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Influenza A(H1N1) 2009 virus: current situation and post-pandemic recommendations (WHO, Feb 18 2011, edited)

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  • Influenza A(H1N1) 2009 virus: current situation and post-pandemic recommendations (WHO, Feb 18 2011, edited)

    Influenza A(H1N1) 2009 virus: current situation and post-pandemic recommendations (WHO, Feb 18 2011, edited)

    [Source: World Health Organization, Weekly Epidemiological Record, full PDF document (LINK). Extract, edited.]

    Weekly epidemiological record
    Relevé épidémiologique hebdomadaire

    18 february 2011, 86th year / 18 février 2011, 86e année
    No. 8, 2011, 86, 61–72

    Influenza A(H1N1) 2009 virus: current situation and post-pandemic recommendations

    Since its emergence in April 2009, pandemic influenza A(H1N1) 2009 virus has affected all parts of the world, causing the first influenza pandemic since 1968. Following the observation that 1 or 2 epidemic periods had occurred in most countries and in most WHO Regions, WHO declared on 10 August 2010 that the world was moving into the post-pandemic period. Based on experiences with past pandemics, the pandemic influenza A(H1N1) 2009 virus was expected to take on the behaviour of a seasonal influenza virus and continue to circulate. However, localized outbreaks or epidemics that include serious disease in younger age groups were expected to continue during the immediate post-pandemic period.(1)

    At the time of WHO’s announcement, India, New Zealand and several countries in the Caribbean were experiencing widespread, and in some places intense, transmission of the pandemic influenza A(H1N1) 2009, while other countries were reporting active circulation of seasonal influenza A(H3N2) and type-B viruses.(2)

    Current situation – January 2011

    The northern hemisphere is now in the winter period for the circulation of seasonal influenza viruses. As was seen during 2010 in the southern hemisphere winter season, a mixed pattern is being observed.(3)

    Influenza activity in North America has been primarily associated with influenza A(H3N2) virus with some cocirculation of influenza type-B virus. In the United Kingdom, severe and fatal cases of influenza have been reported, associated predominantly with pandemic influenza A(H1N1) 2009 virus, and up to 25% of intensive-care beds in the United Kingdom have been occupied by influenza patients.

    Severe disease associated with pandemic influenza A(H1N1) 2009 virus, and to a lesser extent with influenza type-B, has also been increasingly reported on the European continent and in areas of the Middle East. In the tropics, several countries in southern Asia have seen increasing activity during January 2011, mainly associated with the pandemic influenza A(H1N1) 2009 virus.

    Other tropical areas of the world and the temperate countries of the southern hemisphere are currently reporting little circulation of influenza virus.

    Laboratory data indicate that the recommended trivalent vaccines are still well matched to the circulating viruses, and resistance to oseltamivir remains infrequent and sporadic.(3)

    WHO recommendations for the post-pandemic period

    While the level of concern has now diminished, vigilance on the part of national health authorities remains important. Such vigilance is especially critical in the immediate post-pandemic period, when the behaviour of the pandemic influenza A(H1N1) 2009 virus as a seasonal virus cannot be reliably predicted. WHO has published several recommendations for activities to be undertaken during the post-pandemic period; these are summarized below.

    1. Surveillance

    The purpose of surveillance during the post-pandemic period is to provide information that will enable the timely detection of an epidemic or pandemic, detect changes in the behaviour of circulating viruses, inform decisions on health policy and clinical management, and guide the selection of strains for vaccines.(4)

    The early detection activities that individual Member States carry out will vary according to their resources.

    Specific signal events that should trigger an investigation include:
    • abrupt, unexpected changes in the trend of severe respiratory disease observed by routine surveillance systems;
    • clusters of severe respiratory disease or pneumonia occurring in families, workplaces or social networks;
    • an unexpected pattern of respiratory disease or pneumonia, such as an increase in apparent mortality, a shift in the age group associated with severe influenza, or a change in the pattern of the clinical presentation of influenza-associated disease;
    • health-care workers developing severe respiratory disease;
    • unusually high levels of sales of pharmaceuticals used to treat respiratory disease;
    • respiratory disease in humans that is associated with illness in animals;
    • human cases of infection with an influenza virus not currently circulating in human populations.

