How Effective Are Flu Shots?

If the virus in flu shots is inactive or dead, how come some people who get the shot come down with the flu? How effective are the shots overall?

– December 5, 2017

The effectiveness of the flu vaccine varies from year to year and also depends on the age and health of recipients. According to the U.S. Centers for Disease Control and Prevention (CDC), the vaccine can reduce the risk of flu by 40 to 60 percent among the overall population as long it is matched to the most prevalent flu viruses circulating in a given year.

The younger and healthier you are, the more likely the vaccine is to protect you. Older people with weaker immune systems and those with chronic health problems may not have as high a response to it as younger people. The CDC notes, however, that some data suggest that even if the shots don’t protect all seniors from contracting the flu, being vaccinated may lessen the severity of the illness. And the CDC maintains that for this high-risk group some protection is better than none. Incidentally, evidence from a recent British study investigating the effect of mood and behavior on the effectiveness of the flu vaccine showed that seniors who were in a good mood when they got the shot produced more antibodies to the virus than those whose mood wasn’t as good.

Some studies also suggest that flu shots are more effective in people getting them for the first time compared to those who have had repeated shots over the years. But other research shows that getting vaccinated two years in a row seems to provide additional protection.

While the CDC advocates annual flu shots for everyone six months of age or older, I recommend them mainly for people over 65, anyone with a weakened immune or respiratory system, nursing home residents and health care workers who have regular contact with patients.

Some people, regardless of age, should avoid flu shots for health reasons. If you have had a severe allergic reaction to the vaccine which is likely to contain small amounts of egg protein) or have Guillain-Barré Syndrome, you should not have flu shots with either inactivated or live viruses. The CDC recommends avoiding live flu virus shots if you have any long-term heart, kidney, liver or nervous system problems, have asthma or breathing problems, are pregnant or have a weakened immune system. Children who have had wheezing episodes and those who are taking aspirin or products containing aspirin should not have the shots.

If you’re not feeling well, have had other vaccines within the past four weeks, have taken antiviral influenza medication within the past 48 hours or have a very stuffy nose, it is best to postpone getting a flu shot.

Andrew Weil, M.D.


U.S. Centers for Disease Control and Prevention, “Vaccine Effectiveness – How Well Does the Flu Vaccine Work.” October 3, 2017,

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