Mexique, qui signale une tendance ? la diminution
de la transmission.
(...)
-
------
Weekly epidemiological record - 19 JUNE 2009, 84th YEAR - No. 25, 2009, 84, 249?260 (http://www.who.int/wer)
[Original Document: LINK. EDITED.]
New influenza A (H1N1) virus: global epidemiological situation, June 2009
On 11 June 2009, WHO raised the level of pandemic alert from phase 5 to phase 6, indicating that an influenza pandemic is under way,(1) the first in 41 years. Phase 6 is characterized by sustained human-tohuman transmission caused by community-level outbreaks in at least 1 country in ≥2 WHO regions.
Designation of this phase indicates that containment of the virus to a particular geographical area is no longer possible.
During previous pandemics, influenza viruses took >6 months to spread as widely as the new influenza A (H1N1) pandemic virus has taken to spread in <6 weeks since the first cases were detected in California (USA) in 2009.(2)
This report summarizes the global epidemiological situation of new influenza A (H1N1) virus as of 11 June 2009.
The descriptive epidemiology of the pandemic presented in this report includes the distribution by age and sex of laboratory-confirmed cases, the geographical spread of the pandemic virus, global and regional transmission patterns, morbidity and mortality patterns, and information about the co-circulation of new A (H1N1) virus and seasonal influenza viruses in selected countries.
Methods
WHO has compiled data from case-based and aggregated reports on human cases of new influenza A (H1N1) virus infection provided to the Organization by States Parties under the International Health Regulations (2005),(3) published reports and from information posted on the official web sites of national health authorities, including media releases. The case definitions used in this report can be found in the Interim WHO guidance for the surveillance of human infection with new influenza A (H1N1) virus.(4)
The terminology used in this report reflects the indicators of influenza activity recommended by WHO for global surveillance in pandemic phase 6 and during subsequent waves of disease activity. WHO has developed epidemiological and virological indicators of influenza activity to monitor the evolution of the pandemic.(5)
Geographical spread refers to the number and distribution of sites reporting influenza activity. Transmission describes the pattern of spread within countries. Intensity is an estimate of the overall level of respiratory disease activity in the population. Trend refers to the level of respiratory disease activity as a proxy for influenza activity (increasing, unchanged or decreasing) over time. Impact refers to the degree of disruption of health-care services as a result of influenza virus activity.
Geographical spread and transmission dynamics
Map 1 shows the global distribution of cases of new influenza A (H1N1) virus infection. As of 11 June 2009, 28 119 laboratory-confirmed cases, including 144 deaths, had been reported to WHO from Canada (4), Chile (2), Costa Rica (1), the Dominican Republic (1), Mexico (108), Colombia (1) and the United States (27) (Table 1).
Although 74 countries in 5 WHO regions have been affected, approximately 90% of reported cases have occurred in the Americas. Outside of North America, the majority of cases have been reported by Chile (6.1% of total cases), Australia (4.4%), the United Kingdom (2.4%), Japan (1.8%), Spain (1.2%), and Panama (0.8%).
No cases have been reported from any of the 46 countries in the WHO African Region.
International travel has facilitated the geographical spread of new influenza A (H1N1) virus from the initial foci of infection in Mexico and the United States to many countries throughout the world, seeding urban centres(6,7) with a high intensity of transmission before wider geographical spread within countries.
Recent international travel remains an important risk factor for disease. As of 3 June 2009, approximately 50% of the cases (n=415) reported among European Union countries were associated with travel to Mexico (n=133) or the United States (n=90) in the week before the onset of symptoms. Since 1 June 2009, the Health Protection Agency (United Kingdom) has observed a sharp increase in the proportion of locally-acquired infections in the UK.(8) This pattern is being observed in other countries with a rapidly
evolving epidemic. In Canada, among 1630 cases reported as of 4 June 2009, only 182 (10.7%) had a history of travel in the 7 days prior to symptom onset.(9)
Most countries, with the exception of Mexico, are reporting an increasing trend in the number of cases (Table 1).
