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Human infection with new influenza A (H1N1) virus: Mexico, update, March?May 2009 (WHO WER, June 5, 2009, edited)

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  • Human infection with new influenza A (H1N1) virus: Mexico, update, March?May 2009 (WHO WER, June 5, 2009, edited)

    Human infection with new influenza A (H1N1) virus: Mexico, update, March?May 2009 (WHO WER, June 5, 2009, edited)

    Weekly epidemiological record - 5 JUNE 2009, 84th YEAR - No. 23, 2009, 84, 213?236

    [http://www.who.int/wer - Original Document: LINK. EDITED.]


    Human infection with new influenza A (H1N1) virus: Mexico, update, March?May 2009


    On 12 April 2009, the Government of Mexico responded to a request by WHO for verification of an outbreak of acute respiratory infections in the small rural community of La Gloria, Veracruz. During 15?17 April, the Ministry of Health received informal notification of clusters of rapidly progressive severe pneumonia occurring mostly in the Federal District (metropolitan Mexico City) and San Luis Potosi. In response, national surveillance for acute respiratory infections and pneumonia was intensifi ed on 17 April. During 22?24 April, new influenza A (H1N1) virus infection, previously identifi ed in 2 children in the United States,1 was confirmed in several patients. This report updates a previous report(2) on the outbreak in Mexico of human cases of new influenza
    A (H1N1) virus infection and summarizes public health actions taken to date by the Government of Mexico to monitor and control the outbreak.


    During 1 March?29 May 2009, national surveillance identified 41 998 people with acute respiratory infections, of whom 5337 (12.7%) were confirmed with new influenza A (H1N1) virus infection by real-time reverse transcription?polymerase chain reaction (rRT-PCR). As of 29 May, 97 patients with laboratory-confirmed infection had died. Epidemiological evidence to date suggests that the outbreak likely peaked nationally in late April, although localized cases continue to be identified.


    Enhanced surveillance

    The outbreak of acute respiratory infections in La Gloria, Veracruz (population 2155) was characterized by a large number of cases (616 inhabitants; 28.5% of the population) reported during 5 March?10 April 2009. This outbreak was probably of mixed cause: subsequent testing of respiratory specimens collected at the time identified 3 patients as positive for different seasonal influenza virus strains (2 influenza A (H3N2) and 1 influenza B) and 1 patient as positive for new influenza A (H1N1) virus with an adenovirus coinfection. A large majority of the respiratory illnesses from this outbreak remain undiagnosed. No severe cases or deaths were observed.

    During March and April 2009, clusters of rapidly progressive severe pneumonia were identified in Mexico City, San Luis Potosi and other cities. These included 47 cases and 12 deaths; 4 of the deaths were positive for new influenza A (H1N1) virus infection. In response to the La Gloria outbreak and the pneumonia clusters, the Mexico national committees of epidemiological surveillance and emerging infectious diseases released an epidemiological alert on 17 April to enhance national surveillance for acute respiratory infections and severe pneumonia. Active case-finding was implemented in hospitals throughout the country, including daily zero-reporting (requiring facilities and jurisdictions to report even if no suspected cases had been identified), and monitoring of news media and other sources.

    The Ministry of Health also initiated investigations of outbreaks throughout the country, with the assistance of the WHO Global Outbreak and Alert Response Network, coordinated by the Pan American Health Organization.

    During 18?19 April 2009, a survey conducted in 23 hospitals in Mexico City indicated increased pneumonia-related hospital admissions since 10 April, particularly among young adults. On 21 April, respiratory specimens collected as a result of these enhanced surveillance activities were sent to the National Microbiology Laboratory of the Public Health Agency of Canada and to the Influenza Division at the United States Centers for Disease Control and Prevention (CDC). During 22?24 April, both laboratories identifi ed new influenza A (H1N1) virus infection in specimens collected from Mexican patients.

    The Directorate General of Epidemiology (DGE) established an Internet-based reporting platform to collect case-based epidemiological information and a daily epidemiological bulletin to disseminate the results of ongoing investigations and recommendations from DGE.

    This bulletin was first released on 26 April 2009.

    In May 2009, the Ministry of Health of Mexico revised its case definition of a suspected case of new influenza A (H1N1) virus infection. The initial definition, which included any hospitalized patient with severe acute respiratory illness, was expanded on 1 May to include any person with acute respiratory illness (defined as fever and either sore throat or cough). On 11 May, this definition was refined to include any person with fever, cough, and headache plus at least 1 of the following symptoms: rhinorrhoea, coryza, arthralgia, myalgia, prostration, sore throat, chest pain, abdominal pain or nasal congestion. In children aged <5 years, irritability replaced headache. A laboratory-confirmed case of new influenza A (H1N1) virus infection was defined as any person in whom a respiratory specimen tested positive for new influenza A (H1N1) virus infection by rRT-PCR.

