Pandemic (H1N1) 2009: global situation
Chronology
On 12 April 2009, Mexico responded to WHO’s request for verification of an outbreak of acute respiratory illness (ARI) in the small community of La Gloria, Veracruz. During 15–17 April, the country’s 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, on April 17, Mexico intensified national surveillance for ARI and pneumonia. During 22–24 April, novel influenza A (H1N1) virus infection, previously identified in two children in the United States of America, was confirmed in several patients.
The virus was found to be genetically related to swine influenza A (H1N1) viruses recently circulating in pigs in North America and Europe/Asia, but with a genetic makeup not previously detected among viruses infecting either swine or human populations. It has a mixture of genes from viruses circulating in the two geographical regions and comprises six genes similar to those of the American “triple reassortant” swine viruses (including the haemagglutinin) and two genes encoding the neuraminidase and matrix proteins similar to those of Eurasian swine viruses.
On 24 April, WHO, in line with the International Health Regulations (IHR), declared a public health event of international concern, and advised countries to intensify surveillance for unusual outbreaks of influenza-like illness (ILI) and severe pneumonia. When sustained community transmission of the new virus in Mexico was demonstrated, WHO declared pandemic phase 4 on 27 April. Given the widespread occurrence of the virus at this stage, containment of the outbreak was not considered feasible, and WHO recommended that countries focus on mitigation, and not close borders or restrict international travel.
On 29 April, as there were two affected countries in the same WHO region, WHO declared pandemic phase 5. WHO advised countries immediately to activate their pandemic preparedness plans, to heighten surveillance for the early detection and treatment of cases, and to enhance infection control practices in all health facilities.
When sustained community spread occurred in multiple WHO regions, WHO declared pandemic phase 6 on 11 June 2009. Within nine weeks, all WHO regions had reported cases of pandemic (H1N1) 2009. WHO advised countries with few or no cases to be vigilant, and those with widespread transmission to focus on managing patients, conducting selective laboratory testing and investigating cases. WHO continued to discourage restrictions on travel and border closures.
Clinical disease and risk groups
As of 11 October 2009, there had been more than 399 000 laboratory-confirmed cases of pandemic influenza H1N1 2009 worldwide, of which the large majority had been mild, with patients experiencing ILI. Nevertheless, severe disease and deaths have also occurred and over 4735 deaths have been reported to WHO. Most deaths had been caused by severe viral pneumonia, and 50–80% of severe cases had been reported in people with underlying medical conditions, such as chronic lung disease (including asthma), cardiovascular diseases, diabetes and immunosuppression: similar to the risk factors seen in severe outcomes of seasonal influenza. In addition, severe disease and deaths have been observed in pregnant women (particularly those in the third trimester) and people with obesity.
Precisely documenting the proportion of severe cases of pandemic (H1N1) 2009 with underlying co-morbidities, however, is complicated by a lack of standardization in surveillance and data collection globally. This challenge notwithstanding, a striking feature of this new virus that distinguishes it from circulating seasonal influenza strains is the larger number of otherwise healthy people under 50 years of age that experience rapid progression to severe or fatal illness, characterized by severe pneumonia that destroys lung tissue, and multiple organ failure. These findings form the basis for the current WHO recommendations on priority groups for pandemic vaccine.
Clusters of severe cases have also been observed in indigenous and rural populations around the world, where the prevalence of chronic underlying conditions is often higher and there is poor access to medical care. Australia provides an example. Delays in antiviral treatment have been noted among fatal cases, and may be anticipated to be risk factors for severe outcomes during the upcoming influenza season.
Impact on health care services
The emergence of this pandemic virus has placed some strain on health care services in different locations. At times, higher-than-normal numbers of febrile people have sought care for ILI from general practitioners (GPs) or emergency departments. This has placed a strain on outpatient care providers and has been observed in Member States in the Region since week 16/2009. In the southern hemisphere, Argentina, Australia, Chile, New Zealand and Uruguay reported early regional surges in hospital, emergency department and outpatient visits. Some countries reported transient shortages of hospital beds, equipment or medication. These have tended to be geographically isolated and relatively short lived, however.