These countries are describing in-country differences in patterns and intensity of transmission of new influenza A(H1N1) virus at the sub-national level, with reports of zero cases, imported cases only, limited local transmission associated with institutional outbreaks, notably schools (e.g. Australia,(10) Chile,(7) 14 countries within the WHO European Region,(11) Japan,(12) the Philippines(13) and the United States(6)) to widespread and sustained community-level transmission (Canada, Chile, Mexico and the United States). Australia has reported sustained community-level transmission(14) in the state of Victoria while all other jurisdictions are reporting local transmission. Approximately 40% (666/1521) of the laboratory-confirmed cases in Europe for whom an exposure history was obtainable resulted from local transmission.
Estimates of the basic reproduction number
Early estimates of the basic reproduction number (R0) for new influenza A(H1N1) virus are in the range of 1.4?1.6, while a genetic analysis gave a central estimate of 1.2.(15) Transmissibility is considered to be substantially higher than for seasonal influenza and is comparable with lower estimates of R0 calculated from estimates for the 1918 influenza pandemic of 2?3.(16)
A study in Japan estimates the R0 to be as high as 2.3, with a mean generation time of 1.3?4.0 days.(17) As the R0 is higher than previously reported, the authors suggest that high contact rates among adolescents compared with other populations may be one of the main drivers of the 2009 pandemic. Further modelling is under way to better describe transmission dynamics during the explosive outbreaks seen in school settings.
Distribution of cases by age and sex
To date, the vast majority of cases in all countries have occurred among adolescents and young adults. Males and females are similarly affected in all countries. Early cases were associated with students and recent travel to Mexico and the United States during the Spring holidays in the northern hemisphere. Fig. 1 shows the median age and range of cases reported to WHO from selected countries as of 11 June 2009. Data compiled by WHO from Chile, countries of the European Union and the European Free Trade Association, Japan, Panama and Mexico indicate that approximately 25% of cases were aged 0?9 years, 36% were aged 10?19 years, 17% were aged 20?29 years, 9% were aged 30?39 years, 7% were aged 40?49 years and 5% were aged >50 years. In Canada,(9) among cases for whom information was available (n=1695), 5.9% were aged 0?4 years, 10.6% aged 5?9 years, 57.8% aged 10?29 years and 0.7% >65, with a median age of 17 years (range, <1?80 years). In Chile, the median age is 13 years (range 1?65 years). In Japan, approximately 80% of cases are aged between 10 and 19 years. In the Philippines, most of the cases occurred among those aged 5?24 years (range, 1?64 years), and in the UK the median age of cases is 12 years (mean 20 years, range 0?73 years).
As of 21 May 2009, the median age of cases in countries reporting to the WHO European Region(18) was 24 years (range 0?69 years). Overall in the WHO Western Pacific Region, the median age of the approximately 215 cases for whom age is known is 23 years (inter-quartile range 18?30 years).
At this stage of the pandemic, it remains unclear whether younger age groups are over-represented among reported cases given the increased susceptibility in younger, immunologically naive populations, the fact that there has been insufficient time for the virus to fully penetrate beyond the social networks of known cases or that there are biases in disease surveillance.
Global morbidity and mortality
Globally, most cases have been mild, although severe disease has been reported both among known groups at higher risk of adverse outcomes following influenza and in previously healthy young adults, leading WHO to classify the severity of this pandemic to date as ?moderate?. In Kobe City (Japan),(12) of the 49 laboratory-confirmed cases reported to WHO as of 25 May 2009, most patients recovered quickly from their illness.
The duration of onset and resolution of fever was 1?8 days (median, 3 days).
Hospitalization
The United States,(19,20) Mexico,(19) Canada, Chile, Panama, Scotland and Australia have reported cases requiring hospitalization for the medical care of new influenza A (H1N1) virus infection rather than for diagnosis and quarantine. In the United States, of 399 patients with laboratory-confirmed new A (H1N1) influenza infection identified as of 5 May 2009 and for whom hospitalization status was known, 36 (9%) required hospitalization. Of the 22 hospitalized patients for whom data were available, 4 (18%) were aged <5 years, 1 patient (4%) was pregnant and 9 patients (41%) had co-morbidities.(21)
As of 11 June 2009, the New York City Department of Health and Mental Hygiene (USA) had notified 567 hospital admissions and 16 deaths. The hospitalized patients were younger than those who would normally experience severe illness during seasonal influenza outbreaks; approximately 79% of patients were aged <50 years; 46% were aged <18 years and 20% were aged <5; only 5% of hospitalized cases were aged >65 years.(22)
As of 30 May 2009, the Ministry of Health of Canada(9) had notified 84 hospital admissions. The median age of hospitalized patients was 17 years (range, 1?78 years), 13 of whom (15.5%) were admitted to intensive care. Information regarding underlying medical conditions is available for 40 (47.6%) of hospitalized cases, 12 of whom (30.0%) had underlying medical conditions including lung disease, chronic heart disease and other underlying comorbidities.
