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Clinical and Epidemiological Overview of 2009-H1N1 Influenza A

Transmission characteristics: In general, household secondary attack rates (a measure of the risk of someone being infected with a disease by an ill close contact) for 2009-H1N1 influenza A are slightly lower than attack rates for seasonal influenza. This suggests that pharmaceutical and nonpharmaceutical mitigation measures may appreciably limit the spread of 2009-H1N1 influenza A prior to the development of an effective vaccine. Indeed, the use of antiviral medications (which can reduce viral shedding) to treat cases or prevent influenza in household contacts may already have decreased secondary attack rates.

Age profile: Age-specific frequency of cases is highest among school-age children and young adults; the lowest frequency of cases occurs among the elderly.

Symptoms: Most people infected with 2009-H1N1 influenza A virus experience uncomplicated influenza-like illness, with full recovery within a week, even without medical treatment.

Severe cases: Small subsets of 2009-H1N1 influenza A patients rapidly develop very severe progressive pneumonia, which in turn is often associated with failure of other organs, or marked worsening of underlying asthma or chronic obstructive airway disease. Primary viral pneumonia is the most common finding in severe cases and a frequent cause of death. This is markedly different from severe cases of seasonal influenza, which tend to involve secondary bacterial infections.


aThe early treatment of bacterial infections may prevent severe complications and death. Antiviral treatment is recommended to treat infection and has also been used prophylactically in persons who have been exposed to H1N1 virus.

and H5N1 avian influenza virus) had been detected in any of the 2009-H1N1 influenza A virus isolates.

Several speakers described the recent Southern Hemisphere influenza season, during which the clinical and epidemiological characteristics of the morbidity and mortality associated with the 2009-H1N1 influenza A pandemic, as reflected in Box WO-3, remained essentially unchanged. Some Southern Hemisphere countries experienced simultaneous or serial epidemics of multiple viral diseases, as shown in Box WO-4. In many, but not all, cases the 2009-H1N1 influenza A virus eventually dominated other seasonal influenza strains. Much as Ruiz-Palacios found co-infections with multiple respiratory viruses (including parainfluenza 1, 2, and 3; respiratory syncytial virus [RSV]; and the coronavirus that causes bronchitis) in a majority of Mexican patients with severe disease, one might expect to find similar co-infections in other locations with multiple viral epidemics.

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