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Pfizer’s COVID-19 vaccine is 90% effective at preventing hospitalization and death six months after the second dose, but its effectiveness at preventing infection dropped to 47%, a study posted in the Lancet medical journal reported.

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The data about the drug, released Monday, was used by federal health officials to decide on the need for a booster shot for the company’s vaccine.

According to the Lancet article, the vaccine’s ability to prevent hospitalization and death from COVID-19 remained at 90% for at least six months. The drug’s effectiveness against contracting the virus fell to 47% six months after the second dose.

That means that while a person may be more likely to contract the disease six months after a second shot of the vaccine, the vaccine would still be effective in protecting them from serious COVID-19 symptoms that require hospitalization and could lead to death.

The Pfizer vaccine maintained its effectiveness against severe disease, even against the highly-contagious delta variant of the coronavirus, the Lancet article noted.

“Our variant-specific analysis clearly shows that the vaccine is effective against all current variants of concern, including delta,” said Luis Jodar, senior vice president and chief medical officer at Pfizer vaccines.

The study was conducted by researchers from Pfizer and Kaiser Permanente, who looked at the electronic health records of 3.4 million people between December 2020, when the vaccine was first approved for use, and August of 2021.

The more than 3 million people who were studied were members of Kaiser Permanente Southern California.

The analysis showed that vaccine effectiveness against the delta variant was at 93% the first month after a person had received a second dose of the vaccine. The effectiveness against delta declined to 53% after four months.

Against other coronavirus variants, the vaccine’s effectiveness declined to 67% effectiveness, from an original 97%.

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

“If it was, we would probably not have seen high protection after vaccination, because vaccination would not be working in that case. It would start low and stay low.”