Serological testing for influenza infection alone, rather than molecular testing methods, may have underestimated the true number of influenza infections [24] . This, along with the relatively high vaccination uptake, will have reduced the number of SCII and thus the effective study power. With an attack rate of 10% in non-healthcare workers, the sample size of 736 analysed would not have been sufficient to detect a relative risk below 1.8 (80% power, 5% significance).