Modelling the impact of varicella vaccination in Hong Kong

Universal varicella vaccination (UVV) was introduced in Hong Kong in July 2014 with no catch-up vaccination programme (CUP), following private vaccination since late 1990s. This PhD aims at understanding the impact of private and public varicella vaccination on the epidemiology of varicella, and to evaluate the varicella vaccination strategies in Hong Kong.

First, I conducted analyses on the epidemiology of varicella and zoster. Vaccine uptake in preschool children reached 50% before UVV introduction, and first dose uptake quickly rose to 98% after UVV. Under private vaccination, varicella notifications reduced significantly in children aged 4 years or below, and the reduction became more substantial five years after UVV. On the other hand, there was a shift in the burden of varicella to slightly older age groups with a corresponding increase in incidence before UVV, which persisted after UVV. Serological data also showed age shifts in infections before UVV, albeit starting before vaccine licensure. By 2020, seroprevalence in UVV ineligible individuals aged 5 to 25 years reduced, leaving a larger pool of adolescents and young adults susceptible to varicella infections.

Next, I estimated that one-dose varicella vaccination was moderately effective, and two dose vaccination was highly effective in preventing notified varicella with no evidence of waning protection in the first four years since vaccination.

Finally, I modelled the impact of varicella vaccination in Hong Kong on varicella epidemiology. The model predicted an upsurge of infections among adolescents and young adults after the lifting of non-pharmaceutical interventions against COVID-19. This upsurge could be reverted if a timely CUP was offered to non-UVV cohorts. In summary, this PhD demonstrated the direct and indirect effect of vaccination on varicella transmission and disease burden, which illustrated the complexities around varicella vaccine use and the usefulness of closely monitored epidemiology.