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.