Giuseppe
Emeritus
[Source: World Health Organization, Weekly Epidemiological Record, full PDF document: (LINK). Edited.]
Weekly epidemiological record / Relev? ?pid?miologique hebdomadaire
15 JUNE 2012, 87th year / 15 JUIN 2012, 87e ann?e - No. 24, 2012, 87, 233?240 - http://www.who.int/wer
Review of the 2011?2012 winter influenza season, northern hemisphere
This report summarizes the chronology, epidemiology and virology of the winter influenza season in the temperate regions of the northern hemisphere. The review covers influenza activity data collected from October 2011 until the end of April 2012. The data presented have been derived primarily from reports published by national ministries of health or other official bodies reporting on their behalf, or reported to WHO through FluNet and FluID.(1, 2)
North America Transmission
Influenza transmission in the northern hemisphere temperate regions can start as early as October and typically peaks around January, although considerable variation is seen from year to year. The 2011?2012 season started unusually late in most of North America, the latest in nearly 30 years in the United States (USA), with activity generally at inter-seasonal levels from October to late November (Figure 1). All 3 countries of North America (Canada, USA and Mexico) began to experience increasing influenza virus detections in early December; however transmission remained generally low until the end of the year. In the USA, as in most previous influenza seasons, influenza-like illness (ILI) activity was initially concentrated in the southern states, and then increased progressively in the central and western states. In the 3 countries, peak transmission occurred first in Mexico, in late January, followed by the USA,(3) in mid-March, and then by Canada in late March to early April.(4)
The distribution of influenza virus types and subtypes differed in these 3 countries. Influenza A(H1N1)pdm09 accounted for nearly all virus detections throughout the season in Mexico. In contrast, influenza A(H3N2) was the most commonly detected virus for most of the season in the USA with A(H1N1)pdm09 accounting for only 27% of influenza A viruses for which subtype data were available. Notably, regional differences were observed within the USA. In the southwestern part of the country A(H1N1)pdm09 viruses were more common than A(H3N2).
Although influenza type B viruses accounted for only 13% of virus detections overall in the USA, as the numbers of A(H3N2) virus detections declined in early April, detections of influenza B increased slightly, accounting for about one third of detections during the last few weeks of the season. In Canada, influenza B viruses accounted for >53% of virus detections nationally, a pattern which persisted throughout the season.
As in the USA, there was regional variation in virus distribution, and influenza A viruses outnumbered influenza B viruses in the western provinces of British Columbia and Alberta by >5 to 1. Nationally, 32% of influenza A viruses with subtype information were A(H1N1)pdm09 for the season.
Illness and mortality
In the USA, influenza activity was considerably less intense than in previous years; clinical consultations for ILI, reported hospitalizations, pneumonia and influenza mortality, and reported influenza-associated paediatric deaths were all lower than in recent years. The percentage of outpatient visits to sentinel physicians for ILI reached the national baseline of 2.4% but never exceeded it, a pattern which has not been observed in at least the last 15 years. Laboratory confirmed influenza-associated hospitalizations reported through the Emerging Infections Program, covering 80 counties in 10 of the 50 states of the USA were lower than the previous year (8.6 per 100 000 population as of 30 April versus 21 per 100 000 population in 2010?2011) and mortality attributed to pneumonia and influenza (P&I) in the 122 Cities Mortality Reporting System slightly exceeded the epidemic threshold (1.645 standard deviations above the weekly mean) only once this season and was below the weekly historical 5-year average for much of the season. The number of influenza-associated paediatric deaths was low during this season. Excluding the 2009 pandemic year, since the 2003?2004 season when paediatric influenza-associated death surveillance began, a median of 68 deaths per season (range 30?152) have been reported; as of 30 April 2012, only 24 deaths have been reported this season. Fewer severity-related data are available for Canada but the influenza season appeared similar to that in previous years. ILI consultation rates remained near the 15-year average throughout the 2011?2012 season. The total number of reported influenza or ILI institutional outbreaks peaked at 59 in mid-March, the highest peak reported since 2005 with the exception of the 2009 autumn/winter; however, the cumulative number of outbreaks did not notably differ from that in the previous season. The number of laboratory-confirmed influenza-associated hospitalizations reported (1674 as of 12 May 2012) from 7 reporting provinces was higher in the current season than previous years since 2005 (median 583, range 365?1464); however, the number of influenza-associated deaths decreased from 185 in the 2010?2011 season to 83 by the end of April this season.
