Lancet study finds Pfizer vaccine effectiveness wanes over time

But it remains effective at keeping people out of hospital

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The effectiveness of the Pfizer vaccine in preventing infection from COVID-19 wanes after six months, according to a study published Tuesday in The Lancet.


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But the study also shows full vaccination prevents people from getting so sick they have to be admitted to a hospital.

“Our results show high effectiveness of (Pfizer-BioNTech) against hospital admissions up until 6 months after being fully vaccinated,” the authors wrote. “Effectiveness against SARS-CoV-2 infections waned during the 6 months of this study.”

Doctors tracked 3.5 million people vaccinated with Pfizer-BioNTech. They found, after a second shot, the vaccine was 73% effective in preventing infection.

That fell to 47% after five months.

The vaccine was 93% effective at preventing hospital admissions and remained at that level.

“Is there waning over time? The answer is probably yes. There probably is some degree of vaccine effectiveness that wanes with time,” said Dr. Isaac Bogoch, of the University Health Network. “This study probably over-estimates the degree of how much the vaccine wanes.”


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When it comes to tackling the dominant Delta variant in Canada, the study found Pfizer was 93% effective at first.

After four months, that fell to 53%.

The medical community has been discussing whether booster shots are necessary.

The Lancet study touched on that, saying: “Our findings underscore the importance of monitoring vaccine effectiveness over time and suggest that booster doses might eventually be needed to restore the high levels of protection observed early in the vaccination program.”

Bogoch said Canada does not need a broad booster campaign right now.

“There might be a point where we all need a third dose, but that time is not now,” he said. “It’s currently only select populations that would benefit from a third dose.”


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He added that with Ontario 82% fully vaccinated and 86% with at least a single dose, the province is in a much better position than some to weather the fourth wave.

Some countries have already started administering booster shots.

The study’s authors admitted their survey has limitations because it did not take into account post-vaccination behaviours such as whether people continued to wear masks and maintained social distancing when around others.

  1. In this file photo Brandon Rivera, a Los Angeles County emergency medical technician, gives a second does of Pfizer-BioNTech Covid-19 vaccine to Aaron Delgado, 16, at a pop up vaccine clinic in the Arleta neighborhood of Los Angeles, California, August 23, 2021.

    Pfizer submits trial data on vaccine use in children aged 5 to 11 to Health Canada

  2. Vials with Pfizer-BioNTech and Moderna coronavirus disease (COVID-19) vaccine labels are seen in this illustration picture taken March 19, 2021.

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  3. Vials with Pfizer-BioNTech and Moderna coronavirus disease (COVID-19) vaccine labels are seen in this illustration picture taken March 19, 2021.

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“This is what’s called an observational study,” Bogoch said. “It is not a randomized control trial.”


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