One hospitalized case was pregnant.
In Chile, 1.7% of total laboratory-confirmed cases (29/1694) required hospitalization.(7)
As of 8 June 2009, of 291 cases reported to WHO from countries of the European Union and the European Free Trade Association, 105 patients (36%) required hospitalization. In several countries (including Austria, Belgium, France and Romania), cases were hospitalized for isolation purposes.(23)
As of 11 June 2009, the WHO Western Pacific Region had notified 116 hospitalizations among 1077 cases (11%) for whom data were available; however, it is unclear what proportion of patients were admitted for severe disease. No hospitalizations have been reported by countries in the WHO South-East Asia Region or the WHO Eastern Mediterranean Region.
Co-morbidities
Although a large proportion of cases have been reported as having >1 underlying health condition (such as asthma, cardiovascular disease and pregnancy),(24) the true extent of co-morbidities on the severity of disease is currently under investigation in several countries. Of the 567 hospitalized patients in New York City,(22) 80% had at least 1 known risk factor for severe illness or complications due to influenza. Asthma is the most common risk factor, present in 41% of all laboratory-confirmed hospitalized cases. Other important risk factors include pregnancy (28% of 142 women of childbearing age hospitalized with confirmed disease), children aged <2 years (12%) and patients with diabetes (11%), immunodeficiency (9%) and cardiovascular disease (9%).
Community-based studies
A community-based telephone survey in New York City (USA) has estimated that 6.9% of the population developed an influenza-like illness during 1?20 May 2009. The city-wide survey found district-level differences in the prevalence of this illness ranging from 9.4% to 3.6% of respondents. While not all those with influenza-like illness had new influenza A (H1N1) virus infection, these results suggest that infections caused by A (H1N1) were common at that time.
Secondary household attack rates
The rates of secondary attack of seasonal influenza range from 5% to 15%. Current estimates of the secondary household attack rates of new influenza A (H1N1) range from 22% to 33%.
Impact on health services of new influenza A (H1N1) virus infection
To date, the overall impact of new influenza A (H1N1) virus infection on health-care services in Canada, Chile, Mexico and the United States is considered to be low. However, the impact on some sub-national health services has been moderate (that is, the demands on health-care services have caused some stress to systems above usual levels, but below maximum capacity [PAHO, unpublished data, 2009].
Co-circulation of new influenza A (H1N1) virus and seasonal influenza viruses
The impact of seasonality on the circulation and severity of new influenza A (H1N1) virus is an important and unanswered question. Although it is still too early to draw conclusions, several influenza surveillance systems have provided evidence of new influenza A (H1N1) virus and seasonal influenza viruses co-circulating in Australia, Canada, Chile, Europe, Mexico and the United States. In Chile, new influenza A (H1N1) virus is replacing the circulating seasonal influenza viruses.(25) By the end of May 2009, 90% of influenza isolates tested were found to be new A (H1N1) influenza virus; that proportion was 65% of isolates in the week ending 2 June 2009. Based on the antigenic and/or genetic characterization of 3548 influenza viruses reported to the European Influenza Surveillance Scheme up to epidemiological week 23, only 18 (0.5%) were characterized as new influenza A (H1N1) virus (see http://www.eiss.org/cgi-fi les/bulletin_v2.cgi). In the United States, new influenza A (H1N1) virus accounted for 77.8% (2071) of the 2663 influenza A viruses characterized during epidemiological week 22 (see http://www.cdc.gov/fl u/weekly/).
Editorial note.
For the 2009 pandemic influenza virus, WHO has observed the following preliminary epidemiological patterns:
- younger age groups are more affected than those affected during outbreaks of seasonal influenza;
- although global geographical spread is extensive, transmission remains relatively localized for most countries.
