Previous-infection and vaccine population immunities in a specific country can be defined as the overall protection against infection at a given point in time in the full national population. These immunity patterns demonstrate the need for the concept of population immunity to track evolution of overall immune protection over time in a given population. Similarly, COVID-19 primary-series and booster vaccination induce protection against SARS-CoV-2 infection, but this protection also wanes with time since last dose. Effectiveness of previous infection and vaccination against severe, critical, or fatal COVID-19 were generally >80% throughout the study duration.Įvidence before this study SARS-CoV-2 infection induces protection against reinfection, but this protection wanes with time since last infection. Booster effectiveness dropped immediately after Omicron emergence from 83.0% (95% CI: 65.6 -91.6%) in November 2021 to 32.9% (95% CI: 26.7-38.5%) in December 2021, and continued to decline thereafter. After Omicron emergence, effectiveness dropped suddenly from 52.7% (95% CI: 46.5-58.2%) in November 2021 to negligible levels in December 2021. Effectiveness declined linearly by ∼1 percentage point every 5 days. Primary-series effectiveness against infection was 84.0% (95% CI: 83.0-85.0%) in April 2021, soon after introduction of vaccination, before waning gradually to 52.7% (95% CI: 46.5-58.2%) by November of 2021. Findings Previous-infection effectiveness against reinfection was strong before emergence of Omicron, but declined with time after a wave and rebounded after a new wave.
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