As the New York Times reports, Iowa State University researchers conducted two experiments to determine if hitting the gym after getting a flu shot would prove beneficial.
In the first study, healthy individuals were given a flu shot and then assigned to either go for a 90-minute moderately paced jog, ride a bike for 15 or rest quietly for 90 minutes. A month later, researchers measured participants' influenza antibodies. Gretchen Reynold writes:
Those volunteers who had exercised after being inoculated, it turned out, exhibited “nearly double the antibody response” of the sedentary group, said Marian Kohut, a professor of kinesiology at Iowa State who oversaw the study, which is being prepared for publication. They also had higher blood levels of certain immune system cells that help the body fight off infection.
To test how much exercise really is required, Dr. Kohut and Justus Hallam, a graduate student in her lab, subsequently repeated the study with lab mice. Some of the mice exercised for 90 minutes on a running wheel, while others ran for either half as much time (45 minutes) or twice as much (3 hours) after receiving a flu shot.
Four weeks later, those animals that, like the students, had exercised moderately for 90 minutes displayed the most robust antibody response. The animals that had run for three hours had fewer antibodies; presumably, exercising for too long can dampen the immune response. Interestingly, those that had run for 45 minutes also had a less robust response. “The 90-minute time point appears to be optimal,” Dr. Kohut says.
Although more research is needed to understand how working out after receiving a flu shot may increase your resistance to the flu, researchers say the effects could be attributed to exercise increasing blood circulation and pumping the vaccine away from the injection site to other areas of the body. Another explanation could be that physical activity invigorates the body’s overall immune system, which may increase the vaccine's effects.