By Manas Mishra

(Reuters) – The effectiveness of Pfizer (NYSE:) Inc/BioNTechSE vaccine for preventing coronavirus infection dropped to 47% from 88% after six months, according to Monday’s data. These were the same numbers that U.S. healthcare agencies used to decide on whether booster shots are necessary.

Before peer review, the data had already been released earlier in August.

It was found that even though the vaccine is highly contagious, the effectiveness of the vaccine in preventing death and hospitalization was at least 90%.

Researchers concluded that this drop in efficacy is not due to more contagious varieties, but rather waning efficacy.

Kaiser Permanente researchers analyzed electronic health records from approximately 3.4 Million Kaiser Permanente Southern California residents between December 2020 (when the vaccine was first made available) and August 2021.

Luis Jodar (senior vice president, chief medical officer, Pfizer vaccines) stated that “our variant-specific analysis clearly demonstrates that the [Pfizer/BioNTech] vaccine is effective against any current variants of concern including Delta.”

One limitation to this study was the lack of information on the adherence of masking guidelines and the occupations of the population. This could have had an impact on the frequency of testing or the likelihood of being exposed to the virus.

After the first month of vaccination, 93% were effective against Delta variant. However, after four months, the effectiveness dropped to 53%. The efficacy against other coronavirus variants dropped to 67%, from 97%.

“To us, that suggests Delta is not an escape variant that is completely evading vaccine protection,” said study leader Sara Tartof with Kaiser Permanente Southern California’s Department of Research & Evaluation.

If it were, there would likely not been high vaccine protection. Vaccination would not work in this case. It would be low at the beginning and then stay low.

The risk of testing for variants in vaccine-vaccinated subjects is higher, so it could be possible to underestimate the effectiveness of the study.

A booster dose of Pfizer/BioNTech vaccine has been approved by the U.S. Food and Drug Administration for those who are older and at risk. Researchers have asked for additional data to determine if boosters should ever be used for anyone.