It is the beginning of flu season in the northern hemisphere, which means it's time to get your flu vaccine. Each year the debate about the safety and efficacy of the flu vaccine is renewed, mostly by anti-vaccine ideologues, despite the fact that the evidence shows that the vaccine is both safe and effective.


There are two basic types of flu vaccine in common use, the trivalent inactivated vaccine (TIV), which is injected, and live attenuated influenza vaccine (LAIV), which is generally given as an inhaled nasal spray. The question of efficacy - how well do these vaccines work, is deceptively complex for several practical reasons.

First, many people get sick during flu season without getting the actual flu virus. Most people who get the flu do not get the diagnosis confirmed with a laboratory test. Therefore studies can vary depending on whether they count confirmed cases of flu or just clinically suspicious cases.

There is also a variable response to the vaccine itself. Not everyone who gets the vaccine develops antibodies. An antibody response is one way to measure the effectiveness of the vaccine.

Researchers can also count complications, like the need to be hospitalized, secondary pneumonia, and death. There are also different populations to consider, such as the elderly, children, and vulnerable populations such as those who are already ill.

There are both observational studies and experimental studies. In the former researchers imply see what happens in a population and compare vaccinated vs unvaccinated individuals. In the latter researcher actually expose subjects to the flu virus to see how well the vaccine protects them from getting an infection.

And finally, the efficacy of the flu vaccine varies from year to year. Each season researchers need to make their best guess as to which flu strains are coming around, and then target that season's flu vaccine against those strains. The measured efficacy varies greatly depending on how well the researchers guessed.

In the end we have an imperfect and complicated patchwork of evidence. But there is a consistent signal of effectiveness in the data. If you get the vaccine, your chance of getting the flu and complications are reduced.

Marc Crislip, writing for Science-Based Medicine, wrote an excellent review of the evidence as it stood in 2009 . He concluded:

"You can conclude that neither the vaccine nor the data is perfect, and decide the vaccine is not useful.

Or you can look at the preponderance of data, with all the flaws,  nuance, subtleties and qualifiers, and conclude the flu vaccine is of benefit.  The vaccine decreases the probability of morbidity and mortality.  It is a good thing."

This week in the Lancet was just published a comprehensive systematic review and meta-analysis of studies looking at flu vaccine efficacy. They report:

"Influenza vaccines can provide moderate protection against virologically confirmed influenza, but such protection is greatly reduced or absent in some seasons. Evidence for protection in adults aged 65 years or older is lacking. LAIVs consistently show highest efficacy in young children (aged 6 months to 7 years). New vaccines with improved clinical efficacy and effectiveness are needed to further reduce influenza-related morbidity and mortality."

They found that the overall efficacy of the TIV was 59%, the LAIV 83%, and the H1N1 vaccine 69%. They consider that to be "moderate" protection, which is a reasonable interpretation. These are average numbers. The actual protection is higher in years where there is a good match between the vaccine and the circulating strains, and low in years where there is a poor match.

When they write that "protection in adults aged 65 years or older is lacking" they mean that they could find no high quality studies, not that the vaccine doesn't work in this population.

The bottom line is that the vaccine works, it provides moderate protection but variable from season to season. There is room for improvement, mostly in predicting which strains to target, but also in the vaccine itself.

Given that the flu causes hundreds of thousands of hospitalizations each year in the US alone, and 10-30 thousand deaths, even moderate protection is worthwhile. In medicine we must always consider the risk vs benefit. The benefit is clear - what about the risk?

Risk of the flu vaccine

Vaccines are among the most studies medical interventions we have, with the highest safety. Millions of vaccine have been given with only rare side effects. No one claims the vaccine is without risk, but the evidence clearly shows the risk is very small.

The most likely serious reaction to the flu vaccine is from an allergic reaction. Those who are allergic to eggs or who have had a reaction to a vaccine in the past should not get the flu vaccine. If symptoms of an allergic reaction occur after getting the vaccine, then immediate medical attention should be sought. But for most people only slight or no symptoms will occur.

There is also the question of Guillain Barre Syndrome (GBS). There was an outbreak of GBS follow the swine flu vaccine in 1976. However, since then there has been no clear associated between the flu vaccine and GBS. One study did show a small association, with an addition 1 case of GBS per million vaccines. GBS normally has an incidence of about one per hundred thousand per year - 10 times the extra risk of the flu vaccine.

There was some concern about the recent addition of the H1N1 strain to the flu vaccine, since the swine flu was an H1N1 strain. But careful monitoring found no additional risk of GBS from the H1N1 vaccine.

It should also be noted that you cannot get the flu from the TIV vaccine (the shot), because the virus is dead. The LIAV is a live attenuated virus, so it is theoretically possible to get an infection from this vaccine, if a spontaneous mutation occurs that restores virulence to the attenuated virus (which is rare but possible).

Much of the fear-mongering concerning the flu vaccine on the part of anti-vaccine activists have little to do with the actual small but real risks of the vaccine. Instead they largely focus on thimerosal and other imagined toxins in the vaccine.

Thimerosal is a mercury-based preservative that is present in small amounts in the multi-dose flu vaccines, but not the single dose of LIAV vaccines. The dose of ethylmurcury in the multi-dose vaccines is 25mcg, far below the safety limits for a single exposure. Ethylmercury, it should be noted, is far less toxic than methylmercury (the kind of mercury found in tuna fish) and is cleared from the body very quickly.

Further there is a large amount of evidence for the safety of thimerosal in vaccines. Fears that it is associated with autism or other neurological disorders have been shown to be unwarranted.


The flu vaccine is both safe and effective. Side effects are rare and can be mostly avoided if people with a history of egg allergy or vaccine sensitivity avoid the vaccine. The CDC also recommends that those with a history of GBS avoid the vaccine, although the association between the current flu vaccine and GBS is in doubt, and at worst results in one additional case per million vaccines.

Fear mongering about the safety of the flu vaccine is not justified by the evidence. This, however, has not stopped anti-vaccine activists from distorting the facts and evidence for their propaganda purposes.

The effectiveness of the flu vaccine is sufficient to justify its use. I am happy to reduce my chance of getting the flu by 59%, even at the low end. But there is clear room for improvement in the technology.


Steven Novella, M.D. is the JREF's Senior Fellow and Director of the JREF’s new Science-Based Medicine project.

Dr. Novella is an academic clinical neurologist at Yale University School of Medicine. He is the president and co-founder of the New England Skeptical Society and the host and producer of the popular weekly science show, The Skeptics’ Guide to the Universe. He also authors the NeuroLogica Blog.