May 22, 2009

Serum Cross-Reactive Antibody Response to a Novel Influenza A (H1N1) Virus After Vaccination with Seasonal Influenza Vaccine

As of May 2009, a total of 5,469 confirmed cases of human infection with a novel influenza A (H1N1) virus was documented in 47 states and the District of Columbia. In addition, the virus spread to 41 countries, with a total of 4,774 cases reported in countries outside the United States. Because producing a novel influenza A (H1N1) virus vaccine can take several months, determining whether the seasonal influenza vaccine offers any protection against the novel influenza A (H1N1) virus is important. Using stored serum specimens collected during previous vaccine studies, CDC assessed the level of the antibody to the novel influenza A (H1N1) virus in children and adults before and after vaccination with influenza season vaccines. The results indicated that before vaccination, no antibody to the novel influenza A (H1N1) virus existed among children. Among adults, before vaccination, antibody was detected in a third of those aged over 60 years. Among adults, seasonal vaccination resulted in a twofold increase in antibody response to the novel influenza A (H1N1) virus. These data suggest that receipt of recent seasonal influenza vaccines is unlikely to elicit a protective antibody response to the novel influenza A (H1N1) virus.