    If the results of an investigation meet the reporting criteria as defined by International Health Regulations 2005, the event should be reported within 24 hours to the WHO focal point for these regulations.

    Routine surveillance for respiratory disease plays a critical role in defining expected baseline rates of disease, understanding the overall burden of illness relative to other diseases, and describing the groups at risk for severe disease. Routine monitoring will also provide data that are useful for health-care planning. Member States are encouraged to use standard WHO case definitions for surveillance.(5, 6, 7)

    Routine surveillance systems should collect at least a limited set of epidemiological and clinical data including:
    • the number of cases of influenza-like illness, severe acute respiratory infection, acute respiratory infection or pneumonia detected by sentinel sites, depending on the surveillance strategy adopted by the Ministry of Health;
    • the number of deaths from respiratory illness detected at each sentinel site;
    • data on risk factors, including the proportion of severe cases with pre-existing respiratory disease, heart disease, diabetes, neurological disorders, liver disease or immunodeficiency, or were pregnant;
    • the total number of outpatients seen and inpatients admitted at the above sentinel sites (this will provide the denominator data for calculating the proportion of patients who have respiratory disease) or the population of the catchment area of the sentinel sites.

    Member States are requested to share these data globally.

    2. Laboratory and virology

    The collection and submission of clinical samples during the post-pandemic period should follow procedures similar to those of the pre-pandemic period. Samples should be collected and tested during routine epidemiological surveillance (described above). Detailed information on methods for detecting and characterizing pandemic influenza A(H1N1) 2009 virus, influenza A(H3N2) and influenza B viruses is available from WHO.(8, 9)

    Representative specimens should be submitted to the WHO collaborating centres for influenza for additional characterization and analysis.

    The criteria for selecting specimens or virus isolates to send to the collaborating centres are available from WHO(10) and should include:
    • representative8 specimens collected within 4–6 weeks or virus isolates;
    • all viruses that yield lower titres than expected in the haemagglutination inhibition (HI) test;
    • any virus that cannot be subtyped (these should be sent to a WHO collaborating centre as quickly as possible);
    • a subset of clinical specimens or isolates from severe cases and unusual outbreaks.

    The laboratory results of testing should be shared with WHO through the FluNet(11) global reporting system or through the regional databases linked with FluNet, such as EuroFlu.(12)

    Member States are encouraged to continue to test for antiviral resistance in pandemic influenza A(H1N1) 2009 viruses.(8) Events of epidemiological and virological significance relevant to monitoring the antiviral susceptibility of pandemic influenza A(H1N1) 2009 virus that should be reported to the WHO include:
    • any indication of person-to-person transmission of pandemic influenza A(H1N1) 2009 that are resistant to antiviral medicines, including the occurrence of clusters of cases;
    • changes in the results of routine virological surveillance
      • for example, detecting higher than normal instances of antiviral resistance in pandemic influenza A(H1N1) 2009 viruses;
    • detection of novel mutations and genetic markers for antiviral resistance.

    It is also important to document complementary clinical information, such as the clinical background of the cases in which resistance was detected. Viruses found or suspected to be resistant to antiviral medicines should be sent to a WHO collaborating centre for further characterization.

    3. Vaccination

    WHO strongly recommends vaccination to protect against pandemic influenza A(H1N1) 2009 virus(13, 14) as well as the other seasonal influenza strains(15) in keeping with national priorities and with regard to groups recognized to be at higher risk of severe disease. The principal aim of vaccination is to reduce morbidity and mortality, and to reduce transmission of the virus within communities.

    Trivalent vaccines are designed to protect against all 3 currently circulating influenza viruses.(16, 17)

    For individuals targeted for immunization, WHO recommends annual revaccination because (i) the duration of protection is uncertain and may be shorter in groups considered to be at higher risk, such as elderly people and immunocompromised patients, and (ii) viruses usually drift antigenically so that protection from prior vaccination cannot be assured from one season to the next.

