Flu Vaccine 36% Effective, But Some Age Groups Fare Much Worse

In interim assessment of vaccine effectiveness, young children do better

This year’s vaccine has been 36% effective against viruses that have made for a particularly trying flu season––but some age groups are receiving “no statistically significant protection,” according to interim estimates released by the Centers for Disease Control and Prevention (CDC).

Vaccine effectiveness (VE) has been highest for children 6 months to 8 years of age (59%, 95% confidence interval [CI], 44% to 69%) and adults 18 to 49 years of age (33%, CI, 16% to 47%), according to estimates published in Morbidity and Mortality Weekly Report. VE in other age groups was lower: ages 9 to 17, 5% (CI, –38% to 34%); ages 50 to 64, 17% (CI, –15% to 40%), and 65 or older, 18% (CI, –25% to 47%).

“These early VE estimates underscore the need for ongoing influenza prevention and treatment measures,” the report says. “CDC continues to recommend influenza vaccination because the vaccine can still prevent some infections with currently circulating influenza viruses, which are expected to continue circulating for several weeks. Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths.”

The estimates are based on a relatively small sample of medically attended acute respiratory illness among 4,562 children and adults enrolled at five sites in the U.S. Influenza Vaccine Effectiveness Network. Test results confirmed flu in 1,712 (38%) of this group.

As predicted, VE has been weakest against the H3N2 variant of influenza A. Overall, VE against H3N2 was 25% (CI, 13% to 36%), boosted by high VE in young children. VE was reported at 51% (CI, 29% to 66%) among those 6 months to 8 years of age. But beyond that, VE against H3N2 was sharply lower: ages 9 to 17, –8% (CI, –62% to 29%); ages 18 to 49, 20% (CI, –4 % to 38%); ages 50 to 64, 12% (CI, –26% to 39%); and age 65 or older, 17% (CI, –35% to 49%).

VE was higher against influenza A H1N1: 67% overall (CI, 54% to 76%). Again, VE varied by age: 78% (CI, 63% to 87%) among those under 18 years of age; 51% (CI, 20% to 70%) among those 18 to 64 years; and 34% (CI, –96% to 78%) among those 65 and older. Influenza B VE for those age groups was 36% (CI, 1% to 58%), 50% (CI, 28% to 66%), and 25% (CI, –62% to 66%), respectively, with overall VE of 42% (CI, 25% to 56%). Small sample sizes in some age groups resulted in wide confidence intervals, the CDC explains.

In this sample, influenza A accounted for 81% of the positive flu tests, while B viruses accounted for 19%. Among the A viruses, H3N2 made up 85% of the positive tests and H1N1 comprised 16%. Forty-three percent of participants with positive test results had received the flu vaccine.

Sources: MMWR; February 15, 2018.