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Jeff Cercone
By Jeff Cercone January 4, 2022

Danish study shows decline in vaccine protection against omicron is restored with Pfizer booster

If Your Time is short

  • A Danish study shows a sharp drop in vaccine effectiveness over time against the omicron variant of COVID-19 in people with two doses of the Pfizer or Moderna vaccine.

  • But it also shows protection is quickly restored with a Pfizer booster dose. There was not enough data about a Moderna booster.

  • The study’s authors said that their findings prove a need to ramp up vaccination and booster doses to fight the variant.

A recent study from Denmark on the Pfizer and Moderna two-dose vaccines showed a sharp and quick decline in their effectiveness against the omicron variant of COVID-19.

But some social media posts and articles are misrepresenting what the study shows, its authors said.

An Instagram post by Teens Against Mandates shows a headline that reads "Yale study: Vaccinated people more likely to be infected than those without the jab"

This post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

The poster links in the comments to an article in the Rogue Review. That article links to a study posted on, a site that allows users to post preprints of unpublished manuscripts that have not been peer reviewed.

The headline calling it a "Yale study" is not correct. MedRxiv was founded by Cold Spring Harbor Laboratory, Yale University and BMJ, a medical research site for health care providers. The site states that "no endorsement of a manuscript’s methods, assumptions, conclusions, or scientific quality" is implied by the laboratory, Yale or BMJ.

The study was conducted by researchers on the Infectious Disease Preparedness Group at Denmark’s Statens Serum Institut, which states on its website that it’s "responsible for the Danish preparedness against infectious diseases."

The headline also misrepresents the study’s findings. Nowhere does it suggest that vaccinated people are more likely to be infected than unvaccinated people.

"Interpretation that our research is evidence of anything but a protective vaccine effect is misrepresentative," Astrid Blicher Schelde, one of the study’s authors wrote in an email to PolitiFact. 

The study measures the effectiveness of the Pfizer and Moderna COVID-19 vaccines against the omicron and delta variants in Danish residents up to five months after a primary vaccine series and uses data from Nov. 20 through Dec. 12, about the time the omicron variant was first reported by researchers in South Africa.

The authors wrote that the study showed vaccine effectiveness against both variants after two doses, though they were each more effective against delta. It also found that the vaccine effectiveness declined rapidly against both variants after several months, but that it was restored after a Pfizer booster dose.

"In light of the exponential rise in Omicron cases, these findings highlight the need for massive rollout of vaccinations and booster vaccinations," they wrote.

The Pfizer vaccine was 55.2% effective against omicron in the first 30 days after two doses, but that dropped to -76.5% after 90 days, the study found. The Moderna vaccine also showed a steep drop in effectiveness from 36.7% to -39.3% in the same time period.

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A Pfizer booster restored vaccine effectiveness to 54.6%, but there was not enough data on a Moderna booster, the researchers said.

The Rogue Review story pointed to the two negative numbers, and said these figures show that those vaccinated with Pfizer are 76.5% more likely to get omicron than the unvaccinated, and that Moderna vaccine recipients are 39.3% more likely to get omicron than the unvaccinated. That’s not what the study concluded.

In the discussion section of the study, the authors explained that the negative numbers suggest that different behavior "and/or exposure patterns in the vaccinated and unvaccinated" caused underestimation of the vaccines’ effectiveness.

"This was likely the result of omicron spreading rapidly initially through single (super-spreading) events causing many infections among young, vaccinated individuals," they wrote.

Blicher Schelde said there are several reasons why the number might be negative. First, vaccinated people may test more than unvaccinated people, she said.

Also, the data is from the first generations of omicron cases in Denmark, which occurred disproportionately among those who were traveling internationally and people in their social and professional circles who were largely vaccinated, she said. Therefore, there was likely an overrepresentation of vaccinated people.

Finally, discrepancies in risk behavior between vaccinated and unvaccinated people will lead to an underestimate of vaccine effectiveness, she wrote, pointing out that the "increasingly small cohort of unvaccinated people" in Denmark may take extra precautions because they are not vaccinated.

In Denmark, 78.4% of the population have received two doses of a vaccine and 49.5% have received a booster dose, as of Jan. 4.

"To conclude, the vaccines’ protective effect may be low against infection with omicron after four months, but it is most unlikely to be negative," Blicher Schelde wrote.

Fact checkers from Lead Stories and Reuters also disputed similar claims about the Danish study.

Our ruling

An Instagram post claimed that a Yale study showed that vaccinated people are more likely to be infected with omicron than people who are not vaccinated.

The claim misrepresents the source and findings of the study. It was done by Danish researchers at the Statens Serum Institut, not by Yale researchers. 

The study shows a sharp decline in vaccine effectiveness against omicron over time in people who have received two doses. It also shows that effectiveness is restored with a booster dose of Pfizer, although there is not enough data on a Moderna booster. The authors said the study did not conclude that vaccinated people are more likely to be infected with omicron than people who are not vaccinated.

They said their findings prove the need for more vaccinations and booster doses to combat the rapid rise of omicron.

We rate this claim False.

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Danish study shows decline in vaccine protection against omicron is restored with Pfizer booster

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