COVID-19 Infection Risk Increases Gradually 90 Days After Second Vaccine Dose


Investigators have confirmed that the risk of COVID-19 infection increases from 90 days following a second dose of the Pfizer-BioNTech vaccine, according to a study published in The BMJ. The researchers said these results reinforce the importance of a third booster dose of the vaccine.

To conduct the study, the investigators examined electronic health records for 80,057 adults who received a PCR test at least 3 weeks after their second injection and had no evidence of previous COVID-19 infection. Among those tested, 9.6% had a positive test result. These individuals were matched to negative controls of the same age and ethnic group who were tested in the same week.

According to the investigators, the rate of positive results increased with time elapsed following the patient’s second dose. Across all age groups, 1.3% of participants tested positive 21-89 days after a second dose. This increased to 2.4% after 90-119 days, 4.6% after 120-149 days, 10.3% after 150-179 days, and 15.5% after 180 days or more.

After accounting for other factors that could potentially influence rates of infection, the investigators found a significantly increased risk of infection with time elapsed since a second dose. Compared to the risk of infection within the initial 90 days following a second dose, the risk of infection across all age groups increased by 2.37 times after 90-119 days, 2.66 times after 120-149 days, 2.82 times after 150-179 days, and 2.82 times after 180 days or more.

Potential limitations of the study include its observational design, as well as the possibility that other unmeasured factors—including household size, population density, or virus strain—may have had an effect on infection rates. However, the large sample size and uniform vaccine type suggest the results are robust.

The investigators concluded that in individuals who received 2 doses of the Pfizer-BioNTech vaccine, protection appears to decrease over time, and the risk of breakthrough infection increases progressively following the initial 90 days. They added that the results suggest the consideration of a third vaccine dose might be warranted, and that examining the risk of infection relative to the time since vaccination could provide suggestions for proper timing of a booster dose.


Study finds gradual increase in covid infection risk after second vaccine dose [news release]. EurekAlert; November 24, 2021. Accessed November 30, 2021.

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