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Observed Protection Against SARS-CoV-2 Reinfection Following a Primary Infection: A Danish Cohort Study Using Two Years of Nationwide PCR-Test Data
22 Pages Posted: 10 Mar 2022
More...Abstract
Background: The level of protection after a SARS-CoV-2 infection against reinfection and COVID-19 disease remains important with much of the world still unvaccinated.
Methods: Analysing nationwide, individually referable, Danish register data including RT-PCR-test results, we conducted a cohort study using Cox regression to compare SARS-CoV-2 infection rates before and after a primary infection among still unvaccinated individuals, adjusting for sex, age and residency region. The prevalence of infections classified as symptomatic or asymptomatic was compared for primary infections and reinfections. The study also assessed protection against each of the main viral variants after an earlier variant primary infection by restricting follow-up time to distinct, mutually exclusive periods during which each variant dominated.
Findings: Until 1 July 2021 the estimated protection against reinfection was 83.5% (95%CI: 82.2%–84.6%); but lower for the 65+ year-olds (72.0%; 95%CI: 56.1%–82.2%). First-time cases who reported no symptoms were more likely to experience a reinfection (OR: 1.48; 95%CI: 1.36–1.62). By autumn 2021, when infections were almost exclusively by the Delta variant, the estimated protection of a recent infection was 91.3% (95%CI: 89.7%–92.7%) compared to 71.3% (95%CI: 66.8%–75.2%) after a first infection over a year earlier. With Omicron, a first infection in the past 3-6 months gave an estimated 43.1% (95%CI: 41.6%-44.4%) protection, whereas a first infection longer than 12 months earlier provided only 14.6% (12.7-16.4%) protection.
Interpretation: SARS-CoV-2 infection offered a high level of sustained protection against reinfection, comparable with that offered by vaccines, but decreased with the introduction of new main virus variants; dramatically so when Omicron appeared. Protection was lower among the elderly but appeared more pronounced following symptomatic compared to asymptomatic infections. Decreases in protection against reinfection, seemed primarily to be driven by viral evolution.
Funding Information: None.
Declaration of Interests: All authors declare no competing interests.
Ethics Approval Statement: This study was performed under the authority task of the Danish national infectious disease control institute, Statens Serum Institut. It used data from existing Danish national COVID-19 surveillance systems and did not require ethical approval.
Keywords: COVID-19, Cohort Studies, Immunity, reinfection, herd immunity, SARS-CoV-2 variants, epidemic
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