As this year's flu season continues to take its toll, those procrastinators now hurrying to get a flu shot might want to know that exercise can amplify the flu vaccine's effect.

For maximum benefit, 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.

The more antibodies someone develops, the better their protection against the flu, generally speaking.

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 brisk walking or a balance and stretching routine. After 10 months, the walkers had improved their aerobic fitness and, after receiving flu shots, had higher average influenza antibody counts 20 weeks after the vaccine than the group who had stretched.

But that experiment involved almost a year of exercise training, a prospect daunting to some people and not very 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 boost 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 90-minute jog or bike ride 15 minutes after receiving their flu shot. Other volunteers sat quietly for 90 minutes after their shot. A month later the researchers checked for influenza antibodies.

Turns out the volunteers who had exercised had “nearly double the antibody response” of the sedentary group, said Marian Kohut, the ISU professor who oversaw the study.

To test how much exercise is required, Kohut and Justus Hallam, a graduate student in her lab, then repeated the study with lab mice. Some of the mice exercised for 90 minutes on a running wheel, while others ran for either half that time or twice as much after receiving a flu shot.

Four weeks later, the 90-minute exercisers showed the most robust antibody response.

“The 90-minute time point appears to be optimal,” Kohut said.

Unless you prefer weights, perhaps.

In another set of studies, undertaken at the University of Birmingham in England, healthy adult volunteers lifted weights for 20 minutes several hours before a flu shot, focusing on the arm that would be injected. A control group did not exercise before their shot.

After four weeks, the weightlifters had higher antibody levels, although the effect was more pronounced for women than for men.

Overall, said Kate Edwards, a co-author of the weight-training study, “we think that exercise can help vaccine response by activating parts of the immune system.”

The bicep curls the weightlifters did probably induced inflammation in the arm muscles, which may have primed the immune response there, she said.

As for the 90 minutes of post-shot jogging or cycling, said Kohut, that probably sped blood circulation and pumped the vaccine away from the injection site. The exercise probably also goosed the body's overall immune system, she said, which helped the vaccine's effect.

She cautioned that data about exercise and flu vaccines is incomplete.

For now, she said, the best course of action is to get a flu shot, because any degree of protection is better than none, and if possible schedule a visit to the gym that same day.

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