Quantifying the causal impact of funding bedside antigen testing on the incidence of respiratory syncytial virus infection in Japan: a difference-in-differences study

BACKGROUND: To estimate the causal impact of a change in the national health insurance policy to cover the cost of respiratory syncytial virus (RSV) antigen testing on the incidence of RSV infection by age-groups, we analyzed the sentinel datasets of RSV infection in Japan from 2009–2017. METHODS: The causal effect of introducing RSV antigen testing on increasing the reported incidence of RSV infection was quantified using a quasi-experimental difference-in-differences (DID) design and influenza as a control group. RESULTS: Examining sentinel and virus surveillance datasets, only clinically mild cases have selectively increased from 2012/13. The median estimated incidences of influenza and RSV infection among children aged 0–4 years, the age group that dominates cases of both diseases, were estimated at 5,607 and 1,761 per 100,000 individuals, respectively. Exploring sentinel datasets, only the incidence of RSV infection abruptly increased from 2012/13. Using an age-dependent model, the estimated causal effect on the increase in RSV annual incidence was greatest among children aged 5–9 years, with an estimated additional 1,912 cases per 100,000 individuals (95% CI: 418–3,406). CONCLUSIONS: Owing to financial support of bedside RSV antigen testing from 2012/13, the incidence of RSV infection has been elevated. The recent increasing trend in RSV infection incidence should not be directly perceived as an increase in natural infections with RSV.