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Ask the Experts

Flu Shot: Does Time of Day Matter?

by Jeanine Barone  

Q. Will getting my flu shot in the morning make it more effective?

A. Possibly, at least according to a recent study published in the journal Vaccine, which found that getting vaccinated in the morning may rev up the immune system more than when the vaccine is given later in the day. The study, from the University of Birmingham in the U.K., involved almost 300 people ages 65 and older. Half were given the flu vaccine between 9 a.m. and 11 a.m.; the other half between 3 p.m. and 5 p.m. A month later, the researchers measured the antibodies in the participants’ blood that were produced in response to the vaccine. (The researchers chose one month because it typically takes two weeks to a month for antibody levels to peak.) Antibody levels were significantly higher in the morning group.

The flu shot works by stimulating the immune system to produce antibodies that battle the strains of influenza included in that year’s vaccine. The effectiveness of the vaccine depends in part on the age and health of the person getting vaccinated. Various aspects of the immune system decline with age, so anything that could boost the efficacy of the vaccine for older people would be a boon.

This new finding goes along with research by the same scientists, published in Psychophysiology in 2008, which found that older men who got the flu shot (as well as young men who got the hepatitis A vaccine) had a better antibody response when vaccinated in the morning than in the afternoon. For unknown reasons, morning vaccination did not improve the immune response of female participants in this older research.

Like many things in human biology, aspects of the immune system fluctuate during the course of the day. But the clinical significance and practical implications of these daily fluctuations for vaccinations remain largely unclear. For instance, it’s not known if people who get the flu vaccine in the morning are actually less likely to develop the flu than those vaccinated in the afternoon.

Bottom line: Until more definitive studies show that getting the flu vaccine in the morning is more protective, we simply encourage you to get vaccinated at whatever time is convenient. But if you have the choice, opt for morning.

Also see Can My Pet Give Me the Flu?