Effectiveness of vaccination against SARS-CoV-2 infection and Covid-19 hospitalisation among Finnish elderly and chronically ill-An interim analysis of a nationwide cohort study

Background In Finland, both mRNA and adenovirus vector (AdV) Covid-19 vaccines have been used after the vaccination campaign started on December 27, 2020. Vaccination of the elderly and chronically ill was prioritized and the interval between doses set to 12 weeks. The objective of this interim analysis was to evaluate first and second dose vaccine effectiveness (VE) in a real-world setting. Methods During the first five months of the campaign, a register-based cohort study was conducted in the Finnish elderly aged 70+ years and those aged 16-69 years with medical conditions predisposing to severe Covid-19 (chronically ill). Using Cox regression, VE against SARS-CoV-2 infection and Covid-19 hospitalisation was estimated comparing the hazard in the vaccinated with that in the unvaccinated. Results The cohorts included 901092 elderly (89% vaccinated) and 774526 chronically ill (69% vaccinated) individuals. Three weeks after the first dose, mRNA VE against infection was 45% (95% confidence interval, 36-53%) and 40% (26-51%) in elderly and chronically ill; mRNA VE against hospitalisation was 63% (49-74%) and 82% (56-93%). In chronically ill, AdV VE was 42% (32-50) and 62% (42-75%) against infection and hospitalisation, respectively. One week after the second dose, mRNA VE against infection was 75% (65-82%) and 77% (65-85%) in elderly and chronically ill; mRNA VE against hospitalisation was 93% (70-98%) and 90% (29-99%). Conclusions Covid-19 vaccines protect against SARS-CoV-2 infection and Covid-19 hospitalisation. A single dose provides moderate protection in elderly and chronically ill, although two doses are clearly superior.