In the USA, hospitalization rates for influenza were highest among those aged ≥65 years (30/100 000), followed by children aged 0?4 years (14/100 000).
Influenza A was identified in 87% of laboratory-confirmed influenza-associated hospitalizations and of those influenza A viruses with subtype information, 75% were A(H3N2), approximately the same proportions as reported for ILI cases. The most commonly reported underlying medical conditions among adults hospitalized with influenza were chronic lung diseases (of which 52% were asthma), cardiovascular disease and obesity. The most commonly reported underlying medical conditions in children hospitalized with influenza were chronic lung diseases (of which 76% were asthma) and neurological disorders. Of note, almost half of hospitalized children had no identified underlying medical condition.
Similarly in Canada, 33% of laboratory confirmed influenza-associated hospitalizations and 79% of deaths were among adults ≥65 years of age.
Among children ≤16 years of age, 35% of laboratory confirmed influenza-associated hospitalizations were among those ≤2 years of age. The distribution of influenza virus type or subtype varied between age groups in Canada. Among children <20 years of age, influenza B was detected in a higher proportion of ILI cases than influenza A (59% B versus 41% A), the reverse was true in adults (34% B versus 56% A). Differences were more pronounced when considering only the type A viruses, particularly at the extremes of age. Of influenza type A associated ILI cases in children <5 years old, 39% were A(H1N1)pdm09 viruses versus only 9% in adults ≥65 years.
Europe and the Middle East Transmission
The influenza season in Europe also started late (Figure 1).(5, 6) During September to November, most countries in Europe reported only low intensity or no transmission of influenza. Belgium, Italy, Norway, Spain, Sweden and Turkey reported an increasing trend in transmission in early December 2011 but transmission did not start in earnest in much of western Europe until after the end of the year and unlike previous years, there was no clear west to east progression in the countries of western Europe. Transmission began slightly later in eastern Europe. Influenza transmission peaked at the end of February or early March in most countries of western Europe. Transmission in the Estonia, Latvia, Russian Federation and Slovakia, and other countries in eastern Europe peaked in April. Increased numbers of ILI cases began to be reported as early as September 2011 in North Africa and the Middle East and peaked between mid-December and mid-January, 6 to 8 weeks earlier than in Europe.
Influenza transmission in Europe, North Africa and the Middle East was primarily associated with influenza A(H3N2) throughout most of the season, however, influenza B detections increased toward the end of the season. Overall, less than 13% of influenza viruses from outpatient specimens were influenza type B in Europe and of the influenza A viruses with subtype information, 98% were A(H3N2). However, an assessment published by the European Centres for Disease Control early in the season found that A(H1N1)pdm09 was overrepresented in severe cases compared with the other 2 circulating viruses in France, Ireland, Spain and the United Kingdom.(7) North Africa and the Middle East had a distribution of virus types and subtypes similar to that in Europe, although the influenza A season was more distinct as it occurred much earlier and was complete by the time that B viruses began circulating in low numbers, Nearly all of the influenza A viruses in North Africa were A(H3N2).
Qatar reported a late season wave of A(H1N1)pdm09 in March and April.
Illness and mortality
Overall, influenza severity indicators were not consistent across Europe. In western Europe, numbers of ILI cases seen in primary care settings were more variable than usual with some countries experiencing relatively few cases scarcely reaching the epidemic baseline (e.g. the United Kingdom and Ireland) while others had more typical seasons (e.g. France and Spain). The number of severe acute respiratory infections reported by 7 participating countries that were positive for influenza was slightly lower than last season (1282 as of 11 May 2012 versus 1548 at the end of the 2010?2011 season). The European Mortality Monitoring Project (EUROMOMO), which pools all-cause mortality data from 15 countries of Europe, reported excess mortality among persons ≥65 years of age in some countries, peaking in February 2012.(8) This was most notable in countries that experienced more community transmission, around the same time as the peak in influenza transmission. Mortality in the 15 to 64 year old age group was notably lower compared to the 2010?2011 season when A(H1N1)pdm09 was the predominant virus circulating in the area.
