A tweet shared tens of thousands of times claims a study found that mRNA Covid-19 vaccines increase the chances of infection with the Omicron variant. But experts said the shots do not increase the risk of infection, and the authors of the Danish study said the claim misinterpreted the data, overlooking key factors including more frequent testing and more potential exposure among the vaccinated, which would account for the higher reported infection rate in that group.
"Holy moly. This study shows that after three months the vaccine effectiveness of Pfizer & Moderna against Omicron is actually negative. Pfizer customers are 76.5% more likely and Moderna customers are 39.3% more likely to be infected than unvaxxed people," says a December 23, 2021 tweet from Canadian commentator Ezra Levant.
The tweet includes a screenshot of data of the Danish study showing that after 91-150 days, Pfizer and Moderna Covid-19 vaccine effectiveness against the Omicron variant falls below zero, meaning more vaccinated than unvaccinated participants were infected.
Levant's claim was retweeted more than 16,000 times, and was also shared as a screenshot on other social media platforms and in languages including Spanish, Portuguese, Lithuanian and Japanese.
The Omicron variant was first detected in late November 2021 by South African scientists, and appeared in Canada in the last days of that month. Canada's National Advisory Committee on Immunization recommends vaccines to all eligible Canadians and booster shots six months after initial vaccination, particularly for those over the age of 50 or anyone older than 18 who did not receive a mRNA jab initially.
The study referred to in the tweet was submitted by Danish researchers on December 22, 2021 to medRxiv, an online platform for preprints, or preliminary reports on studies.
MedRxiv warns that the studies on its site are not peer reviewed and "should not be relied on to guide clinical practice or health-related behavior."
The study Levant cites aimed to determine the effectiveness of the Pfizer and Moderna shots against the Omicron variant up to five months after full vaccination. It found "evidence of protection against infection with the Omicron variant," but that effectiveness is "significantly lower than against Delta infection and declines rapidly over just a few months."
The authors added that effectiveness "is re-established upon revaccination" and say their "findings highlight the need for massive rollout of vaccinations and booster vaccinations."
Palle Valentiner-Branth, one of the authors of the study and head of the vaccine preventable diseases group at Denmark's Statens Serum Institut, told AFP on December 28, 2021: "Interpretation that our research is evidence of anything but a protective vaccine effect is misrepresentative."
Valentiner-Branth offered three explanations for why the vaccine effectiveness estimate in his study could be negative.
The first was that in many places including Denmark, vaccinated individuals are tested more frequently than unvaccinated individuals, causing the incidence rate to be higher.
The second was that Denmark's quick identification of Omicron cases allowed the Nordic country to detect the first infected individuals, who had traveled abroad and were largely vaccinated.
Thirdly, Valentiner-Branth said that the study's estimation of vaccine effectiveness is based on the assumption that vaccinated and unvaccinated people are taking similar precautions against Covid-19. In reality, he said people in Denmark who are unvaccinated may take further precautions and engage in fewer risky activities than those who have received the shots, leading vaccine effectiveness to be underestimated.
In Denmark, proof of vaccination is required for bars, cafes, theatres and other large social gatherings, meaning that these potential points of exposure are closed to those who have not been vaccinated.
Data from the Danish Health Authority showed that as of January 5, 2022, more than 78 percent had received at least two vaccine doses and more than 50 percent a third or booster dose.
Dan Milner, chief medical officer at the American Society for Clinical Pathology, gave similar explanations for the discrepancy in infection rates in the Danish study.
"This estimate is likely highly confounded by behavior and restrictions on the the groups," he explained on January 4, 2022. This difference in sample groups means that "the actual measure of effectiveness is not correctly estimated in this data."
"The take home point of this is that, after three months, you need a booster," Milner concluded, adding that the study also shows how much protection the vaccines provide for the first three months.
Alexandre Naime, head of the Infectious Diseases Department of the Sao Paolo State University in Brazil, dismissed claims that vaccines could increase chances of infection.
"There is no vaccine that induces a greater risk (of infection). This is untrue, it is a biological fallacy. Vaccines may not be effective... but it does not mean that they increased the risk," he told AFP.
Richard Kennedy, co-director of vaccine research at Mayo Clinic, also rejected the idea of vaccines increasing likelihood of infection. "There is no rational biologic reason for the vaccine to have 55.2 percent (Pfizer) or 36.7 percent (Moderna) effectiveness at day 30 and then 60 days later increase your susceptibility to disease," he said on January 4, referring to the earlier effectiveness rates in the study.
"It is more reasonable to assume that the study has some flaw in its design," he added, pointing out that the authors of the study did admit the shortcomings.
"Existing data pretty clearly shows that these vaccines are very good at preventing serious illness/hospitalization/death if you are exposed and get infected. They are less effective at preventing infection if you are exposed," he explained.
AFP Fact Check has debunked more than 1,200 inaccurate claims related to Covid-19 here.