As expected, uncertainties remain, including the changes over time of the clinical, virological and epidemiological characteristics of the virus and the impact of seasonality on these parameters. To answer these and other critical questions, epidemiological data collected through pandemic monitoring and special studies are urgently needed and will require:
- early detection of the virus in previously unaffected countries or areas;
- monitoring trends in countries/areas with established outbreaks;
- monitoring changes in the characteristics of the virus (virulence, susceptibility to antiviral agents and evidence of further genetic re-assortment) that could impact clinical case management and public health measures for mitigation;
- regular assessment of the severity of the disease;
- special studies to capture data on the extremely mild end of the clinical spectrum and asymptomatic cases to describe the true extent of new influenza A (H1N1) virus spread and better describe key epidemiological and virological characteristics.
Limitations of the data
Variations among countries in the case definitions and surveillance methods used to detect cases of new A (H1N1) virus infection suggest that there is significant underreporting, with surveillance systems not designed to detect all cases. In many countries, initial cases have been detected through screening upon entry of airline passengers, targeted testing of travellers developing symptoms of influenza-like illness after returning from affected countries and the investigation of outbreaks of such illnesses in schools. Asymptomatic and mild cases are likely to be missed.
The use of travel to an affected country to identify suspected cases of human infection with influenza A (H1N1) virus infection is limited by the speed of spread of the new virus. The current distribution of affected countries is also likely to reflect variations in surveillance systems and laboratory capacity. Furthermore, many of the intensive case-finding activities have been related to the follow-up of contacts of school outbreaks and therefore reported cases at this stage of the pandemic may be biased towards identifying cases in school-aged children.
Lessons learnt from past pandemics
Findings from an evaluation of past pandemics suggest that influenza viruses are unpredictable and that epidemiological patterns will vary within and among countries and during the different waves of pandemics. (26)
All governments and the international community will need to adapt to the changing dynamics of the situation. The timely sharing of information will be a critical component in supporting global mitigation efforts in the coming months and years.
Table 1 Laboratory-confirmed human cases of new influenza A (H1N1) virus infection, as officially reported to WHO by States Parties under the International Health Regulations (2005),* by WHO region (data as of 11 June 2009)
[WHO region ? n (%) ? No. of cases n (% of total) ? No. of deaths (n) ? Increasing or decreasing trends]
- African ? 0 - 0 - 0 - 0
- Americas ? 27 - 24 - 231 - (86.2) - 144 - Increasing(b)
- Eastern Mediterranean ? 6 - 37 - (0.1) - 0 - Increasing
- European(a) ? 30 - 1 - 801 - (6.4) - 0 - Increasing
- South-East Asia ? 2 - 12 - (0.04) - 0 Increasing
- Western Pacific ? 9 - 2 038 - (7.2) - 0 - Increasing
- Total 74 - 28 - 119 - 144
(*) International Health Regulations (2005), 2nd ed. Geneva, World Health Organization, 2005 (available at http://whqlibdoc.who.int/publication...80410_eng.pdf; accessed June 2009).
(a) Data have been updated retrospectively for the European Region (15 June 2009).
(b) With the exception of Mexico, which is reporting a decreasing trend in transmission of infl uenza A (H1N1) virus infection. ? ? l?exception du Mexique, qui signale une tendance ? la diminution
de la transmission.
- Statement to the press by WHO Director-General Dr Margaret Chan, 11 June 2009 (available at http://www.who.int/mediacentre/news/.../en/index.html ; accessed June 2009).
- Swine influenza A (H1N1) infection in two children, Southern California, March?April 2009. Morbidity and Mortality Weekly Report, 2009, 58:467?470 (available at http://www.cdc.gov/mmwr/preview/mmwr...m58d0421a1.htm ; accessed June 2009).
- International Health Regulations (2005), 2nd ed. Geneva, World Health Organization, 2005 (available at http://whqlibdoc.who.int/publication...580410_eng.pdf ; accessed June 2009).
- Interim WHO guidance for the surveillance of human infection with swine influenza A (H1N1) virus [29 April 2009]. Geneva, World Health Organization, 2009 (available at http://www.who.int/csr/disease/swine...2009_04_29.pdf ; accessed June 2009).
- Human infection with new influenza A (H1N1) virus: updated interim WHO guidance on global surveillance [in preparation]. Geneva, World Health Organization, 2009.
- United States Centers for Disease Control and Prevention web site (http://www.cdc.gov/flu/weekly/ ).
