Pandemic H1N1 2009 Overview

Last updated December 16, 2010. At the current time, this content is considered historical and will not be updated until further notice.

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Cases of pH1N1 2009 influenza were first identified in mid-April 2009 in California and soon thereafter in Texas and Mexico (CDC 2009: Swine influenza A [H1N1] infection in two children—southern California, March-April 2009; CDC 2009: Update: swine influenza A [H1N1] infections—California and Texas). The earliest recognized case occurred in Mexico with illness onset on Mar 17, 2009 (CDC 2009: Outbreak of swine-origin influenza A (H1N1) virus infection—Mexico, March-April 2009).

• Following initial recognition, the virus quickly spread around the globe, and on Jun 11, 2009, the World Health Organization (WHO) declared the onset of an influenza pandemic.
• An interactive map showing the timeline of events is available on the WHO Web site (WHO 2009: Timeline of influenza A [H1N1] cases).
• An analysis of global air traffic patterns illustrates how the virus spread via air travel from its likely source in Mexico to other areas of the world, most notably the United States (Khan 2009).

The WHO declared the onset of an influenza pandemic in June 2009 for the following reasons:

• Once the first human infections with the new H1N1 virus were confirmed in April 2009, analysis of samples showed that this new virus had not circulated in humans previously. This virus was genetically distinct from other H1N1 viruses causing disease since 1977.
• Epidemiologic patterns of disease occurrence were seen that were not typical during seasonal epidemics of influenza.
• The new H1N1 pattern of illness and death differed significantly from the patterns seen with seasonal influenza. Seasonal influenza typically affects the frail elderly, but this virus targeted a younger population. In addition, a frequent cause of death was viral pneumonia caused directly by the virus. In seasonal viruses, most cases of pneumonia result from secondary bacterial infections.
• The new virus crowded out other circulating influenza viruses and appeared to have shifted the old H1N1 out of circulation. This occurrence is a feature of pandemic activity.
• Early findings suggested that antibodies from seasonal influenza did not protect against the new virus. (Later findings showed that about a third of the elderly had some protection but the younger population was unprotected.)

During the first influenza season following recognition of the pandemic strain, five WHO National Influenza Centers (NICs) in the Southern Hemisphere collected data to examine circulation of pandemic and seasonal influenza strains (Blyth 2010).

• The overall proportion of influenza A–positive specimens from May to October 2009 subtyped as the pH1N1 2009 virus ranged from 53% in Johannesburg, South Africa, to 85% in Melbourne, Australia.
• For specimens received from August to October 2009, the proportion of influenza viruses typed as pH1N1 2009 ranged from 92% to 96%.
• The pH1N1 2009 virus significantly displaced seasonal influenza A H1N1, and to a lesser extent A H3N2 viruses, circulating in the Southern Hemisphere. Complete replacement of seasonal influenza A strains, however, was not observed.

From the start of the pandemic in April 2009 through late July 2010, the total number of specimens reported positive for influenza by NIC laboratories around the globe was 652,849. Of those, 491,766 (75.3%) were of the pH1N1 2009 strain (WHO 2010: Pandemic (H1N1) 2009—update 110). The cumulative number of deaths from pandemic H1N1 influenza reported to WHO regional offices as of July 18, 2010, was at least 18,366.

The CDC estimated the burden of the H1N1 pandemic in the United States as follows (CDC 2010: Updated CDC estimates of 2009 H1N1 influenza cases, hospitalizations and deaths in the United States, April 2009 – April 10, 2010):

• Between 43 million and 89 million cases of 2009 H1N1 occurred in the United States between April 2009 and April 10, 2010. The mid-level in this range is about 61 million people infected with pH1N1 2009.
• Between about 195,000 and 403,000 pH1N1-related hospitalizations occurred between April 2009 and April 10, 2010. The mid-level in this range is about 274,000 hospitalizations related to pH1N1 infection.
• Between about 8,870 and 18,300 pH1N1-related deaths occurred between April 2009 and April 10, 2010. The mid-level in this range is about 12,470 pH1N1-related deaths.

The overall case-fatality rate for persons infected with pH1N1 2009 appears to be slightly less than 0.5%, although estimates have varied somewhat (Vaillant 2009, Writing Committee of the WHO Consultation on Clinical Aspects of Pandemic H1N1 2009 Influenza). Approximately half of the patients who died had underlying medical conditions, most notably pregnancy and obesity (Vaillant 2009). In addition, most deaths occurred in younger populations.

One recent report estimated the mortality and years of potential life lost (YLL) attributable to the pH1N1 2009 influenza in the United States (Viboud 2010). Estimates were based on pneumonia and influenza mortality surveillance data from 122 US cities, and the age distribution of laboratory-confirmed pandemic deaths, which had a mean of 37 years.

• The authors estimated that between 7,500 and 44,100 deaths in the United States were attributable to the pandemic strain between May and December 2009.
• The authors also estimated that between 334,000 and 1,973,000 years of life were lost during that time. The upper range of YLL estimates for 2009 exceeded the burden of the 1968 pandemic adjusted to the year 2000 population. The high estimates of YLL for the 2009 pandemic are reflective of the mean age at death (ie, 37 years), which is substantially younger than the 1957 and 1968 pandemics (mean ages at death of 64.6 and 62.2 years, respectively).

On Aug 10, 2010, the WHO declared that the H1N1 situation had moved into the post-pandemic period.

• During the summer months of 2010, pH1N1 2009 activity continued at low levels in a number of areas of the world, with more intense activity in India and New Zealand.
• Also during summer 2010, a number of different influenza strains (including pH1N1 2009, influenza A H3N2, and influenza B viruses) were circulating in the Southern Hemisphere.
• The pH1N1 2009 virus is expected to continue to circulate in the coming years and follow a typical seasonal pattern.

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