Northern Asia Transmission
The influenza season in the northern temperate areas of Asia began at the end of November 2011, one month later than in the previous season (Figure 1).(9) Transmission peaked in Japan and the Republic of Korea during mid to the end of January 2012 and about 4 weeks later in Mongolia and northern China. Influenza A(H3N2) was the predominant virus in Japan and Republic of Korea, accounting for almost all of the influenza A viruses detected. Both countries reported increased numbers of influenza type B virus isolates towards the end of the season although in Republic of Korea, this secondary transmission was quite marked. In contrast, the season in northern China and Mongolia was predominantly influenza B initially with transition to A(H3N2) late in the season.
Illness and mortality
Reporting rates for ILI visits in northern China, Mongolia and Republic of Korea were all similar to those in previous seasons; however, Japan experienced the highest number of influenza-confirmed cases since 2002, except for the 2009 pandemic period. In Mongolia, the proportion of hospitalizations for pneumonia and the reported number of pneumonia deaths were lower than during the 2010?2011 season.
Antigenic testing
The seasonal trivalent vaccine for 2011?2012 contained the same 3 viruses as the 2010?2011 northern hemisphere vaccine: A/California/7/2009 (H1N1)-like virus, A/Perth/16/2009 (H3N2)-like virus and B/Brisbane/60/2008-like virus (B Victoria lineage). Early in the season, nearly all of the influenza A viruses detected globally were antigenically similar to the vaccine viruses.(10) However, increasing antigenic diversity was noted in A(H3N2) viruses in the latter part of the season. These viruses had reduced titre cross-reactivity with antiserum produced against the A/Perth/16/2009 virus but higher titres against A/Victoria/361/2011-like reference viruses. In Europe this was associated with lower vaccine effectiveness than in previous seasons in well controlled field observational studies.(11, 12) Because of antigenic heterogeneity within influenza A(H3N2) viruses circulating during this influenza season, the updated trivalent influenza vaccine for the northern hemisphere will contain an A/Victoria/361/2011-like virus.(13) Influenza B viruses of both the B/Victoria and the B/Yamagata lineages circulated during this influenza season in nearly equal proportions in some areas. The increasing proportion of viruses of the B/Yamagata lineage prompted a change in the next season vaccine composition to include a Yamagata virus (B/Wisconsin/1/2010-like virus).
Antiviral sensitivity testing
Very little resistance to neuraminidase inhibitors was found in influenza viruses in the 2011?2012 season. In Europe, Canada and China resistance to neuraminidase inhibitors (oseltamivir and zanamivir) was undetectable throughout the season. However, oseltamivir resistance was reported in 1.5% (16) of the 1067 influenza A(H1N1) pdm09 viruses tested in the USA late in the season. Information related to oseltamivir exposure is available for 14 of the 16 cases, of which 3 had taken oseltamivir for 1 day or more at the time of specimen collection, 2 had family members using oseltamivir, and 9 had no exposure to oseltamivir. Of the 16 oseltamivir resistant viruses, 11 were from the state of Texas. Of 371 influenza A(H1N1)pdm09 specimens tested from Texas, 11 (3.0%) collected from January to April 2012 were resistant to oseltamivir. All oseltamivir resistance was associated with a H275Y substitution in the neuraminidase gene. Countries performing sensitivity analysis for resistance to adamantanes (amantadine and rimantadine), continued to report high levels of resistance to adamantanes in A(H1N1)pdm09 and A(H3N2) viruses.
Summary
The 2011?2012 influenza season was notably late in most of the northern hemisphere temperate zone, with the exception of North Africa. The season was remarkably mild in the USA, where influenza A(H3N2) was the predominant virus circulating (Map 1). Severity data are limited in other parts of the world, however, the available data indicate that the season did not notably exceed levels seen in previous recent seasons in the rest of the temperate zone and was milder than usual in many, but not all, European countries. Of note, a few countries of Europe reported excess mortality among individuals ≥65 years of age which coincided with local transmission of A(H3N2), and the predominant virus circulating varied widely fin different countries. In North America Mexico reported almost exclusively influenza A(H1N1)pdm09, which was also seen in Central America, while the USA reported much more A(H3N2) except in the states bordering Mexico. Canada had predominance of type B nationally, which increased late in the season, but reported more A(H3N2) in the western provinces.