- Ministry of Health of the Government of Chile, weekly report 12 June 2009 (available at http://www.redsalud.gov.cl/minsalaud...rte12junio.pdf ; accessed June 2009).
- United States Centers for Disease Control and Prevention web site (http://www.cdc.gov/flu/weekly/ ). Health Protection Agency, Health Protection Scotland, National Public Health Service for Wales, HPA Northern Ireland Swine influenza investigation teams. Epidemiology of new influenza A (H1N1) virus infection, United Kingdom, April?June 2009. Eurosurveillance, 2009, 14(22):pii=19232 (available at http://www.eurosurveillance.org/View...rticleId=19232 ; accessed June 2009).
- Public Health Agency of Canada. FluWatch, 24?30 May 2009, week 21 (available at http://www.phac-aspc.gc.ca/fluwatch/.../index-eng.php ; accessed June 2009).
- Australian Department of Health and Ageing web site (https://www.healthemergency.gov.au/i...ontent/updates ).
- Update on influenza A (H1N1), European Region of the World Health Organization. Copenhagen, WHO European Region, 2009 (available at http://www.euro.who.int/influenza/AH1N1/20090523_1 ; accessed June 2009).
- See No. 24, 2009, pp. 237?244.
- Department of Health of the Republic of the Philippines web site (http://www.doh.gov.ph/ ).
- WHO Regional Office for the Americas (AMRO/PAHO). Update ? new virus influenza A (H1N1), regional report 24 May 2009 (available at http://new.paho.org/hq/index.php?opt...id=1〈=en ; accessed June 2009).
- Fraser C et al. Pandemic potential of a strain of influenza A (H1N1): early findings. Science Express, 11 May 2009 ? doi: 10.1126/science.1176062 (abstract available at http://www.sciencemag.org/cgi/content/abstract/1176062 ; accessed June 2009).
- Chowell G, Nishiura H, Bettencourt L. Comparative estimation of the reproduction number for pandemic influenza from daily case notification data. Journal of the Royal Society: Interface, 2007, 4(12):155?166 ? doi: 10.1098/rsif.2006.0161 (abstract available at http://rsif.royalsocietypublishing.org/content/4/12/155 ; accessed June 2009).
- Nishiura H et al. Transmission potential of the new influenza A (H1N1) virus and its age-specificity in Japan. Eurosurveillance, 2009, 14(22):pii=19227 (available at http://www.eurosurveillance.org/View...rticleId=19227 ; accessed June 2009).
- Update on influenza A (H1N1), European Region of the World Health Organization, April?May 2009. Copenhagen, WHO European Region, 2009 (available at http://www.euro.who.int/influenza/AH1N1/20090523_1 ; accessed June 2009).
- Novel influenza A (H1N1) virus infections?worldwide, May 6, 2009. Morbidity and Mortality Weekly Report, 2009, 58:453?458 (available at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5817a1.htm ; accessed June 2009).
- Alert #21: novel H1N1 influenza?update, June 2, 2009. New York City Department of Health and Mental Hygiene, 2009 (available at http://www.nyc.gov/html/doh/download...009/09md21.pdf ; accessed June 2009).
- Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team. Emergence of a novel swine-origin infl uenza A (H1N1) virus in humans. New England Journal of Medicine, 2009, 360:2605?2615.
- Press release #42-09: community transmission of H1N1 fl u appears to decline in New York City. New York City Department of Health and Mental Hygiene, 2009 (available at http://www.nyc.gov/html/doh/html/pr2009/pr042-09.shtml ; accessed June 2009).
- Ammon A et al. Preliminary analysis of influenza A (H1N1)V individual and aggregated case reports from EU and EFTA countries. Eurosurveillance, 2009, 14(23) (available at http://www.eurosurveillance.org/View...rticleId=19238 ; accessed June 2009).
- See No. 21, 2009, pp. 185?189
- Ministry of Health of Chile. Reporte diario de situation de influenza A-H1N1, 2 June 2009 (available at http://www.redsalud.gov.cl/portal/ur...011f01019b.pdf ; accessed June 2009).
- Miller MA et al. The signature features of influenza pandemics?implications for policy. New England Journal of Medicine, May 7, 2009 ? 10.1056/NEJMp0903906 (available at http://content.nejm.org/cgi/content/full/NEJMp0903906 ; accessed June 2009).
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