Study: Two Doses of Pfizer, BioNTech COVID-19 Vaccine Effectively Prevent COVID-19 Hospitalization for at Least 6 Months


Vaccine efficacy was 93% against the Delta variant at 1 month after the 2-dose vaccine regimen, although it fell to 53% after 4 months.

New research published in The Lancet has found that the efficacy of the 2-dose COVID-19 vaccine from Pfizer and BioNTech against COVID-19-related hospitalization for all variants remains high for at least 6 months.

Notably, the vaccine’s efficacy against all SARS-CoV-2 infections declined from 88% to 47% over the 6-month study period. However, efficacy against hospitalizations remained at 90% overall and for all variants. These findings are consistent with data from the CDC and underscore the importance of improving COVID-19 vaccination rates, according to the study.

“Our study confirms that vaccines are a critical tool for controlling the pandemic and remain highly effective in preventing severe disease and hospitalization, including from the Delta and other variants of concern,” said lead author Sara Tartof, PhD, MPH, with the Kaiser Permanente Southern California (KPSC) Department of Research and Evaluation, in the press release. “Protection against infection does decline in the months following a second dose.”

In the study, investigators analyzed nearly 3.5 million electronic health records from the KPSC health system between December 4, 2020, and August 8, 2021, to assess the vaccine’s efficacy against SARS-CoV-2 infections and related hospitalization. During the study period, 5.4% of people were infected with SARS-CoV-2, 6.6% of whom were hospitalized. The average time between full vaccination and infection was 3 to 4 months.

Furthermore, genome sequencing and viral lineage analysis of 8911 PCR-positive SARS-CoV-2 samples from the study cohort found that the Delta variant comprised 28% of the overall proportion of positive sequences. During the study period, the proportion of positive cases attributed to the Delta variant increased from 0.6% in April 2021 to nearly 87% by July 2021.

Vaccine efficacy against the Delta variant at 1 month after the 2-dose vaccine regimen was 93%, although it fell to 53% after 4 months. Efficacy against other variants was 97% at 1 month post-vaccination and fell to 67% after 4 months. Notably, efficacy against Delta variant-related hospitalizations remained at 93% for the duration of the study period.

Researchers found no difference in waning between the SARS-CoV-2 variant types, although the study authors said that because the Delta variant became the dominant strain in the middle of the study period, analyses with longer follow-up are necessary.

“Our variant-specific analysis clearly shows that the BNT162b2 vaccine is effective against all current variants of concern, including Delta,” said Luis Jodar, PhD, PharmD, senior vice president and chief medical officer of Pfizer Vaccines, in the press release. “COVID-19 infections in people who have received 2 vaccine doses are most likely due to waning and not caused by Delta or other variants escaping vaccine protection.”


The Lancet: Two doses of Pfizer-BioNTech vaccine are highly effective against COVID-19 hospitalizations for at least 6 months. News release. Eurekalert; October 4, 2021. Accessed October 5, 2021.

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