H1N1 pandemic less severe among adults with history of flu

Couch RB. Clin Infect Dis. 2011;doi:10.1093/cid/cir809.

Issue: December 2011

The severity and intensity of the 2009 H1N1 influenza pandemic was reduced among adults with a previous history of seasonal influenza infection, according to recent findings published in Clinical Infectious Diseases.

“This study has indicated that a high level of immunity existed among adults aged 18 years and older despite relatively low levels of antibody,” Robert B. Couch, MD, of the department of molecular virology and microbiology at Baylor College of Medicine, and colleagues wrote in the study. “The 2009 pandemic influenza experience indicated that the full susceptibility with high illness rates among healthy persons was only in children.”

For the prospective study, the rate for influenza was assessed among healthy adults aged 18 to 49 years at Texas A&M University. Researchers investigated the role of having prior seasonal H1N1 infections on infection occurrence and illness in adults with pandemic H1N1. Those who received the 2009 pandemic H1N1 vaccine were excluded from the study.

Of 513 participants, 77 reported moderate to severe illness during the surveillance period. Thirty-one were infected with pandemic H1N1 virus and 30 with a rhinovirus.

Pandemic H1N1 occurred at a frequency of 23%; the rate decreased with increasing anti-pandemic H1N1 antibody, as well as increasing antibody to the recent seasonal H1N1 virus, according to the study.

“Pre-existing antibody to pandemic H1N1 virus, responses to a single vaccine dose, a low infection-to-illness ratio, and a short duration of illness and virus shedding among those with influenza indicated presence of considerable preexisting immunity to pandemic H1N1 in the population,” the researchers wrote.

Disclosure: This work was supported by the National Institute of Allergy and Infectious Diseases (Public Health Service Contract NO1-AI-30039).

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