Updated: October 5, 2021 7:18:47 am
A new study has reconfirmed the effectiveness of the Pfizer-BioNTech vaccine (BNT162b2) as far as hospitalisation is concerned, and across all variants of SARS-CoV-2, the virus that causes Covid-19. The study from Kaiser Permanente and Pfizer, published in The Lancet, has found that two doses are 90% effective against hospitalisations for all variants, including Delta, for at least six months.
It was conducted on subjects in the United States.
Effectiveness of the Pfizer vaccine against all SARS-CoV-2 infections declined over the study period, falling from 88% within one month after receiving two vaccine doses, to 47% after six months. However, effectiveness of the vaccine against hospitalisation remained at 90% overall and for all variants, the study found.
These findings are consistent with preliminary reports from the US Centers for Disease Control and Prevention (CDC) and those of the Israel Ministry of Health, which had found reductions of BNT162b2 against infection after approximately six months, the study authors noted.
Researchers analysed 3,436,957 electronic health records from the Kaiser Permanente Southern California (KPSC) health system between December 4, 2020 and August 8, 2021. During the study period, 5.4% (184,041 people) were infected with SARS-CoV-2. Among those infected, 6.6% (12,130) were hospitalised. 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% 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, confirming the Delta variant had become the dominant strain in the United States.
Vaccine effectiveness against the Delta variant infections at one month after two doses of BNT162B2 was 93% and fell to 53% after four months. Effectiveness against other variants at one month after receiving two doses was 97% and declined to 67% after four months. Effectiveness against Delta-related hospitalisations remained high (93%) for the duration of the study period.
The authors, however, note that analyses with longer follow-up to measure the rate of waning for Delta compared to other variants are warranted.
The researchers say the study underscores the importance of monitoring vaccine effectiveness to determine which populations should be prioritised to receive booster shots. “While this study provided evidence that immunity wanes for all age groups that received the vaccine, the CDC Advisory Committee on Immunization Practices has called for additional research to determine if booster shots should be made available to all age groups eligible for this vaccine. In line with the recent FDA and CDC recommendations and considerations for booster shots should take global Covid-19 vaccine supply into account as people in many countries around the world have not yet received a primary vaccination series,” the study’s lead author Dr Sara Tartof, from Kaiser Permanente, said in a statement.
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