Three RCTs, with a total of 967 participants (488 in the influenza vaccination group and 479 in the control group), were included in the review. Methodological quality of included studies was rated as high for two studies and moderate for the remaining study. No studies used intention-to-treat analysis. The moderate quality study had a relatively high drop-out rate (22%) and did not provide adequate information on sequence generation, allocation concealment and the blinding of investigators and outcome assessors.
One study reported laboratory-confirmed influenza infections. The risk of infection was significantly lower in the vaccination group, RR 0.12 (95% CI 0.04 to 0.41) and vaccine efficiency 88% (95% CI 59 to 96%).
One study reported incidence of influenza-like illness and showed no significant difference between the vaccine and control groups. A second study reported the number of influenza-like illness episodes and found no significant difference between the vaccine and control groups. The third study reported no significant difference in days with influenza-like illness symptoms between the vaccine and control groups.
There was no significant difference in the mean number of working days lost between vaccinated and unvaccinated health care workers (two studies, Ι²=0%).
All three included studies reported absenteeism and other adverse effects after the receipt of an influenza vaccination. Adverse effects were mainly localised, including pain at the site of the injection to erythema with or without induration, and were mild and transitory. There were no reports of life-threatening or persistent adverse reactions.