Effectiveness of mRNA BNT162b2 COVID-19 Vaccine Up to 6 Months

Quick Takes

  • This is a retrospective analysis of the effectiveness of the BNT162b2 vaccine against SARS-CoV-2 infection and COVID-19 hospitalizations in a cohort of 3,436,957 members of Kaiser Permanente Southern California.
  • Vaccine effectiveness against hospitalization was +90% for up to 6 months, while effectiveness against infection steadily declined to 50% after 5 months.
  • Reduction in the vaccine’s effectiveness in preventing infections was observed irrespective of the SARS-CoV-2 variant and is likely due to waning immunity rather than the variant escaping protection.

Study Questions:

How effective is the BNT162b2 (Pfizer–BioNTech) vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections and coronavirus disease 2019 (COVID-19)–related hospital admissions by time since vaccination?


The authors retrospectively reviewed medical records of members (≥12 years of age) of the health care organization Kaiser Permanente Southern California (CA, USA) to assess BNT162b2 vaccine effectiveness against SARS-CoV-2 infections and COVID-19–related hospital admissions for up to 6 months. The study timeline was between December 14, 2020–August 8, 2021). The primary exposure was full vaccination with BNT162b2, defined as receiving two doses of BNT162b2 within ≥7 days after the second dose. Individuals were considered partially vaccinated if they received only one dose within ≥14 days after the first dose or if they received two doses within <7 days after the second dose. The primary outcomes were SARS-CoV-2 polymerase chain reaction (PCR)-positive tests and COVID-19–related hospital admissions. Variant-specific effectiveness was also examined.


The study included 3,436,957 members (median age 45 years, 52.4% female, 40.5% Hispanics). During the study period, 184,041 (5.4%) participants were infected with SARS-CoV-2, among whom 12,130 (6.6%) were admitted to the hospital. Hospitalized patients were older and had a higher burden of comorbidities. Effectiveness against SARS-CoV-2 infections was 73% (95% confidence interval [CI], 72–74) and 90% (95% CI, 89–92). Effectiveness against infections declined from 88% (95% CI, 86–89) during the first month after full vaccination to 47% (95% CI, 43–51) after 5 months. Among sequenced infections (n = 8,911), vaccine effectiveness against infections of the delta variant was high during the first month after full vaccination (93% [95% CI, 85–97]), but declined to 53% (95% CI, 39–65) after 4 months. Effectiveness against other (non-delta) variants the first month after full vaccination was also high at 97% (95% CI, 95–99), but waned to 67% (95% CI, 45–80) at 4–5 months. Vaccine effectiveness against hospital admissions for infections with the delta variant for all ages was high overall (93% [95% CI, 84–96]) up to 6 months.


The BNT162b2 vaccine is highly effective at preventing hospitalization for COVID-19 up to 6 months post-vaccination, regardless of the SARS-CoV-2 variant.


Besides supporting the multiple other studies showing that vaccines are indeed effective at preventing SARS-CoV-2 infections and hospitalizations for COVID-19, this study offers insight on the relative effectiveness across time and variants in a real-world setting. The findings are reassuring. Effectiveness against severe COVID-19 requiring hospitalization did not wane at 6 months across age groups and variants, remaining at +90%. A decline in effectiveness at preventing infections was noted irrespective of the SARS-CoV-2 variants, suggesting it is due to waning immunity rather than differences in variant virulence. These findings have two important implications: 1) a booster vaccine will inevitably be required to stem resurgence of SARS-CoV-2 infections, and 2) the booster may not need to be variant-specific, as the current form of the vaccine appears to be effective regardless of strain.

Keywords: Comorbidity, Coronavirus, Coronavirus Infections, COVID-19, COVID-19 Vaccines, Delivery of Health Care, Integrated, Immunization, Secondary, Mass Vaccination, Polymerase Chain Reaction, Primary Prevention, RNA, Messenger, SARS-CoV-2, Vaccination, Vaccines, Virulence

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