This season's flu vaccine is so far one of the most effective since the government began evaluating influenza vaccine performance in 2003, according to preliminary data from the Centers for Disease Control and Prevention.
The vaccine was 59% effective overall, the CDC announced late Wednesday at an advisory committee meeting in Atlanta. The vaccine typically has an efficacy rate of 47% to 60%. Last year's was just 23% because of a mutation in the virus' dominant strain.
This season's vaccine was 51% effective against H1N1 viruses that were responsible for most cases of influenza this season, according to preliminary data. It was 76% effective against all influenza B viruses, and 79% effective against the B/Yamagata lineage of B viruses.
The CDC does not yet have enough data to estimate the vaccine's effectiveness by age group or how well it performs against the H3N2 variant and B/Victoria lineage. The estimates, which can change over the course of the season, are based on data from a network of five study sites at universities and hospitals across the U.S. between Nov. 2, 2015, and Feb. 12.
Vaccinemakers were advised to swap out two strains this season to account for last season's “drifting virus.” For the first time ever, GlaxoSmithKline chose to only offer quadrivalent vaccines, which include four strains of the virus. Traditional vaccines have only covered three strains, but patients have pushed for more availability of quadrivalent vaccines and payers have offered reimbursement.
CDC officials said positive flu tests have been elevated since mid-January. For the past 13 years, seasons have averaged about 13 weeks in length, ranging from just one week to 20 weeks.
"Flu activity this season started a bit later and has been lower so far than we've seen during the previous three seasons, but activity is still on the upswing and expected to continue for several weeks," Dr. Joseph Bresee, chief of the CDC's epidemiology and prevention branch, said in a statement.