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Vaccine efficacy of BNT162b significantly reduced after 5 months

A large retrospective study has found that while the BNT162b vaccine efficacy is initially high it significantly reduces over 5 months.

In the first clinical trial, the BNT162b COVID-19 vaccine was shown to provide 95% protection against COVID-19 in those 16 years of age and older. In addition, a real-world study has confirmed the efficacy observed in the trial, with two doses of BNT162b2 being highly effective in preventing both symptomatic and asymptomatic infections, COVID-19-related hospitalisations, severe disease and death. However, there have been concerns that approved COVID-19 vaccines might show reduced efficacy over time prompting the need for a booster dose.

In an attempt to provide answers as to whether vaccine efficacy wanes over time, a team from the Department of Research and Evaluation, Kaiser Permanente, California, USA, undertook a retrospective cohort study. The team analysed electronic health records for individuals 12 years and older to assess BNT162b effectiveness against COVID-19 infection and related hospitalisations for up to six months. The study outcomes were COVID-19 infections (based on a positive PCR test) and COVID-19-related hospitalisations over a 6 month period.


A total of 3,436,957 individuals with a median age of 45 years (52.4% female) were included in the final analysis. There were 184 041 (5·4%) participants infected with COVID-19, of whom, 12130 (6·6%) were admitted to hospital. Among those fully vaccinated, the vaccine efficacy against infection was 73% (95% CI 72 – 74) and against hospital admission 90% (95% CI 89 – 92). Over a period of 5 months however, vaccine efficacy against infection reduced from 88% (in the first month) to 47% after 5 months. Despite this, vaccine efficacy against hospitalisation remained stable over the study period.

The study also examined vaccine efficacy against the COVID-19 delta variant and 4 months after full vaccination, vaccine effectiveness against this variant declined from 93% to 53%.

The authors concluded that their findings underscored the importance of monitoring vaccine efficacy over time and suggested that a booster dose might be needed to restore high levels of protection.


Tartof SY et al. Effectiveness of mRNA BNT162b2 COVID-19 vaccine up to 6 months in a large integrated health system in the USA: a retrospective cohort study. Lancet 2021