Global H1N1 influenza infection rate as high as 21 percent

A team of researchers from Australia, Hong Kong and Japan estimate that the global cumulative incidence of pandemic H1N1 influenza infection before vaccines was 11 to 21 percent.

The incidence of H1N1, which was estimated as closer to 50 percent by planners envisioning a worst case scenario, differed widely by age group, ranging from 34 to 43 percent in school-age children and dropping to only about three percent in older adults, CIDRAP News reports.

The researchers say that the "lower than anticipated" overall incidence of the infection was probably due to elderly people having been previously exposed to relatives of the pandemic virus.

The researchers found 12 studies from eight countries that met their criteria of publication between April 2009 and December that included samples taken before the pandemic and after the first wave but before vaccines became widely available in late 2009.

In preschool children aged zero to four, the incidence was 16 to 28 percent. The incidence in those older than 65 years old was only two to three percent, according to CIDRAP News.

"This review confirms that cumulative incidence of infection during the first and second [pandemic] waves prior to the availability of a pandemic specific vaccine anywhere in the world, fell well below the assumption of an upper estimate of a 50 percent clinical attack rate that had informed the pandemic planning of the United Kingdom and other countries," the report said, according to CIDRAP News. "The lower than anticipated cumulative incidence was likely due to the significant number of older people protected by neutralizing antibodies that cross-reacted with pH1N1."

The authors wrote that using leftover diagnostic serum samples is less than ideal due to potential biases related to the risk of infection and to vaccination history in hospital patients, from whom most of the samples come. They wrote that serum samples from healthy blood donors yielded similar estimates of the incident of infection.

"Serological studies need to be routine in order to be sufficiently timely to provide support for decisions about [pandemic] vaccination, and revised pandemic plans might consider a more integrated role for serological studies," the researchers concluded, CIDRAP News reports.