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researcher of Statens Serum Institute in Denmark published a pre-print study suggesting that the second dose of Pfizer and Moderna’s vaccines may increase the chance of infection after 90 to 150 days. The same study suggests that a third Pfizer shot restores efficacy indefinitely.

The large number of so-called breakthrough infections after the second vaccine dose indicates that the effectiveness of vaccines in clinical trials may have been misjudged.

The Danish study

The study, published in Medrxiv preprint in December to await peer review, examines protection from infection with the omicron or delta variants of SARS-CoV-2 five months after a dose of Pfizer’s vaccines or Moderna.

The lead author, Dr. Christian Holm Hansen, Assistant Professor of Medical Statistics and Epidemiology at the London School of Hygiene and Tropical Medicine, and his co-authors are from the Department of Epidemiology and Infectious Disease Prevention and the Department of Infectious Disease Preparedness at Statens Serum Institut in Denmark. The study received no external funding.

The authors extracted data on positive polymerase chain reaction (PCR) test results and vaccination dates from the Danish registries between 20 November and 12 December 2021. The authors compared the rate of COVID-19 infections, both omicron and delta variants, in unvaccinated individuals to those who received two doses of either Pfizer’s or Moderna’s vaccines.

As of December 12, there were 5,767 omicron cases in Denmark. The researchers found that the vaccine’s effectiveness against omicron was 55% (Pfizer) and 37% (Moderna), respectively. The effectiveness of the vaccine declined rapidly over a period of five months.

Initial vaccine efficacy against Delta was 87% for Pfizer and 88% for Moderna, but decreased over the five months to 54% and 65%, respectively.

The study found that after a Pfizer booster dose, VE rose again to 55% against Omicron and 81% against the Delta variants. The authors noted that they lacked similar data about Moderna.

Controversially, the authors found negative vaccine efficacy—a higher likelihood of infection—against omicron between 90 and 150 days post-vaccination. The authors calculated a vaccine efficacy of -76.5% for Pfizer and -39% for Moderna over this period.

“The negative estimates over the last period indicate different behavioral and/or exposure patterns in the vaccinated and unvaccinated cohorts, leading to an underestimation of VE,” the authors said, implying that unvaccinated individuals may behave more cautiously and engage in a were at lower risk, or that more vaccinated people were getting tested for COVID-19 or engaging in behaviors that put them at higher risk.

Data from the UK’s Office for National Statistics (ONS) shows that fully vaccinated individuals and those who have received a booster dose are over four times more likely to test positive for Omicron than unvaccinated individuals. Although these data are only preliminary, they support the claim that Omicron is evading vaccines. The data say nothing about the severity of omicron cases or whether the vaccines protect against severe infections and severe symptoms.

Reuters reviewed the ONS data and claimed that it does not show that vaccinated people are more likely to become infected than unvaccinated, only the likelihood of an infected person becoming infected with omicron. “That means it doesn’t tell us that the vaccinations make the situation worse overall, just that they increase the likelihood that a vaccinated person will be infected with omicron.”

Another pre-print paper titled “The BNT162b2 mRNA vaccine against SARS-CoV-2 reprograms both adaptive and innate immune responses” is coming concluded that the Pfizer vaccine causes dysregulation of the immune system. The paper concludes that the Pfizer vaccine reprograms innate immune responses “which could contribute to a decreased innate immune response to the virus.”

Do vaccines make us more vulnerable?

Since the Danish study is still in the preliminary edition and is still awaiting peer review, it is not conclusive. The study also examined a small cohort of people in Denmark using publicly available data and is not large in scope.

The negative vaccine efficacy could be due to different behaviors or exposure patterns in vaccinated and unvaccinated individuals, leading to “an underestimate of vaccine efficacy,” the authors said, or because omicron is highly transmissible and spreads quickly. dr Astrid Blicher Schelde of Statens Serum Institut, one of the study’s authors, emailed PolitiFact citing other reasons, including higher testing rates in vaccinated than unvaccinated people. Schelde also said the dates of the first omicron cases could be skewed by the more cautious behavior of unvaccinated foreign travelers.

Schelde said that vaccinated people are not necessarily more likely to be infected with omicron because more vaccinations and booster doses are needed to combat omicron and other COVID-19 variants.

The British health authority also reported that a double dose of the vaccines from AstraZeneca, Pfizer and Moderna brought no or negative results after five months. A booster dose increased the vaccine’s effectiveness to 40-60%. The report warns that these results should be interpreted with caution given the small number of vaccinations and possible bias due to populations with high omicron exposure.

A Norwegian report published by Euro Surveillance in December 2021 also reported negative efficacy of vaccines against omicron. The Norwegian case study focused on a “super spreader” event where guests at a Christmas party were infected with omicron. The authors hypothesized that this high rate of transmission between fully vaccinated individuals was due to the venue, crowding, and loud conversations.

Should we be worried?

Although the authors of the Danish study explained the reason for the vaccine’s negative effectiveness in their study, other studies found similar results, with the effectiveness dependent on booster shots.


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