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COVID-19 Management

Data on Waning of Pfizer’s COVID-19 Vaccine

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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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Related ObG Topics:

COVID-19 mRNA Vaccine Effectiveness in the Real World Including in Those Partially Immunized
COVID-19 Real-World Vaccine Effectiveness in Health Care Personnel
Vaccine Effectiveness Against Hospitalization in the U.S. for Moderna, Pfizer and J&J

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