How well does the flu shot work?

Q: BuckMD, can you share some information of flu vaccine efficacy?

A: Thanks for your question.  Vaccine efficacy is measured in a number of ways. Depending on the organism, sometimes we can measure antibody or other immune response markers in the blood of vaccinated vs. unvaccinated people.  Sometimes, we compare clinical outcomes, such as death or hospitalization rates, or missed work or school days, in the two populations. 

With flu vaccine, most experts look at the level of a particular antibody (hemagglutination inhibition antibody, or HI) to say whether the vaccine gives enough response to protect a person against getting the flu.  Most of this year’s flu vaccines produce a protective level of these antibodies in 90% or more of human test subjects. 

However, this percentage is inflated somewhat due to pre-existing immunity.  If I get vaccinated against a particular strain this year, and it happens that I also was vaccinated against that strain 3 years ago, then I might have a higher antibody level than if not previously vaccinated. 

Looking more at how many people get sick as a measure of efficacy, the vaccines protect about 60-70% of those immunized. (Am. J. Epidemiol. (2008) 168 (12): 1343-1352. doi: 10.1093/aje/kwn259 First published online: October 29, 2008, accessed 11/2/10) These studies are much more difficult to perform. 

For more details, take a look at each vaccine’s package insert information at the FDA website:


For an intricate discussion of viruses, read:

Mandell: Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 7th ed., 2009.


Virology, in Microbiology and Immunology Online, University of South Carolina, 2009,, accessed 11/2/10.

Roger Miller, MD, OSU Student Health Services