The season was predominantly associated with A(H3N2) in Europe and North Africa, though influenza B did in crease slightly late in the season.
Temperate countries of Asia had both influenza B peaks and A(H3N2) peaks, with influenza B appearing first in China and Mongolia followed by A(H3N2) and the reverse sequence in the Republic of Korea and Japan.
Few data were available regarding the age distribution of viruses, however, in Canada it appeared that A(H1N1) pmd09 continues to show a marked tendency to affect younger age groups disproportionately.
Notably, A(H3N2) has now begun to drift, requiring a change in vaccine composition for the northern hemisphere 2012?2013 influenza season, and the proportion of influenza B viruses belonging to the B/Yamagata lineage has also increased prompting a change in the B virus for the next vaccine.
The great majority of the viruses tested this season were sensitive to oseltamivir. However, the late-season appearance of a number of cases with oseltamivir resistant A(H1N1)pdm09 viruses in Texas, most of which had no direct or indirect exposure to the drug, raises some concern. A cluster of 29 oseltamivir resistant viruses was reported in New South Wales, Australia in the 2011 southern hemisphere winter season but did not result in onward persistence of the resistant virus.
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(1) FluNet is available online at: http://www.who.int/influenza/gisrs_laboratory/flunet/en/ accessed June 2012.
(2) FluID is available online at: http://who.int/influenza/surveillance_monitoring/fluid/en/ accessed June 2012.
(3) For additional information, see the United States Centers for Disease Control and Prevention?s FluView at http://www.cdc.gov/flu/weekly/ accessed June 2012.
(4) For additional information, see the Public Health Agency of Canada?s FluWatch at http://origin.phac-aspc.gc.ca/fluwatch/ accessed June 2012.
(5) For additional information see WHO/Europe influenza surveillance (EuroFlu.org) is available at http://www.euroflu.org/index.php accessed June 2012.
(6) For additional information see The European Centre for Disease Prevention and Control?s Weekly influenza surveillance overview available at: http://ecdc.europa.eu/en/healthtopics/seasonal_influenza/epidemiological_data/pages/weekly_influenza_surveillance_overview.aspx accessed June 2012
(7) Beaut? J et al. Overrepresentation of influenza A(H1N1)pdm09 virus among severe influenza cases in the 2011/12 season in four European countries. Eurosurveillance. 2012;17(9)
ii=20105. (Available at http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20105 accessed June 2012.)
(8) Mazick A et al. Excess mortality among the elderly in 12 European countries, February and March 2012. Eurosurveillance. 2012;17(14)ii=20138. (Available at http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20138 accessed June 2012.)
(9) WHO Regional Office for the Western Pacific?s Influenza situation update is available at http://www.wpro.who.int/health_topics/influenza accessed June 2012.)
(10) For additional information on vaccine strain selection see Recommended composition of influenza virus vaccines for use in the 2012?2013 northern hemisphere influenza season available at http://www.who.int/influenza/vaccines/virus/recommendations/2012_13_north/en/index.html accessed June 2012
(11) Jim?nez-Jorge S et al. Early estimates of the effectiveness of the 2011/12 influenza vaccine in the population targeted for vaccination in Spain, 25 December 2011 to 19 February 2012. Eurosurveillance. 2012;17(12)ii=20129. (Available at http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20129 accessed June 2012.)
(12) Kissling E, Valenciano M, I-MOVE case?control studies team. Early estimates of seasonal influenza vaccine effectiveness in Europe among target groups for vaccination: results from the I-MOVE multicentre case?control study, 2011/12. Eurosurveillance. 2012;17(15)ii=20146. (Available at http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20146 accessed June 2012.)
(13) Recommended composition of influenza virus vaccines for use in the 2012?2013 northern hemisphere influenza season. Geneva, World Health Organization 2012. (Available at: http://www.who.int/influenza/vaccines/virus/recommendations/201202_recommendation.pdf accessed June 2012.)
