Federal health officials have completed their selection of the three viral strains to be included in next season's flu vaccine, marking the first time in 20 years that they're calling for a complete reformulation of the inoculation.
Vaccine strains are chosen nearly a year in advance because of the long preparation time needed for manufacturing, a laborious process that involves growing the doses in millions of chicken eggs.
Three deactivated viral strains are included in the annual recipe to provide protection against the dominant strains in circulation. Usually, only one or two of those strains are changed each year, but global flu trackers have found that influenza has been particularly widespread this season in the Northern and Southern hemispheres, driven by strains that emerged after the current vaccine was formulated.
"We've enhanced surveillance here and around the world," Dr. Joseph Bresee, chief of the prevention branch in the influenza division of the Centers for Disease Control and Prevention, told Newsday last week.
CDC epidemiologists and their counterparts from the World Health Organization track global flu prevalence and circulating strains to pinpoint the most likely strains that will cause infection during the next flu season. In this country, the U.S. Food and Drug Administration selects among the offerings -- as it did yesterday -- the strains to be included in next season's vaccine.
Yesterday, the FDA announced the selection of A-Brisbane/59, A-Brisbane/10 and B-Florida for the 2008-09 flu season. Viruses are named based on their sites of origin.
The three distinct strains that make up the annual vaccine represent two forms of type A influenza, dubbed H1N1 and H3N2, and one form of type B.This season's flu vaccine has been under a spotlight -- and on a hot seat -- because its recipe does not include two key circulating strains, A-Brisbane/10 and B-Florida. The mismatch between the strains in circulation and those in the vaccine are being blamed for the substantial rise in flu cases nationwide. Infection levels have remained high since January.