BACKGROUND AND PURPOSE:

• The COVID-19 Forecasting Team (The Lancet, 2023) systematically synthesized studies to estimate protection from past infection by SARS-CoV-2

METHODS:

• Systematic review and meta-analysis
• Study inclusion criteria
  • Cohort studies and test-negative case-control studies
  • Compared risk reduction of COVID-19 among individuals with a past SARS-CoV-2 infection vs those without a previous infection
• Study design
  • Primary outcome was based upon variants and time since infection
  • Bayesian meta-regression was used to estimate the pooled estimates of protection
  • Risk-of-bias assessment was evaluated using quality-assessment tools
• Primary outcome
  • Effectiveness of past infection by outcome against infection, symptomatic disease, and severe disease

RESULTS:

• 65 studies (19 countries)
• Protection from infection and any symptomatic disease was
  • High for ancestral, alpha, beta, and delta variants
  • Lower for the omicron BA.1 variant
    • Against re-infection: 45.3% (95% uncertainty interval (UI), 17.3 to 76.1)
    • Against symptomatic disease: 44.0% (95% UI, 26.5 to 65.0)
• For all variants, protection against re-infection declined over time
  • From ancestral, alpha, and delta variants, this protection declined over time, but remained at 78% for 40 weeks
  • Protection against re-infection by the omicron BA.1 variant declined more rapidly | At 40 weeks: 36.1% (95% UI, 24.4 to 51.3)
• For all variants, mean pooled effectiveness against severe disease was greater than 78%
• Protection against severe disease remained high for all variants
  • Ancestral, alpha, and delta variants at 40 weeks: 90.2% (95% UI, 69.7 to 97.5)
  • Omicron BA.1 at 40 weeks: 88.9% (95% UI, 84.7 to 90.9)

CONCLUSION:

• Protection against re-infection was high and remained high for ancestral SARS-CoV-2 variants
  • For Omicron BA.1, this protection waned more quickly, and was around 36% at 40 weeks
• Protection against severe disease was high for all variants, and remained high for all
• The authors state

Our analysis suggests that the level of protection from past infection by variant and over time is at least equivalent if not greater than that provided by two-dose mRNA vaccines

Learn More – Primary Sources:

Past SARS-CoV-2 infection protection against re-infection: a systematic review and meta-analysis