BACKGROUND AND PURPOSE:

• Hansen et al. (Lancet, 2021) used Denmark’s national PCR-test data from 2020 to estimate protection towards repeat infection with SARS-CoV-2

METHODS:

• Population-level observational study
• Data source
  • Danish Microbiology Database
  • Includes individual-level data on patients who had been tested for SARS-CoV-2 in 2020  
• Participants
  • People tested for SARS-CoV-2 | PCR tests done on symptomatic individuals by referral within the national health-care system and parallel national testing system non-symptomatic individuals
  • Exclusion: Individuals with first positive test between the two surges, and those who died before the second surge
• Exposures
  • Previous confirmed infection
• Study design
  • The authors analyzed reinfection rates during the second surge of COVID-19
    • First surge: March to May, 2020
    • Second surge: September to December, 2020
  • Alternative cohort analysis: Infection rates throughout the year between those with vs without a previous confirmed infection at least 3 months earlier
  • Differences related to age, sex, and time since infection were also assessed
• Primary outcome
  • Protection against repeat infection (1-rate ratio [RR])

RESULTS:

• 525,339 people were tested during the first surge and eligible for follow-up in second surge
  • PCR positive: 2.11% (n=11,068)
• Second surge positivity
  • Tested positive during first surge and positive again in second surge
    • 0.65% (95% CI, 0.51 to 0.82)
  • Tested negative during first surge, and positive in second surge
    • 3.27% (95% CI, 3.22 to 2.32)
  • Adjusted RR: 0.195 (95% CI, 0.155 to 0.246)
  • Protection against repeat infection: 80.5% (95% CI, 75.4 to 84.5)
• Alternative cohort analysis provided similar estimates
  • Adjusted RR for reinfection: 0.212 (95% CI, 0.179 to 0.251)
  • Estimated protection: 78.8% (95% CI, 74.9 to 82.1)
• ≥65 years
  • Observed protection against repeat infection was lower vs general population: 47.1% (95% CI, 24.7 to 62.8)
• No difference in estimated protection against repeat infection by sex
  • Male 78.4% (95% CI, 72.1 to 83.2)
  • Female 79.1% (95% CI, 73.9 to 83.3)
• There was also no evidence of waning protection over time
  • 3–6 months follow-up: 79.3% (95% CI, 74.4 to 83.3)
  • ≥7 months follow-up: 77.7% (95% CI, 70.9 to 82.9)

CONCLUSION:

• In the general population, protection against repeat SARS-CoV-2 infection was approximately 80%
• Protection was lower for ≥65 at approximately 47%
• Protection did not wane over 2020
• The authors conclude that while protection was “robust” overall, reduced protection in those 65 and older

…highlights the need to implement protective measures for the older population in the form of effective vaccines and enhanced physical distancing and infection control, even in those known to be previously infected

Furthermore, our data indicate that vaccination of previously infected individuals should be done because natural protection cannot be relied on

Learn More – Primary Sources:

Assessment of protection against reinfection with SARS-CoV-2 among 4 million PCR-tested individuals in Denmark in 2020: a population-level observational study

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