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Weekly epidemiological record / Relev? ?pid?miologique hebdomadaire
15 JUNE 2012, 87th year / 15 JUIN 2012, 87e ann?e - No. 24, 2012, 87, 233?240 - http://www.who.int/wer
Review of the 2011?2012 winter influenza season, northern hemisphere
This report summarizes the chronology, epidemiology and virology of the winter influenza season in the temperate regions of the northern hemisphere. The review covers influenza activity data collected from October 2011 until the end of April 2012. The data presented have been derived primarily from reports published by national ministries of health or other official bodies reporting on their behalf, or reported to WHO through FluNet and FluID.(1, 2)
North America Transmission
Influenza transmission in the northern hemisphere temperate regions can start as early as October and typically peaks around January, although considerable variation is seen from year to year. The 2011?2012 season started unusually late in most of North America, the latest in nearly 30 years in the United States (USA), with activity generally at inter-seasonal levels from October to late November (Figure 1). All 3 countries of North America (Canada, USA and Mexico) began to experience increasing influenza virus detections in early December; however transmission remained generally low until the end of the year. In the USA, as in most previous influenza seasons, influenza-like illness (ILI) activity was initially concentrated in the southern states, and then increased progressively in the central and western states. In the 3 countries, peak transmission occurred first in Mexico, in late January, followed by the USA,(3) in mid-March, and then by Canada in late March to early April.(4)
The distribution of influenza virus types and subtypes differed in these 3 countries. Influenza A(H1N1)pdm09 accounted for nearly all virus detections throughout the season in Mexico. In contrast, influenza A(H3N2) was the most commonly detected virus for most of the season in the USA with A(H1N1)pdm09 accounting for only 27% of influenza A viruses for which subtype data were available. Notably, regional differences were observed within the USA. In the southwestern part of the country A(H1N1)pdm09 viruses were more common than A(H3N2).
Although influenza type B viruses accounted for only 13% of virus detections overall in the USA, as the numbers of A(H3N2) virus detections declined in early April, detections of influenza B increased slightly, accounting for about one third of detections during the last few weeks of the season. In Canada, influenza B viruses accounted for >53% of virus detections nationally, a pattern which persisted throughout the season.
As in the USA, there was regional variation in virus distribution, and influenza A viruses outnumbered influenza B viruses in the western provinces of British Columbia and Alberta by >5 to 1. Nationally, 32% of influenza A viruses with subtype information were A(H1N1)pdm09 for the season.
Illness and mortality
In the USA, influenza activity was considerably less intense than in previous years; clinical consultations for ILI, reported hospitalizations, pneumonia and influenza mortality, and reported influenza-associated paediatric deaths were all lower than in recent years. The percentage of outpatient visits to sentinel physicians for ILI reached the national baseline of 2.4% but never exceeded it, a pattern which has not been observed in at least the last 15 years. Laboratory confirmed influenza-associated hospitalizations reported through the Emerging Infections Program, covering 80 counties in 10 of the 50 states of the USA were lower than the previous year (8.6 per 100 000 population as of 30 April versus 21 per 100 000 population in 2010?2011) and mortality attributed to pneumonia and influenza (P&I) in the 122 Cities Mortality Reporting System slightly exceeded the epidemic threshold (1.645 standard deviations above the weekly mean) only once this season and was below the weekly historical 5-year average for much of the season. The number of influenza-associated paediatric deaths was low during this season. Excluding the 2009 pandemic year, since the 2003?2004 season when paediatric influenza-associated death surveillance began, a median of 68 deaths per season (range 30?152) have been reported; as of 30 April 2012, only 24 deaths have been reported this season. Fewer severity-related data are available for Canada but the influenza season appeared similar to that in previous years. ILI consultation rates remained near the 15-year average throughout the 2011?2012 season. The total number of reported influenza or ILI institutional outbreaks peaked at 59 in mid-March, the highest peak reported since 2005 with the exception of the 2009 autumn/winter; however, the cumulative number of outbreaks did not notably differ from that in the previous season. The number of laboratory-confirmed influenza-associated hospitalizations reported (1674 as of 12 May 2012) from 7 reporting provinces was higher in the current season than previous years since 2005 (median 583, range 365?1464); however, the number of influenza-associated deaths decreased from 185 in the 2010?2011 season to 83 by the end of April this season.
