School closures and mitigation of influenza B, Hong Kong, 2018

Abstract

Background: In Hong Kong, school closures have been used to mitigate SARS in 2003, seasonal influenza in 2008, and pandemic influenza in 2009. On 8 February 2018 the authorities in Hong Kong elected to close schools to mitigate an epidemic of influenza B/Yamagata, one week earlier than the scheduled Chinese New Year school holiday. Methods: We analyzed surveillance data on influenza activity in Hong Kong in order to infer the impact of school closures on community transmission. We estimated transmissibility through the effective reproduction number, Rt, assuming that the serial interval distribution was a Weibull distribution with mean 3.2 days and standard deviation 1.3 days. We examined changes in transmissibility during the school closure period, and simulated epidemics with and without school closures from 8-15 February to estimate the impact of the closures on incidence. Results: The estimate of Rt was 1.03 (95% CI: 0.73, 1.34) before the start of the school closure and it was reduced to 0.87 (95% CI: 0.54, 1.21) during the closure week, corresponding to a 16% (95% CI: 10%, 26%) reduction in transmissibility. We then simulated incidence under the counterfactual scenario of no school closures between 8 and 15 February, estimating that closures led to a reduction by 4.2% (95% CI 1.5%, 6.7%) in the cumulative incidence of infections. Conclusions: School closure after the epidemic peak had a small effect on transmission, and we estimated a 4.2% reduction in overall incidence of infections. There were approximately 400 laboratory-confirmed influenza deaths by the end of the 2017/18 winter season, fewer than those in recent A(H3N2) epidemics, but still indicating a moderate to high impact of influenza in the 2017/18 winter. A reduction in incidence of infections by 4.2% might have reduced hospitalisations and deaths by a similar fraction, with the caveat that most infections occur in children while most deaths occur in older adults.