Background: Influenza is an infectious virus affecting both humans and animals. In humans, symptoms present as fever, cough, sore throat, runny nose, headache, muscle and joint pain, and malaise. The epidemiological profile of influenza is influenced by multiple factors, including transmissibility of the virus and the susceptibility of the population. Annually, influenza is estimated to infect 5% to 10% of adults, with higher rates in winter seasons in countries with seasonal variation. Exercise could be an intervention to enhance immune response and limit influenza incidence and its related complications.

Objectives: To assess the efficacy and safety of short and long-term exercise prior to influenza vaccination in enhancing influenza prevention in adults.

Search methods: We searched CENTRAL (2015, Issue 11), which contains the Cochrane Acute Respiratory Infections Group's Specialised Register, MEDLINE (1966 to 3 November 2015), Embase (1974 to 3 November 2015), CINAHL (1981 to 3 November 2015), LILACS (Latin American and Caribbean Health Sciences, 1982 to 3 November 2015), PEDro (1980 to 3 November 2015), SPORTDiscus (1985 to 3 November 2015), the WHO International Clinical Trials Registry Platform (ICTRP) and ClinicalTrials.gov (November 2015).

Selection criteria: Randomised controlled trials (RCTs) of short- and long-term exercise prior to influenza vaccination for the general adult population were eligible for inclusion.

Data collection and analysis: Two review authors independently extracted and checked data from the included trials using a standard form. We used the random-effects model due to differences in the type, duration, intensity and frequency of exercise in the analysis.

Main results: We included six trials published between 2007 and 2014 that randomised 599 adult participants. Study size ranged from 46 to 158 participants. Participants were aged between 18 years and 80 years; we could not derive gender proportions, as participants' sex was not reported in all studies. One study was available in abstract form only.We did not find a significant difference in outcomes between people who exercised and those who did not exercise before receiving influenza vaccination.Pre-vaccination exercises included endurance activities such as walking or using a treadmill, and resistance activities included biceps curls and lateral raises. Five of the studies provided one session of exercise between 25 and 50 minutes. In five studies, exercise was undertaken on the same day as the vaccination. One study provided exercise over a period of eight weeks before vaccination, with one 2½ hour supervised session, plus daily home exercise practice of 45 minutes. Exercise intensity ranged from 55% to 85% of maximal heart rate. Control group participants undertook a range of activities, including quiet rest, sitting, reading, meditation or unspecified activity.One study reported numbers of people who contracted influenza; no significant difference was reported between exercise and no-exercise participants. None of the included studies reported complications related to influenza illness. Only one study, which we assessed as providing low-quality evidence, reported numbers of people who experienced adverse events. This study reported no significant difference in outcomes between people who exercised and those who did not. No studies reported numbers of working days or days lost related to influenza illness. Only two studies reported participant-centred outcomes.Overall, study quality was unclear; we assessed five of the six included studies to have at least four unclear 'Risk of bias' domains (allocation concealment, blinding of outcome assessment, selective reporting and other bias). Insufficient reporting in four studies about selective reporting did not provide enough information to enable judgement; only two studies were included in trials registers.

Authors' conclusions: From the available evidence, we found that exercising before influenza vaccination is neither beneficial nor harmful. However, study data were limited and of low quality. Small sample sizes, study design limitations, exercise types, and focus on biochemical rather than participant-centred outcomes strongly influenced our findings.