BACKGROUND AND PURPOSE:

• Tartof et al. (Lancet, 2021) evaluated the overall and variant-specific effectiveness of Pfizer’s COVID-19 vaccine against infection and hospital admissions by time since vaccination

METHODS:

• Retrospective cohort study
  • Kaiser Permanente Southern California 
• Participants
  • Individuals ≥12 years of age
• Exposure
  • COVID-19 vaccination with Pfizer’s vaccine
  • Full vaccination: Having received two vaccine doses ≥7 days after the second dose
  • Partial vaccination:
    • Only one dose ≥14 days after the first dose or if they received two doses <7 days after the second dose
• Study design
  • Effectiveness calculations were based on hazard ratios from adjusted Cox models
• Primary outcome
  • Infection confirmed with PCR testing
  • COVID-19 related hospital admissions

RESULTS:

• 3,436,957 individuals
  • Median (IQR) age: 45 (29 to 61) years
  • 52.4% female
• Pfizer vaccination effectiveness in fully vaccinated individuals
  • Effectiveness against infection: 73% (95% CI, 72 to 74)
  • Effectiveness against hospital admission: 90% (95% CI, 89 to 92)
• Effectiveness against infections declined after 5 months
  • 1 month after full vaccination: 88% (95% CI, 86 to 89)
  • 5 months after full vaccination: 47% (95% CI, 43 to 51)
• Delta variant: Among sequenced infections, vaccine effectiveness against infections was high during the first month after full vaccination but declined by 4 months
  • 1 month after full vaccination: 93% (95% CI, 85 to 97)
  • 4 months after full vaccination: 53% (95% CI, 39 to 65)
• Non-delta variant: Effectiveness also decreased after 4 to 5 months
  • 1 month after full vaccination: 97% (95% CI, 95 to 99)
  • 4 to 5 months after full vaccination: 67% (95% CI, 45 to 80)
• Vaccine effectiveness against hospital admissions for infections remained high after 6 months including delta variant
  • Delta: 93% (95% CI, 84 to 96)
  • Non-delta: 95% (95% CI, 90 to 98)

CONCLUSION:

• Pfizer’s vaccine effectiveness against infection appears to decline after approximately 6 months since full vaccination
  • This decline is more pronounced among infections with the delta variant
• However, effectiveness against hospital admission remained high (93%) at 6 months, even with infections with delta
• The authors state

Reduction in vaccine effectiveness against SARS-CoV-2 infections over time is probably primarily due to waning immunity with time rather than the delta variant escaping vaccine protection

Learn More – Primary Sources:

Effectiveness of mRNA BNT162b2 COVID-19 vaccine up to 6 months in a large integrated health system in the USA: a retrospective cohort study

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