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Phys Ed Gretchen Reynolds on the science of fitness.

As this year’s influenza season continues to take its toll, those procrastinators now hurrying to get a flu shot might wish to know that exercise may amplify the flu vaccine’s effect. For maximal potency, the exercise should be undertaken at the right time and involve the right dosage of sweat, according to several recent reports.

Flu shots are one of the best ways to lessen the risk of catching the disease. But they are not foolproof. By most estimates, the yearly flu vaccine blocks infection 50 to 70 percent of the time, meaning that some of those being inoculated gain little protection. The more antibodies someone develops, the better their protection against the flu, generally speaking. But for some reason, some people produce fewer antibodies to the influenza virus than others do.

Being physically fit has been found in many studies to improve immunity in general and vaccine response in particular. In one notable 2009 experiment, sedentary, older adults, a group whose immune systems typically respond weakly to the flu vaccine, began programs of either brisk walking or a balance and stretching routine. After 10 months, the walkers had significantly improved their aerobic fitness and, after receiving flu shots, displayed higher average influenza antibody counts 20 weeks after a flu vaccine than the group who had stretched.

But that experiment involved almost a year of dedicated exercise training, a prospect that is daunting to some people and, in practical terms, not helpful for those who have entered this flu season unfit.

So scientists have begun to wonder whether a single, well-calibrated bout of exercise might similarly strengthen the vaccine’s potency.

To find out, researchers at Iowa State University in Ames recently had young, healthy volunteers, most of them college students, head out for a moderately paced 90-minute jog or bike ride 15 minutes after receiving their flu shot. Other volunteers sat quietly for 90 minutes after their shot. Then the researchers checked for blood levels of influenza antibodies a month later.

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.

Unless, that is, you work out before you are inoculated, another set of studies intimates, and use a dumbbell. In those studies, undertaken at the University of Birmingham in England, healthy, adult volunteers lifted weights for 20 minutes several hours before they were scheduled to receive a flu shot, focusing on the arm that would be injected. Specifically, they completed multiple sets of biceps curls and side arm raises, employing a weight that was 85 percent of the maximum they could lift once. Another group did not exercise before their shot.

After four weeks, the researchers checked for influenza antibodies. They found that those who had exercised before the shot generally displayed higher antibody levels, although the effect was muted among the men, who, as a group, had responded to that year’s flu vaccine more robustly than the women had.

Over all, “we think that exercise can help vaccine response by activating parts of the immune system,” said Kate Edwards, now a lecturer at the University of Sydney, and co-author of the weight-training study.

With the bicep curls, she continued, the exercises probably induced inflammation in the arm muscles, which may have primed the immune response there.

As for 90 minutes of jogging or cycling after the shot, it probably sped blood circulation and pumped the vaccine away from the injection site and to other parts of the body, Dr. Kohut said. The exercise probably also goosed the body’s overall immune system, she said, which, in turn, helped exaggerate the vaccine’s effect.

But, she cautions, data about exercise and flu vaccines is incomplete. It is not clear, for instance, whether there is any advantage to exercising before the shot instead of afterward, or vice versa; or whether doing both might provoke the greatest response – or, alternatively, be too much and weaken response.

So for now, she says, the best course of action is to get a flu shot, since any degree of protection is better than none, and, if you can, also schedule a visit to the gym that same day. If nothing else, spending 90 minutes on a stationary bike will make any small twinges in your arm from the shot itself seem pretty insignificant.