Introduction
infection
INFECTION, CNS INFECTIONS, SARS-COV-2 INFECTION
An unprecedented global effort has led to the rapid development and deployment of COVID-19 vaccines.The COVID-19 vaccination programme in Hong Kong (Special Administrative Region, China) initially offered either two doses of CoronaVac (Sinovac Biotech) 28 days apart or two doses of BNT162b2 (Fosun Pharma–BioNTech in mainland China, Hong Kong, Macau, and Taiwan; Pfizer–BioNTech in other regions) 21 days apart, with broad eligibility criteria and availability. Subsequently, adults were recommended to receive a third dose of either vaccine (ie, allowing crossover) as soon as 90 days after the second dose.Safety, immunogenicity, and efficacy are the main aspects evaluated before vaccines are licensed. Few trialsIn addition, Hong Kong had a large wave of infections in early 2022. Most of the population in Hong Kong was unexposed to SARS-CoV-2 infection by any strain until the end of 2021 due to stringent travel restrictions and community-wide non-pharmaceutical interventions. This lack of exposure allowed us to evaluate the comparative efficacy of these different vaccine combinations against infection with the prevalent omicron BA.2 subvariant in a population in whom any previous immunity was from vaccination only.