In the USA, hospitalization rates for influenza were highest among those aged ≥65 years (30/100 000), followed by children aged 0?4 years (14/100 000).
Influenza A was identified in 87% of laboratory-confirmed influenza-associated hospitalizations and of those influenza A viruses with subtype information, 75% were A(H3N2), approximately the same proportions as reported for ILI cases. The most commonly reported underlying medical conditions among adults hospitalized with influenza were chronic lung diseases (of which 52% were asthma), cardiovascular disease and obesity. The most commonly reported underlying medical conditions in children hospitalized with influenza were chronic lung diseases (of which 76% were asthma) and neurological disorders. Of note, almost half of hospitalized children had no identified underlying medical condition.
Similarly in Canada, 33% of laboratory confirmed influenza-associated hospitalizations and 79% of deaths were among adults ≥65 years of age.
Among children ≤16 years of age, 35% of laboratory confirmed influenza-associated hospitalizations were among those ≤2 years of age. The distribution of influenza virus type or subtype varied between age groups in Canada. Among children <20 years of age, influenza B was detected in a higher proportion of ILI cases than influenza A (59% B versus 41% A), the reverse was true in adults (34% B versus 56% A). Differences were more pronounced when considering only the type A viruses, particularly at the extremes of age. Of influenza type A associated ILI cases in children <5 years old, 39% were A(H1N1)pdm09 viruses versus only 9% in adults ≥65 years.
Europe and the Middle East Transmission
The influenza season in Europe also started late (Figure 1).(5, 6) During September to November, most countries in Europe reported only low intensity or no transmission of influenza. Belgium, Italy, Norway, Spain, Sweden and Turkey reported an increasing trend in transmission in early December 2011 but transmission did not start in earnest in much of western Europe until after the end of the year and unlike previous years, there was no clear west to east progression in the countries of western Europe. Transmission began slightly later in eastern Europe. Influenza transmission peaked at the end of February or early March in most countries of western Europe. Transmission in the Estonia, Latvia, Russian Federation and Slovakia, and other countries in eastern Europe peaked in April. Increased numbers of ILI cases began to be reported as early as September 2011 in North Africa and the Middle East and peaked between mid-December and mid-January, 6 to 8 weeks earlier than in Europe.
Influenza transmission in Europe, North Africa and the Middle East was primarily associated with influenza A(H3N2) throughout most of the season, however, influenza B detections increased toward the end of the season. Overall, less than 13% of influenza viruses from outpatient specimens were influenza type B in Europe and of the influenza A viruses with subtype information, 98% were A(H3N2). However, an assessment published by the European Centres for Disease Control early in the season found that A(H1N1)pdm09 was overrepresented in severe cases compared with the other 2 circulating viruses in France, Ireland, Spain and the United Kingdom.(7) North Africa and the Middle East had a distribution of virus types and subtypes similar to that in Europe, although the influenza A season was more distinct as it occurred much earlier and was complete by the time that B viruses began circulating in low numbers, Nearly all of the influenza A viruses in North Africa were A(H3N2).
Qatar reported a late season wave of A(H1N1)pdm09 in March and April.
Illness and mortality
Overall, influenza severity indicators were not consistent across Europe. In western Europe, numbers of ILI cases seen in primary care settings were more variable than usual with some countries experiencing relatively few cases scarcely reaching the epidemic baseline (e.g. the United Kingdom and Ireland) while others had more typical seasons (e.g. France and Spain). The number of severe acute respiratory infections reported by 7 participating countries that were positive for influenza was slightly lower than last season (1282 as of 11 May 2012 versus 1548 at the end of the 2010?2011 season). The European Mortality Monitoring Project (EUROMOMO), which pools all-cause mortality data from 15 countries of Europe, reported excess mortality among persons ≥65 years of age in some countries, peaking in February 2012.(8) This was most notable in countries that experienced more community transmission, around the same time as the peak in influenza transmission. Mortality in the 15 to 64 year old age group was notably lower compared to the 2010?2011 season when A(H1N1)pdm09 was the predominant virus circulating in the area.