    During 2008, in compliance with the requirement to strengthen core capacities under the International Health Regulations (2005), the Ministry of Health of Mexico increased its number of influenza sentinel sites from 380 to 520 and expanded influenza testing capacity to 4 additional states. In mid-April 2009, enhanced surveillance for cases of new influenza A (H1N1) virus infection generated an increase in the number of clinical specimens collected from patients with acute respiratory illness and a surge in testing capacity at the national laboratory from approximately 30 specimens to 900 daily. Enhancement of surveillance also included expanding influenza testing capacity to 8 states with rRT-PCR and 30 states with immunofluorescence.

    As of 29 May 2009, a total of 22 184 specimens had been tested using rRT-PCR, of which 5337 (24%) tested positive for new influenza A (H1N1) virus infection. Of these 5337 laboratory-confirmed cases, 41.9% were aged <15 years, 32.3% were aged 15?29 years, 23.7% were aged 30?59 years and 2.1% were aged >60 years. Among fatal cases, those who died were generally older: 55.7% of deaths occurred among patients aged 30?59 years (Table 1). Some 49% of patients with confirmed infection were female.

    As of 29 May 2009, the Federal District (Mexico City) had the highest number of laboratory-confirmed cases (1804) and deaths (38) of new influenza A (H1N1) virus infection; Mexico State reported 21 deaths (Fig. 1). The peak number of confi rmed cases (375) had onset of illness on 27 April (Fig. 2). As of 29 May, all states in Mexico had reported laboratory-confirmed cases of new influenza A (H1N1) virus infection.


    Control measures

    On 24 April 2009, the Government of Mexico activated the National Pandemic Preparedness and Response Plan and announced the closure of schools in the metropolitan area of Mexico City. Concurrently, the Ministry of Health launched a mass media campaign to promote respiratory hygiene and to alert the public about transmission of influenza. Additional social distancing measures included closure of restaurants and entertainment venues and cancellation of large public gatherings nationwide.

    To date, Mexico continues enhanced national surveillance and early antiviral treatment, coupled with case isolation, to decrease transmission of new influenza A (H1N1) virus infection. Respiratory hygiene and hand-washing are promoted through television and the printed media.

    On 11 May 2009, as schools re-opened, parents were reminded to keep their children at home if they had symptoms of influenza. In addition, a team of teachers and parents screened children at school entrances to determine whether they had fever or respiratory symptoms.

    The ministries of education and health recommended the closure of classrooms where >2 children presented respiratory symptoms and of schools with ill children in >2 classrooms. On the first day of this strategy, 91 357 children were detected as symptomatic. This screening practice was suspended on 23 May.


    Reported by:

    Office of the Secretary of Health, Ministry of Health, Mexico (Secretar?a de Salud, M?xico, http://portal.salud.gob.mx); Directorate General of Epidemiology, Ministry of Health (Direcci?n General Adjunta de Epidemiolog?a, http://www.dgepi.salud.gob.mx); National Institutes for Epidemiological Reference and Diagnostics (Institutos de Diagn?stico y Referencia Epidemiol?gicos; http://www.cenavece.salud.gob.mx/indre/); National Institute of Public Health, Mexico (Instituto Nacional de Sal?d Publica, http://www.insp.mx/); Pan American Health Organization/World Health Organization (http://new.paho.org/hq/); Public Health Agency of Canada (http://www.phac-aspc.gc.ca/); National Institute for Respiratory Diseases (Instituto Nacional de Enfermedades Respiratorias, http://www.iner.salud.gob.mx); National Center for Preparedness, Detection, and Control of Infectious Diseases (http://www.cdc.gov/ncpdcid/); Epidemic Intelligence Service (http://www.cdc.gov/eis/index.html).


    Editorial note.

    Trends in case counts in Mexico suggest that new influenza A (H1N1) virus activity is now decreasing, although localized transmission continues to occur. The epidemic curve of laboratory-confirmed cases remains incomplete because of a backlog of untested specimens. However, data regarding suspected cases(3) also indicate a peak in late April 2009, and delays from case identification to reporting have decreased to a median of <2 days (Mexico Office of the Secretary of Health, unpublished data, 2009).

    Taken together, these data suggest that the outbreak has likely moved beyond a peak nationally, although a pattern of heterogeneous transmission and focal outbreak activity remains.

    Several features of the outbreak in Mexico are consistent with outbreaks of the same strain of new influenza virus circulating in the United States and other countries.