The increasing burden of disease associated with pandemic (H1N1) 2009 has also placed stress on intensive care units (ICUs) in some localities. Clinicians participating in periodic WHO teleconferences agreed that almost all ICU hospitalizations were primarily a result of acute respiratory distress syndrome (ARDS), caused by viral pneumonia; secondary bacterial pneumonia has rarely been diagnosed.
Surveillance
During the early stages of the pandemic, WHO advised countries to be vigilant and enhance surveillance to detect the first cases of infection, and requested them, where possible, to perform detailed clinical, epidemiological and virological investigations of early cases to gain information on the clinical spectrum of the disease, the proportion of cases with severe illness and risk groups for severe outcomes. To facilitate the rapid identification of cases, the Centers for Disease Control and Prevention (CDC) in the United States (the WHO Collaborating Centre for the Surveillance, Epidemiology and Control of Influenza) rapidly distributed kits for the molecular detection of pandemic (H1N1) 2009 to all national influenza centres worldwide.
On 16 July 2009, WHO published updated guidance on pandemic surveillance. After notifying WHO of the first cases of pandemic (H1N1) 2009 virus infection, countries are no longer required to confirm all cases in laboratories. As long as feasible, however, countries are requested to report the number of confirmed cases and deaths, as well as to investigate, at any stage of the pandemic, unexplained clusters of respiratory disease or deaths, or any change in the epidemiological or clinical presentation of the disease seen to date, that may signal a change in severity of the pandemic.
In countries where community spread of the pandemic (H1N1) 2009 virus is occurring, the main aim of surveillance is continuous monitoring of the epidemiological, virological and clinical features of the pandemic. This should include estimating the severity of the pandemic, monitoring for any change in the demographic or epidemiologic profiles of people developing severe illness, tracking geographic spread and disease trends, and monitoring for any changes in viral antigenicity and antiviral sensitivity. Importantly, this assessment should include disease caused by seasonal influenza viruses; a critical role of sentinel influenza surveillance during a pandemic is the continued monitoring of the relative circulation of all influenza subtypes in the population.
Virological surveillance data indicate that the pandemic (H1N1) 2009 virus has become the dominant influenza virus in all countries where it has been circulating: for example, in Argentina, Australia, Chile and Uruguay. Nevertheless, outbreaks of seasonal influenza continued to be reported through the southern hemisphere’s influenza season, and complete replacement of seasonal viruses has not occurred. In South Africa, following a normally timed influenza season (beginning in week 20) that was dominated by A (H3N2) virus circulation, pandemic (H1N1) 2009 emerged in week 30 and rapidly became dominant. During the week of 27 September to 3 October 2009, a total of 28 national influenza centres reported 6378 influenza virus detections to WHO. Of these, 6250 (98%) were influenza A and 128 (2%) were influenza B. Of the influenza A viruses, 1785 (29%) were A (not subtyped). Of the influenza A viruses that were subtyped 4012 (89.5%) were pandemic (H1N1) 2009, 59 (1.3%) were seasonal A (H1), and 394 (8.8%) were A (H3).
Based on analyses of viruses during the first weeks of the pandemic, WHO recommended using virus strain A/California/7/2009 to develop a vaccine. Since then, all viruses analysed to date are homogenous antigenically compared to this vaccine strain.
Most of the pandemic (H1N1) 2009 viruses tested are susceptible to the neuraminidase inhibitor oseltamivir. Of the more than 10 000 pandemic viruses analysed, only 35 resistant viruses have been detected in sporadic incidents mostly related to oseltamivir treatment or prophylaxis regimens, in countries including Denmark, Israel and the United Kingdom in the European Region. In all cases an H275Y mutation was present in the neuraminidase gene and there was no evidence that oseltamivir-resistant pandemic (H1N1) 2009 viruses are circulating within communities or worldwide.
