Flu Vaccine Far More Effective Than Last Year's, CDC Says

February 24, 2016

The vaccine for the 2015-2016 influenza season has proven to be 59% effective in reducing the risk for illness, a far cry from the 18% effectiveness rate reported in the previous season, the Centers for Disease Control and Prevention (CDC) announced today.

Greater vaccine effectiveness counts in the current flu season, which has taken a nasty turn. Although the CDC is reporting less influenza activity so far than in the previous three seasons, the predominant virus in circulation — A(H1N1)pdm09 — has caused severe respiratory illness in young, otherwise healthy but unvaccinated adults, leading to hospitalization and death in some cases.

The CDC calculates that this season's influenza vaccine is 51% effective against the A(H1N1)pdm09 virus responsible for most flu illness. It is even more effective against all influenza B viruses (76%) and against the B/Yamagata lineage of B viruses in particular (79%).

The vaccine effectiveness rates, based on data from a network of university and hospital researchers, were presented today at a meeting of the CDC's Advisory Committee on Immunization Practices, in Atlanta, Georgia. There is not yet enough data to estimate effectiveness by age group or how well the vaccine is working against A(H3N2) or B/Victoria lineage viruses, the CDC said in a news release. It cautions that estimates of effectiveness may change as the flu season heads toward its spring finish line.

Vaccine effectiveness has taken a rollercoaster ride over the years, dipping to 10% in the 2004-2005 season, and peaking since then at 60% in 2010-2011. The CDC originally reported an effectiveness rate of 23% for 2014-2015, but later trimmed it to 18%.

Influenza vaccines can lose much of their protective power when the viruses they were designed to combat undergo genetic drift after the vaccine was formulated. That apparently has not been a problem in 2015-2016.

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