Studies from Spain, Scotland support flu vaccine effectiveness

Flu vaccine syringe and vial
Flu vaccine syringe and vial

Penn State / Flickr cc

Editor's note: This story was revised on Feb 27, 2015, to clarify information about estimated vaccine effectiveness for elderly people in the Scottish study.

A study in Valencia, Spain, suggests that this year's influenza vaccine has been modestly effective in preventing flu-related hospital admissions, in contrast to findings of low vaccine effectiveness this year in the United States and several other countries.

In addition, a study covering nine past flu seasons in Scotland indicates that flu vaccines provided about 60% protection for nonelderly adults but may have offered less protection for the elderly. Both studies were published today in Eurosurveillance.

In January US researchers reported a preliminary estimate that this year's vaccine was only 23% effective overall, and recent vaccine effectiveness (VE) estimates from Canada and the United Kingdom were even lower. Officials have cited a mismatch between circulating and vaccine H3N2 strains as a major factor in the weak performance.

VE in Spain: 33%

The Spanish researchers used a test-negative case-control design to examine vaccine effectiveness (VE) in preventing flu-related admissions at 10 hospitals in Valencia. They enrolled 1,136 patients with flu-like illness, of whom 210 tested positive for flu. Of those, 110 (52%) had been vaccinated. Of those who tested negative, 64% had been vaccinated.

The overall adjusted VE was estimated at 33% (95% confidence interval [CI], 6%-53%); in those 65 and older, VE was 40% (CI, 13%-59%). "The probability of influenza-related admission in vaccinated individuals was 13% (10–15%) compared with 22% (18–27%) in those unvaccinated," the report says.

"Our estimate suggests that the 2014/15 influenza vaccine was moderately effective in preventing hospital admissions related to influenza in a season in which most influenza A(H3N2) viruses were different from the component in the 2014/15 influenza vaccine," it concludes.

9-season study: VE varies by age

In Scotland, researchers used flu test results from clinics and hospitals to examine flu VE from the 2000-01 through the 2008-09 seasons, also using a test-negative case-control design.

They found that vaccination was associated with a 57% (95% CI, 31%-73%) reduction in the risk of confirmed flu overall, but age made a big difference. VE was 60% (95% CI, 22%-79%) for patients younger than 65 and at risk for serious flu complications, but only 19% (95% CI, −104%-68%) for those 65 years and older, which made it not statistically significant. The authors noted that they had low power to detect significant VE in the elderly, with only 354 patients in that age-group.

"Vaccination was associated with substantial, sustained reductions in laboratory-confirmed influenza in the general population and younger patients in clinical at-risk groups," the authors wrote.

See also:

Feb 26 Eurosurveillance report on Spanish study

Feb 26 Eursurveillance report on Scottish study

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