Flu-related absence, a small proportion of all-cause sickness absence

Editor – The recent paper by Pereira et al on potential for improved sickness absence following influenza vaccination in healthcare workers is interesting.¹ We wonder whether the authors conclusions are valid based on the data in their study.

Annual population influenza infection rates are reported at between 5–20%.² On average each flu case takes 3 days absence.² Not all of influenza cases result in absence from work.³ In an average influenza season the expected contribution from influenza on total sickness absence may be 0.1–0.3%.

The vaccine is ineffective against other influenza-like illness (ILI) that are not caused by influenza. Generally the vaccine does not exactly match circulating seasonal flu strains, and other factors affect vaccine response,⁴ which is at best about 60% effective.⁵ Therefore, the impact of the vaccine on improvement of sickness absence can only be between 0.05 to 0.15% (­average 0.1%).

The data analysis in this paper does correspond with the effect modelling outlined above. The authors' conclusion that ‘A 10% increase in vaccination would be associated with a 10% fall in sickness absence rate’ seems misleading based on the proportion of total sickness absence that is due to flu. In an average flu season the total proportion of influenza-related sickness absence rate is likely to be of the order of only a proportion (0.1%) of the all-cause absence rate of 4.5%. It may be that the authors intended to say that a 10% increase in vaccination would lead to a 10% fall in sickness absence in relation to influenza, but not total absence.

It may be time to review the efficacy of healthcare worker influenza vaccination against the desired objectives of public health policy. To aim to vaccinate 100% of a mostly healthy population, of whom at most about 20% may become infected, with an imperfect vaccine to improve sickness absence by 0.1% in the average flu season, seems of marginal benefit.