    These features include human-to-human transmission during a period that is typically the low season for circulation of influenza viruses(4) and an age distribution of laboratory-confirmed cases that includes severe disease and deaths among Mexican children and adults aged <60 years.(4)

    Some deaths have occurred among previously healthy people,(4) and several patients have experienced an aggressive clinical course with severe pneumonia requiring ventilator support and progression to acute respiratory distress syndrome.(2), (5), (6)

    A recently reported serological study suggests that children and younger adults who have no or low levels of serum antibody, respectively, are cross-reactive for the new influenza A (H1N1) virus. Approximately one-third of adults in the United States aged >60 years who were tested had cross-reactive neutralizing antibodies; however, the extent to which such antibodies might be protective remains unknown.(7) The serological data, along with the age distribution of illness and clinical severity from the outbreak in Mexico, support a younger age group as being at risk for infection and serious illness from new A (H1N1) virus infection.

    The current pattern of transmission of new influenza A (H1N1) virus infection in the northern hemisphere includes many localized outbreaks, several of which have occurred among schoolchildren.(8) This pattern is consistent with infl uenza outbreaks that have been occasionally reported outside of the usual influenza season.(9) However, an unprecedented number of such off-season outbreaks are now occurring that also involve extension into the community, as demonstrated by confirmed illness among travellers with no known epidemiological link to focal outbreaks. Similar patterns of off-season outbreaks have been observed with the emergence and sustained transmission of other strains of new influenza A virus infection among humans.(10)

    The recent introduction of new influenza A H1N1 virus into several countries in the southern hemisphere at the beginning of their influenza season, and the presumed susceptibility among much of the population to this new virus, suggest that new influenza
    A (H1N1) virus might become a dominant circulating virus in the southern hemisphere over the coming months. The Government of Mexico continues to coordinate a national response, engage partners, increase surge capacity and implement mitigation measures to slow the spread of transmission with new influenza A (H1N1) virus infection. Investigations are ongoing to monitor virus circulation and to evaluate the impact of mitigation strategies that might help guide prevention and control strategies in Mexico and worldwide.

    As of 1 June 2009, 64 countries worldwide had reported at least 1 laboratory-confirmed case of human infection with new influenza A (H1N1) virus. Because many of these infected individuals may have travelled while being asymptomatic, pre-departure screening has limitations in controlling export of the virus. However, raised awareness in travellers has proven efficacious in prompting spontaneous reporting of influenza-like illness by travellers to health authorities at points of entry.

    1) Update: swine influenza A (H1N1) infections, California and Texas, April 2009. Morbidity and Mortality Weekly Report, 2009, 58:435?437 (available at http://www.cdc.gov/mmwr/preview/mmwr...58d0424a1.htm; accessed June 2009).
    2) 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...58d0421a1.htm; accessed June 2009).
    3) Direcci?n General Adjunta de Epidemiolog?a (Directorate General of Epidemiology). Brote de influenza A H1N1 M?xico. Bolet?n Diario, 2009, 18 May, No. 22 Directorate General of Epidemiology (available at http://www.sld.cu/galerias/pdf/bolet...ion_sobre_infl uenza_a_h1n1__18-05-09.pdf; accessed June 2009).
    4) Viboud C, Alonso WJ, Simonsen L. Influenza in tropical regions. PLoS Medicine, 2006;3:e89 (available at http://www.plosmedicine.org/article/....pmed.0030089; accessed June 2009).
    5) See No. 21, 2009, pp. 185?189.
    6) Hospitalized patients with novel influenza A (H1N1) virus infection?California, April?May, 2009. Morbidity and Mortality Weekly Report, 2009, 58:536?541 (available at http://www.cdc.gov/mmwr/preview/mmwr...58e0518a1.htm; accessed June 2009).
    7) Serum cross-reactive antibody response to a novel influenza A (H1N1) virus after vaccination with seasonal infl uenza vaccine. Morbidity and Mortality Weekly Report, 2009, 58:521?524 (available at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5819a1.htm; accessed June 2009).
    8) Swine-origin influenza A (H1N1) virus infections in a school?New York City, April 2009. Morbidity and Mortality Weekly Report, 2009, 58:470?472 (available at http://www.cdc.gov/mmwr/preview/mmwr...58d0430a1.htm; accessed June 2009).
    9) Kohn MA et al. Three summertime outbreaks of influenza type A. Journal of Infectious Diseases, 1995,172:246?249.
    10) Taubenberger JK, Morens DM. 1918 influenza: the mother of all pandemics. Emerging Infectious Diseases, 2006,12:15?22 (available at http://www.cdc.gov/ncidod/eid/vol12no01/05-0979.htm; accessed June 2009).

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