Flu Vaccine Efficacy Slips From Prior Estimate, CDC Says

June 23, 2017

The influenza vaccine for the 2016-2017 season was 42% effective in preventing an infection from any A or B virus in people of all ages, down from a preliminary estimate of 48% in February, the Centers for Disease Control and Prevention (CDC) announced earlier this week.

The CDC frames the effectiveness rating in terms of reducing a vaccinated person's risk of getting sick and having to visit a clinician on an outpatient basis because of the flu. 

The vaccine worked the best among children aged 6 months to 8 years, at 61%, and the least among individuals aged 18 to 49 years, at 19%. For people aged 65 years or older — a demographic group that's especially vulnerable to the flu — effectiveness stood at 25%.

The CDC reported the efficacy rates at a meeting of its Advisory Committee on Immunization Practices (ACIP) in Atlanta, Georgia, on June 21 and 22.

Vaccine effectiveness varied considerably by influenza virus type. It was at its lowest in all age groups — 34% — for the A/H3N2 virus that dominated this season. Performance picked up for the A/H1N1 pandemic virus (54%), the B/Yamagata virus (55%), and the B/Victoria virus (60%), according to epidemiologist Jill Ferdinands, PhD, in the CDC's Influenza Division.

The CDC adjusts its efficacy estimates for such factors as age, sex, and underlying medical conditions.

The 42% overall effectiveness of the latest influenza vaccine is somewhat lower than the 47% for the 2015-2016 vaccine, but a big improvement over the 23% for the 2014-2015 edition. Vaccine performance suffers when the viruses they're designed to thwart undergo genetic drift after the vaccine is formulated. However, the CDC reported that most of the flu viruses in circulation in 2016-2017 were similar to those in the latest vaccine, which came in trivalent and quadrivalent formulas.

Recombinant Inactivated Flu Vaccine Okay for Pregnant Women

ACIP also approved a set of recommendations on influenza vaccination practices at its 2-day meeting this week. Perhaps most notably, the advisory group agreed that FluMist Quadrivalent (MedImmune) — the only live attenuated influenza vaccine approved by the US Food and Drug Administration (FDA) — not be administered in the 2017-2018 season, owing to a decline in efficacy. ACIP had made the same recommendation for the 2016-2017 season. The CDC is expecting to receive more data to weigh on FluMist, administered as a nasal spray, this fall.

The panel also tweaked its policy about Afluria (Seqirus), an inactivated influenza vaccine in both trivalent and quadrivalent formulas. Although the FDA approved Afluria for anyone aged 5 years or older, ACIP recommended about 6 years ago that no one should receive it until they are aged at least 9 years, based on reports of febrile seizures and reactions among younger children associated with the vaccine's formula for the Southern Hemisphere. With fever rates for the 5-through-8 age group now resembling those for other vaccines, ACIP is moving its recommended start-year for Afluria to 5, in line with the FDA indication.

In another change, ACIP broadened its policy for flu vaccination and pregnant women by saying they could receive any licensed inactivated influenza vaccine, and not only those produced with chicken eggs but also those produced through recombinant technology. The only approved recombinant inactivated vaccine now is Flublok (Protein Sciences), an egg-free quadrivalent. ACIP decided to include Flublok as a recommended vaccine for pregnant women after hearing evidence that its safety profile in comparison with other inactivated influenza vaccines was "reassuring."

Follow Robert Lowes on Twitter @LowesRobert


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