The effectiveness of the Pfizer Inc./Biontech SE vaccine in preventing infection by the coronavirus dropped from 88% to 47% six months after the second dose, according to data published Monday, which US health agencies considered when deciding on the need for booster shots .

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statistics, which was published in the Lancet medical journal, released in August ahead of peer review.

The analysis showed that the vaccine’s effectiveness in preventing hospitalization and death remained as high as 90% for at least six months, even against the highly contagious delta version of the coronavirus.


The researchers said the data suggests the decline is due to declining efficacy rather than more infectious forms.

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Researchers from Pfizer and Kaiser Permanente studied the electronic health records of nearly 3.4 million people who were members of Kaiser Permanente Southern California between December 2020 – when the vaccine first became available – and August 2021.

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“Our type-specific analysis clearly shows that the (Pfizer/BioNTech) vaccine is effective against all current forms of anxiety, including Delta,” said Luis Jodar, senior vice president and chief medical officer for Pfizer Vaccines.

A potential limitation of the study was the lack of data on adherence to masking guidelines and occupations in the study population, which could have affected the frequency of testing and the likelihood of exposure to the virus.

The vaccine’s effectiveness against the delta variant was 93% after the first month, which dropped to 53% after four months. Compared to other coronavirus variants, the efficacy decreased from 97% to 67%.

“To us, this suggests that Delta is not an escape variant that is completely evading vaccine protection,” said study leader Sarah Tartoff of Kaiser Permanente Southern California’s Department of Research and Evaluation.

“If that were the case, we probably wouldn’t see high protection after vaccination, because in that case the vaccination wouldn’t work. It would start low, and stay low.”

The authors caution that tests for variants are more likely to fail in vaccinated individuals, leading to an underestimation of variant-specific effectiveness in studies.

The US Food and Drug Administration has authorized the use of a booster dose of the Pfizer/BioEntech vaccine for older adults and some Americans at high risk of becoming infected. Scientists are calling for more data on whether boosters should be recommended for everyone.

(Reporting by Manas Mishra in Bengaluru; Editing by Bill Burcrot)