Flu Shot Effectiveness


The exact effectiveness of the flu vaccine (efficacy) is hard to determine. There have been many studies each year that have attempted to make the determination.

For 2014-15, a study in Valencia, Spain showed that this year’s influenza vaccine was modestly effective at a 33% vaccine effectiveness.

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 symptoms, 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 found.

The University of MN Center for Infectious Disease Research and Policy states, “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,”.

In Scotland, a recent study  covering the last nine flu seasons, showed that the flu vaccines provided about 60% protection for non-elderly adults, but much less protection for the elderly. Both the Spanish and Scottish studies were published in Eurosurveillance.

In January of 2015, U.S. researchers reported an estimate that the 2014-15 vaccine was only 23% effective overall. Recent vaccine effectiveness 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. (Center for Infectious Disease Research and Policy)


For the elderly, those over 65 years of age, the flu shot efficacy has been reported to be between 9% and 10%. Several studies have been completed for this age group.

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 that 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.

CBS News and other organizations have reported the 2013 flu vaccine having only a 9% efficacy for the older than 65 group. Even the CDC has reported this result. (CBS)

Jane Brody wrote in the New York Times, in March of 2013, on the efficacy as well. “The vaccine’s effectiveness varies by population, too. This year’s vaccine has been only 9 percent effective in protecting the elderly, for instance.”

The 9% figure has been reported to be statistically insignificant by some studies.  This means it is unknown if the vaccine has any effect on the elderly.


Mercola reports that for 2012-13, the flu vaccine was 56% effective, stating: “Did you know that during the 2012-2013 flu season, the flu vaccine’s effectiveness was found to be just 56 percent across all age groups reviewed by the CDC —in essence, the statistical equivalent of a coin toss. In seniors, aged 65 and over, the US flu vaccines were only 9 percent effective.”

For 2013-14, the Center for Disease Control for the United States (CDC) reported a 61% efficacy across all age groups. “Federal health officials reported today that this year’s seasonal influenza vaccine provides an estimated 61% level of protection overall, and said in the same breath that low vaccination coverage among working-age adults may help explain why that age-group has been hit unusually hard by the flu this year.“

This was far greater than the CDC reported efficacy for 2012-13, which was around 51%. In other years the CDC has reported it at below 40%. The CDC states, “The seasonal vaccine is reducing a vaccinated person’s risk of needing to see a clinician about influenza illness by 61% compared with an effectiveness rate of 51% in the 2012-2013 flu season”. Morbidity and Mortality Weekly Report (MMWR) February 21, 2014.
Here are the reported effectiveness rates for various ages, as reported by Eurosurveillance, Europe’s journal on infectious disease prevention (Vol. 20, Issue 8, 26 February 2015):

  • All ages: 57%
  • Under 65 Years of Age: 60%
  • Over 65 Years of Age: 19%

Here are the effectiveness rates by year, overall:

  • 2004/05: 44%
  • 2005/06: 29%
  • 2006/07: 22%
  • 2007/08: 80%
  • 2008/09: 38%