Assessment of Protection Against Reinfection With SARS-CoV-2
- Protection against repeat infection after previous infection with SARS-CoV-2 is approximately 80% or higher in those aged <65 years, but approximately 47% in those aged ≥65 years.
- These data underscore the need for protective measures for the older population with previous infection including vaccination and continued social distancing and infection control.
- Furthermore, these data suggest that previously infected individuals should be vaccinated because natural protection is not close to 100% and cannot be counted on.
What is the degree of protection towards repeat infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) after initial infection?
The investigators conducted a population-level observational study and collected individual-level data on patients who had been tested in Denmark in 2020 from the Danish Microbiology Database. They analyzed infection rates during the second surge of the coronavirus disease 2019 (COVID-19) epidemic, from September 1 to December 31, 2020, by comparison of infection rates between individuals with positive and negative polymerase chain reaction (PCR) tests during the first surge (March–May 2020). For the main analysis, the authors excluded people who tested positive for the first time between the two surges and those who died before the second surge. They did an alternative cohort analysis, and compared infection rates throughout the year between those with and without a previous confirmed infection ≥3 months earlier, irrespective of date. They also investigated whether differences were found by age group, sex, and time since infection in the alternative cohort analysis. Finally, the investigators calculated rate ratios (RRs) adjusted for potential confounders and estimated protection against repeat infection as 1 – RR and plotted a Kaplan-Meier curve of time until infection during follow-up.
During the first surge (i.e., before June 2020), 533,381 people were tested, of whom 11,727 (2.20%) were PCR positive, and 525,339 were eligible for follow-up in the second surge, of whom 11,068 (2.11%) had tested positive during the first surge. Among eligible PCR-positive individuals from the first surge of the epidemic, 72 (0.65%; 95% confidence interval [CI], 0.51–0.82) tested positive again during the second surge compared with 16,819 (3.27%; 95% CI, 3.22–3.32) of 514,271 who tested negative during the first surge (adjusted RR, 0.195; 95% CI, 0.155–0.246). Protection against repeat infection was 80.5% (95% CI, 75.4–84.5). The alternative cohort analysis gave similar estimates (adjusted RR, 0.212; 95% CI, 0.179–0.251; estimated protection, 78.8%; 95% CI, 74.9–82.1). In the alternative cohort analysis, among those aged ≥65 years, observed protection against repeat infection was 47.1% (95% CI, 24.7–62.8). The investigators found no difference in estimated protection against repeat infection by sex (male 78.4% [72.1–83.2] vs. female 79.1% [73.9–83.3]) or evidence of waning protection over time (3–6 months of follow-up, 79.3% [74.4–83.3] vs. ≥7 months of follow-up, 77.7% [70.9–82.9]).
The authors concluded that natural protection, especially among older people, cannot be relied on.
This large national surveillance dataset of individually referable PCR test results estimated the degree to which previous infection with SARS-CoV-2 results in protection against repeat infection. The protection is ≥80% in those aged <65 years, but is approximately 47% in those aged ≥65 years. Since the older age group is also more prone to a serious clinical course and sequelae of illness, these data underscore the need for protective measures for the older population including vaccination and enhanced social distancing and infection control, even in those known to be previously infected. Furthermore, these data suggest that previously infected individuals should be vaccinated because natural protection is not close to 100% and cannot be counted on.
Keywords: Coronavirus, COVID-19, Geriatrics, Infection Control, Polymerase Chain Reaction, Primary Prevention, SARS-CoV-2, Psychological Distance, Vaccination
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