Northern Asia Transmission
The influenza season in the northern temperate areas of Asia began at the end of November 2011, one month later than in the previous season (Figure 1).(9) Transmission peaked in Japan and the Republic of Korea during mid to the end of January 2012 and about 4 weeks later in Mongolia and northern China. Influenza A(H3N2) was the predominant virus in Japan and Republic of Korea, accounting for almost all of the influenza A viruses detected. Both countries reported increased numbers of influenza type B virus isolates towards the end of the season although in Republic of Korea, this secondary transmission was quite marked. In contrast, the season in northern China and Mongolia was predominantly influenza B initially with transition to A(H3N2) late in the season.
Illness and mortality
Reporting rates for ILI visits in northern China, Mongolia and Republic of Korea were all similar to those in previous seasons; however, Japan experienced the highest number of influenza-confirmed cases since 2002, except for the 2009 pandemic period. In Mongolia, the proportion of hospitalizations for pneumonia and the reported number of pneumonia deaths were lower than during the 2010?2011 season.
Antigenic testing
The seasonal trivalent vaccine for 2011?2012 contained the same 3 viruses as the 2010?2011 northern hemisphere vaccine: A/California/7/2009 (H1N1)-like virus, A/Perth/16/2009 (H3N2)-like virus and B/Brisbane/60/2008-like virus (B Victoria lineage). Early in the season, nearly all of the influenza A viruses detected globally were antigenically similar to the vaccine viruses.(10) However, increasing antigenic diversity was noted in A(H3N2) viruses in the latter part of the season. These viruses had reduced titre cross-reactivity with antiserum produced against the A/Perth/16/2009 virus but higher titres against A/Victoria/361/2011-like reference viruses. In Europe this was associated with lower vaccine effectiveness than in previous seasons in well controlled field observational studies.(11, 12) Because of antigenic heterogeneity within influenza A(H3N2) viruses circulating during this influenza season, the updated trivalent influenza vaccine for the northern hemisphere will contain an A/Victoria/361/2011-like virus.(13) Influenza B viruses of both the B/Victoria and the B/Yamagata lineages circulated during this influenza season in nearly equal proportions in some areas. The increasing proportion of viruses of the B/Yamagata lineage prompted a change in the next season vaccine composition to include a Yamagata virus (B/Wisconsin/1/2010-like virus).
Antiviral sensitivity testing
Very little resistance to neuraminidase inhibitors was found in influenza viruses in the 2011?2012 season. In Europe, Canada and China resistance to neuraminidase inhibitors (oseltamivir and zanamivir) was undetectable throughout the season. However, oseltamivir resistance was reported in 1.5% (16) of the 1067 influenza A(H1N1) pdm09 viruses tested in the USA late in the season. Information related to oseltamivir exposure is available for 14 of the 16 cases, of which 3 had taken oseltamivir for 1 day or more at the time of specimen collection, 2 had family members using oseltamivir, and 9 had no exposure to oseltamivir. Of the 16 oseltamivir resistant viruses, 11 were from the state of Texas. Of 371 influenza A(H1N1)pdm09 specimens tested from Texas, 11 (3.0%) collected from January to April 2012 were resistant to oseltamivir. All oseltamivir resistance was associated with a H275Y substitution in the neuraminidase gene. Countries performing sensitivity analysis for resistance to adamantanes (amantadine and rimantadine), continued to report high levels of resistance to adamantanes in A(H1N1)pdm09 and A(H3N2) viruses.
Summary
The 2011?2012 influenza season was notably late in most of the northern hemisphere temperate zone, with the exception of North Africa. The season was remarkably mild in the USA, where influenza A(H3N2) was the predominant virus circulating (Map 1). Severity data are limited in other parts of the world, however, the available data indicate that the season did not notably exceed levels seen in previous recent seasons in the rest of the temperate zone and was milder than usual in many, but not all, European countries. Of note, a few countries of Europe reported excess mortality among individuals ≥65 years of age which coincided with local transmission of A(H3N2), and the predominant virus circulating varied widely fin different countries. In North America Mexico reported almost exclusively influenza A(H1N1)pdm09, which was also seen in Central America, while the USA reported much more A(H3N2) except in the states bordering Mexico. Canada had predominance of type B nationally, which increased late in the season, but reported more A(H3N2) in the western provinces.
