A workout before your jab does not buy protection

Influenza is estimated to infect 5% to 10% of adults each year, with higher rates in winter in countries with seasonal variation, and symptoms range from fever and cough to muscle and joint pain and malaise. Vaccination is the main defence, and there has long been interest in whether a bout of exercise around the time of the jab could sharpen the immune response and improve protection. The idea is appealing because it is free, quick and requires no change to the vaccine itself. This review set out to test whether short- or long-term exercise before influenza vaccination actually prevents influenza or its complications in adults.

The reviewers searched CENTRAL, MEDLINE, Embase, CINAHL, LILACS, PEDro, SPORTDiscus and two trial registries up to November 2015, looking for randomised controlled trials of exercise before influenza vaccination in the general adult population. Six trials published between 2007 and 2014, randomising 599 adults aged between 18 and 80, met the criteria, with study sizes from 46 to 158 participants. Five trials delivered a single session of 25 to 50 minutes, usually on the day of vaccination, using endurance work such as walking or treadmill or resistance work such as biceps curls and lateral raises, at intensities from 55% to 85% of maximal heart rate. One study ran an eight-week programme with a supervised 2½ hour session plus 45 minutes of daily home practice, while control participants rested quietly, sat, read, meditated or did unspecified activity.

There was no significant difference in outcomes between people who exercised before vaccination and those who did not. Only one study reported how many participants actually caught influenza, and it found no significant difference between the exercise and no-exercise groups. None of the six trials reported complications related to influenza illness, and no trial reported working days lost. Just one study, judged to provide low-quality evidence, reported adverse events, and it too found no significant difference — so exercise before vaccination did not appear to cause harm either. Only two of the six studies reported participant-centred outcomes at all.

The review turns an appealing biological hypothesis into an evidence gap rather than a confirmed effect. Overall study quality was unclear, with five of six trials carrying at least four unclear risk-of-bias domains covering allocation concealment, blinding of outcome assessment, selective reporting and other bias, and only two studies registered in trial registers. Because most trials chased biochemical markers such as antibody measures rather than whether anyone got ill, the literature was poorly designed to answer the question that matters. Small samples, varied exercise types, intensities and durations, and the near-total absence of clinical endpoints mean a real effect could have been missed.

There is currently no good reason to believe that timing a workout around your flu jab changes whether you catch flu. Equally, nothing here suggests exercising near the time of vaccination is risky, and the single trial that looked at adverse events found no difference. The sensible reading is that the vaccine, not the warm-up, is doing the work. If you exercise on vaccination day because that is your normal routine, this review gives no reason to stop and no reason to start.