Two doses of Pfizer-BioNTech vaccine highly effective for at least six months, study finds

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Two doses of Pfizer-BioNTech (BNT162b2) are 90 percent effective against novel coronavirus (COVID-19) hospitalizations for all variants, including delta, for at least six months, according to a new study from Kaiser Permanente and Pfizer published in The Lancet.

For the study, researchers analyzed 3,436,957 electronic health records from the Kaiser Permanente Southern California health system between December 4, 2020, and August 8, 2021, to assess BNT162b2 vaccine effectiveness against SARS-CoV-2 infections and COVID-19-related hospitalization. During the study period, 5.4 percent of people were infected with SARS-CoV-2. Among those who were infected, 6.6 percent were hospitalized. The average time since being fully vaccinated was between three to four months.

A whole genome sequencing and viral lineage analysis of 8,911 PCR-positive SARS-CoV-2 samples from the study cohort determined that the delta variant comprised 28 percent 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 percent in April 2021 to nearly 87 percent by July 2021, confirming the delta variant had become the dominant strain in the United States. 

Effectiveness against all SARS-COV-2 infections declined over the study period, falling from 88 percent within one month after receiving two vaccine doses to 47 percent after six months. However, effectiveness against hospitalizations remained at 90 percent overall and for all variants.

Vaccine effectiveness against delta variant infections at one month after two doses of BNT162B2 was 93 percent and fell to 53 percent after four months. Effectiveness against other (non-delta) variants at one month after receiving two doses was 97 percent and declined to 67 percent after four months. Effectiveness against delta-related hospitalizations remained high (93 percent) for the duration of the study period.

Researchers did not observe a difference in waning between SAR-CoV-2 variant types. However, the authors note that because delta became the dominant strain in the middle of the study period, analyses with longer follow-up to measure the rate of waning for delta compared to other variants are warranted.

These findings are consistent with preliminary reports from the U.S. Centers for Disease Control and Prevention (CDC) and the Israel Ministry of Health that found reductions of BNT162b2 against infection after approximately six months.

The authors acknowledge some limitations in their study. The researchers could not determine causal relationships between vaccination and COVID-19 outcomes as vaccination status among the study population was not randomized. The researchers also did not have data on adherence to mask guidelines, social interactions, occupation, and disease rates in the study population, which could impact the likelihood of contracting and being tested for SARS-CoV-2 infection. The researchers also note that vaccine effectiveness estimates against SARS-CoV-2 infections and hospitalizations could be muted by mild or asymptomatic infections and are not directly comparable to effectiveness estimates against symptomatic disease. Sequencing was more likely to fail among the vaccinated due to lower viral loads, which could lead to an overestimate of variant-specific effectiveness.

Regardless, the researchers say this study underscores the importance of improving COVID-19 vaccination rates worldwide and monitoring vaccine effectiveness to determine which populations should be prioritized to receive booster shots.

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