The season was predominantly associated with A(H3N2) in Europe and North Africa, though influenza B did in crease slightly late in the season.
Temperate countries of Asia had both influenza B peaks and A(H3N2) peaks, with influenza B appearing first in China and Mongolia followed by A(H3N2) and the reverse sequence in the Republic of Korea and Japan.
Few data were available regarding the age distribution of viruses, however, in Canada it appeared that A(H1N1) pmd09 continues to show a marked tendency to affect younger age groups disproportionately.
Notably, A(H3N2) has now begun to drift, requiring a change in vaccine composition for the northern hemisphere 2012?2013 influenza season, and the proportion of influenza B viruses belonging to the B/Yamagata lineage has also increased prompting a change in the B virus for the next vaccine.
The great majority of the viruses tested this season were sensitive to oseltamivir. However, the late-season appearance of a number of cases with oseltamivir resistant A(H1N1)pdm09 viruses in Texas, most of which had no direct or indirect exposure to the drug, raises some concern. A cluster of 29 oseltamivir resistant viruses was reported in New South Wales, Australia in the 2011 southern hemisphere winter season but did not result in onward persistence of the resistant virus.
---
(1) FluNet is available online at: http://www.who.int/influenza/gisrs_laboratory/flunet/en/ accessed June 2012.
(2) FluID is available online at: http://who.int/influenza/surveillance_monitoring/fluid/en/ accessed June 2012.
(3) For additional information, see the United States Centers for Disease Control and Prevention?s FluView at http://www.cdc.gov/flu/weekly/ accessed June 2012.
(4) For additional information, see the Public Health Agency of Canada?s FluWatch at http://origin.phac-aspc.gc.ca/fluwatch/ accessed June 2012.
(5) For additional information see WHO/Europe influenza surveillance (EuroFlu.org) is available at http://www.euroflu.org/index.php accessed June 2012.
(6) For additional information see The European Centre for Disease Prevention and Control?s Weekly influenza surveillance overview available at: http://ecdc.europa.eu/en/healthtopics/seasonal_influenza/epidemiological_data/pages/weekly_influenza_surveillance_overview.aspx accessed June 2012
(7) Beaut? J et al. Overrepresentation of influenza A(H1N1)pdm09 virus among severe influenza cases in the 2011/12 season in four European countries. Eurosurveillance. 2012;17(9)
ii=20105. (Available at http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20105 accessed June 2012.)(8) Mazick A et al. Excess mortality among the elderly in 12 European countries, February and March 2012. Eurosurveillance. 2012;17(14)
ii=20138. (Available at http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20138 accessed June 2012.)(9) WHO Regional Office for the Western Pacific?s Influenza situation update is available at http://www.wpro.who.int/health_topics/influenza accessed June 2012.)
(10) For additional information on vaccine strain selection see Recommended composition of influenza virus vaccines for use in the 2012?2013 northern hemisphere influenza season available at http://www.who.int/influenza/vaccines/virus/recommendations/2012_13_north/en/index.html accessed June 2012
(11) Jim?nez-Jorge S et al. Early estimates of the effectiveness of the 2011/12 influenza vaccine in the population targeted for vaccination in Spain, 25 December 2011 to 19 February 2012. Eurosurveillance. 2012;17(12)
ii=20129. (Available at http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20129 accessed June 2012.)(12) Kissling E, Valenciano M, I-MOVE case?control studies team. Early estimates of seasonal influenza vaccine effectiveness in Europe among target groups for vaccination: results from the I-MOVE multicentre case?control study, 2011/12. Eurosurveillance. 2012;17(15)
ii=20146. (Available at http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20146 accessed June 2012.)(13) Recommended composition of influenza virus vaccines for use in the 2012?2013 northern hemisphere influenza season. Geneva, World Health Organization 2012. (Available at: http://www.who.int/influenza/vaccines/virus/recommendations/201202_recommendation.pdf accessed June 2012.)
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