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Risk of Reinfection, Vaccine Protection, and Severity of Infection with the BA.5 Omicron Subvariant: A Danish Nation-Wide Population-Based Study
21 Pages Posted: 18 Jul 2022More...
Background: Estimates of immunity and severity for the SARS-CoV-2 omicron subvariant BA.5 are important to assess the public health impact associated with its rapid global spread despite vaccination. We estimated natural and vaccine immunity and severity of BA.5 relative to BA.2 in Denmark, a country with high mRNA vaccination coverage and free-of-charge RT-PCR testing.
Methods: This was an observational cohort study including residents 18 years or older with an RT-PCR test between 10 April and 20 June, 2022, identified in the national COVID-19 surveillance system database with information since February, 2020, on RT-PCR tests, wholegenome sequencing, vaccinations and hospitalisations with a positive test and COVID-19 as main diagnosis. We calculated the effect of prior omicron infection on the odds ratio (OR) of BA.5 infection among triple-vaccinated individuals (%protection=1-OR). For BA.5 relative to BA.2 we also calculated the OR of triple-vaccination vs none (ie. vaccine effectiveness) and the OR of hospitalisation. ORs were calculated in logistic regression models adjusted for age, time, sex, region, and comorbidities.
Findings: Among 4,809 BA.5 cases and 164,369 test-negative individuals, a prior omicron infection was highly protective against BA.5 (93.6%, 95%CI92.1-94.8; 96 BA.5 and 29,832 test-negative with prior omicron). Among 4,913 BA.5 and 31,874 BA.2 cases, the OR of triple-vaccination was not different (OR1.02, 95%CI0.83-1.26). Although the overall number of hospitalisations due to COVID-19 diagnosis was low and stable during the study period, the proportion of individuals infected with BA.5 increased. In the crude analysis BA.5 was not associated with hospitalisation (OR 1.04 (0.83-1.31) whereas the adjusted OR was 1.65 (1.16-2.34). The adjusted OR was increased in strata of age and calendar period – the two covariates with the largest contribution to confounding of the crude OR.
Interpretation: We found a high protection against BA.5 from prior omicron infection in triple-vaccinated individuals, and similar vaccine effectiveness for BA.5 infection as currently for BA.2. In an analysis adjusted for covariates, BA.5 infection was associated with an increased risk of hospitalisation which needs confirmation and continued surveillance as hospitalisations were low and stable during the study period.
Funding Statement: None.
Declaration of Interests: None.
Ethics Approval Statement: This study was performed under the authority task of the Danish national infectious disease control institute, which allows Statens Serum Institut to perform analyses on data from existing national COVID-19 surveillance systems. According to Danish law, ethical approval or individual consent is not required for anonymized aggregated register-based studies.
Keywords: Infectious diseases, covid-19, vaccine effectiveness, SARS-CoV-2, epidemiology, risk factors
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