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Claim vaccines increase susceptibility to omicron unfounded

December 30, 2021 GMT

CLAIM: A new study found that, after 90 days, the COVID-19 mRNA vaccines increase the chances that someone who is vaccinated will be infected with the omicron variant.

AP’S ASSESSMENT: Misleading. The observational study found that vaccines protect against infection from the variant but wanes over time, and protection is elevated with a booster. While the study results showed “negative” effectiveness 90 days after someone has been fully vaccinated with two doses, a study author and experts say that is likely the result of bias in the data, not an indication that vaccines are harming recipients.

THE FACTS: A new study, which assesses the effectiveness of COVID-19 vaccines against infection with the omicron and delta variants in Denmark, is being cited online to erroneously suggest that it found the vaccines hurt more than they help against omicron.

Multiple conservative blogs ran headlines purporting that the study showed that, after 90 days, the vaccines “increase probability” of being infected with omicron, or make infection “more likely.”

One widely shared tweet claimed that the study “indicates that the mRNA vaccines protect for a few weeks only but then SIGNIFICANTLY AUGMENT Omicron infectivity.”

But one of the authors of the study says its findings are being misrepresented.

“Interpretation that our research is evidence of anything but a protective vaccine effect is misrepresentative,” co-author Christian Holm Hansen, a medical statistician and epidemiologist with the Statens Serum Institut in Denmark and the London School of Hygiene and Tropical Medicine, told The Associated Press in an email.

The study being cited is a preprint — meaning it has not yet been peer-reviewed — that examined data during the first 20 days of omicron being detected in Denmark. It is observational, not a randomized controlled trial, and calculated vaccine effectiveness by comparing infection rates among the vaccinated population versus the unvaccinated.

Hansen said the study shows the Pfizer and Moderna COVID-19 vaccines provide protection against infection with the omicron variant, but it’s lower than the protection afforded against infection with the delta variant — and declines in a matter of months. A booster shot of Pfizer’s vaccine re-establishes that protection, he said.

Importantly, the study did not look at the vaccines’ protection in terms of COVID-19 outcomes. Early research suggests that the Pfizer and Moderna vaccines continue to be effective at preventing serious illness, hospitalization and death against omicron.

The specific point being cited online to suggest the vaccines are harmful concerns the study’s finding that, after 90 days, the vaccine effectiveness against infection with omicron was calculated to be below zero — which would suggest in theory that it increases chances of contracting the virus.

However, the study explains that “the negative estimates in the final period arguably suggest different behaviour and/or exposure patterns in the vaccinated and unvaccinated cohorts causing underestimation” of vaccine effectiveness.

Hansen said there are a “number of reasons” why the estimate might be negative, noting that biases are “quite common” in calculating effectiveness with observational studies. In other words, other factors are causing the vaccine to appear less effective.

The underestimates could be the product of vaccinated individuals being tested more frequently, therefore resulting in a higher incidence rate, he said. Also, behavioral differences, such as vaccinated individuals engaging in more activities that could lead to exposure, could be at play, he said.

Natalie Dean, an assistant professor of biostatistics at Emory University whose research includes methods for evaluating vaccine efficacy, said in an email that while the vaccines are less effective against infection with omicron, there is no real-world evidence that scientists are aware of that would explain the vaccines’ effectiveness actually being below zero.

Instead, she agreed that the negative results in the study were “highly likely to be due to bias in the data.”

As an example, she said, high vaccination rates mean that the “pool of people who are unvaccinated is comparatively small and may be fundamentally different in terms of risk or testing behavior.” More than 78% of people in Demark are fully vaccinated against COVID-19.

Jeffrey Morris, director of the division of biostatistics at the University of Pennsylvania’s Perelman School of Medicine, shared similar observations. He said in an email that the results were “more likely to be an artifact of some selection bias effect than any inherently higher risk.”


This is part of AP’s effort to address widely shared misinformation, including work with outside companies and organizations to add factual context to misleading content that is circulating online. Learn more about fact-checking at AP.