    Where the trivalent vaccine is not available, the monovalent A(H1N1) 2009-based vaccine may be used for protection, but this vaccine will not protect against other circulating seasonal strains.

    4. Clinical management

    Pandemic influenza A(H1N1) 2009 virus continues to cause severe illness and death in all age groups and in otherwise healthy individuals. Other seasonal strains of influenza are also associated with severe illness and death,(18) although these outcomes are less common among younger age groups. Early recognition and prompt treatment of severe, or potentially severe, cases remain important, and WHO guidelines for the use of antiviral medicines, which refer to both seasonal and pandemic influenza, should continue to be followed.(19)

    In particular, WHO recommends that patients who have severe or deteriorating clinical illness should be treated with oseltamivir as soon as possible. Patients with uncomplicated illness who are in a group known to be at higher risk of developing severe or complicated influenza should be treated with oseltamivir or zanamivir as soon as possible. These recommendations apply to all groups of patients, including women who are pregnant, postpartum or breastfeeding. The WHO guidance on other aspects of clinical management for severely ill patients should continue to be followed.(20, 21)


    Experience during the 2010–2011 influenza season in the northern hemisphere has demonstrated that pandemic influenza A(H1N1) 2009 virus is still circulating and, importantly, still causing severe disease in younger people. Continued vigilance is imperative, and the appropriate surveillance, control and treatment practices that allow for the control of both established seasonal influenza strains and pandemic influenza A(H1N1) 2009 virus should be maintained during the post-pandemic period.

    (1) H1N1 in post-pandemic period: Director-General’s opening statement at virtual press conference, 10 August 2010. Geneva, World Health Organization ((LINK); accessed January 2011).
    (2) Influenza: update 113. Geneva, World Health Organization ((LINK); accessed January 2011).
    (3) See (LINK); accessed January 2011.
    (4) Surveillance recommendations for Member States in the post pandemic period: 12 August 2010. Geneva, World Health Organization ((LINK), accessed January 2011).
    (5) WHO Regional Office for Europe guidance for influenza surveillance in humans, Copenhagen, World Health Organization Regional Office for Europe ((LINK), accessed January 2011).
    (6) PAHO-CDC generic protocol for influenza surveillance, 2006. Washington DC, Pan American Health Organization (PAHO) ((LINK), accessed February 2011).
    (7) A practical guide to harmonizing virological and epidemiological influenza surveillance. Manila, WHO Regional Office for the Western Pacific , 2008 (LINK), accessed January 2011).
    (8) WHO information for laboratory diagnosis of pandemic (H1N1) 2009 virus in humans: revised. Geneva, World Health Organization, 2009 ((LINK), accessed January 2011).
    (9) See No. 46, 2010, pp. 453–460.
    (10) Selection of clinical specimens for virus isolation and of viruses for shipment from National Influenza Centres to WHO Collaborating Centres: revised. Geneva, World Health Organization, 2010 ((LINK), accessed January 2011).
    (11) See (LINK); accessed January 2011.
    (12) See (LINK); accessed January 2011.
    (13) See No. 30, 2009, pp. 301–304.
    (14) See No. 22, 2010, pp. 197–212.
    (15) See No. 33, 2005, pp. 279–287.
    (16) See No. 10, 2010, pp. 81–92.
    (17) See No 41, 2010, pp. 402–412.
    (18) Ellis J et al. Virological analysis of fatal influenza cases in the United Kingdom during the early wave of influenza in winter 2010/11. Euro Surveillance, 2011;16(1):pii=19760 ((LINK), accessed January 2011).
    (19) WHO guidelines for pharmacological management of pandemic influenza A(H1N1) 2009 and other influenza viruses. Geneva, World Health Organization, 2010 ((LINK), accessed January 2011).
    (20) Clinical management of adult patients with complications of pandemic influenza A(H1N1) 2009: emergency guidelines for the management of patients with severe respiratory distress and shock in district hospitals in limited- resource settings. Geneva,
    World Health Organization, 2010 ((LINK), accessed January 2011).
    (21) Clinical management of human infection with pandemic (H1N1) 2009: revised guidance. Geneva, World Health Organization, 2009 ((LINK), accessed January 2011).