The effectiveness of the Pfizer and AstraZeneca COVID-19 vaccines turns negative against severe COVID-19 months after administration, according to a new study.
A single dose of the Pfizer vaccine was pegged at negative 121 percent effectiveness on day 84 and negative 85 at day 98. A second Pfizer dose held up better, but still dipped below 50 percent at day 98, researchers concluded.
A single dose of the AstraZeneca vaccine turned negative at day 70 and a second dose turned negative at day 84, according to the paper, which was published in the International Journal of Epidemiology.
Negative effectiveness means a vaccinated person is more likely to experience a condition than an unvaccinated person.
The negative effectiveness estimates in the study mean vaccinated people were more likely to experience severe COVID-19 than unvaccinated people.
Pfizer and AstraZeneca did not respond to requests for comment.
Researchers reached the estimates by analyzing health records from some 12.9 million people in England, Scotland, Wales, and Northern Ireland. The study period was Dec. 8, 2020 to June 30, 2021—one of the reasons boosters, which weren’t available until later in 2021, were not included.
Steven Kerr, a senior research fellow at the University of Edinburgh’s Usher Institute, and the other researchers theorized that the negative estimates stemmed from behavioral differences between the vaccinated and unvaccinated populations.
“We believe that the most likely explanation … is that vaccination caused recipients to believe they were protected, leading them to change their behaviour in ways that increase their chance of contracting the infection,” they wrote. “These changes in behaviours should initially have been outweighed by the protection offered by the immune response stimulated by the vaccine, but as time progressed the protection is likely to have diminished such that the impact of behavioural changes may have become dominant.”
The researchers provided no evidence for the theory.
The group also said that it’s possible the protection one enjoys after recovering from COVID-19 may have played a role, pointing to one of many papers that has concluded that natural immunity is superior to vaccination.
Part of Growing Trend
The study is the latest to estimate negative effectiveness, though the others have examined effectiveness against infection.
Researchers with Moderna, for instance, said in a recent paper that the effectiveness against infection turned negative after several months against BA.2.12.1, BA.2, BA.4, and BA.5, which are all subvariants of the Omicron virus strain.
Another paper, which analyzed effectiveness against infection among children aged 5 to 11, estimated that Pfizer’s vaccine turned negative after 18 or 20 weeks, depending on whether the children had also been infected with COVID-19.
Researchers in Qatar, examining the effectiveness of the Pfizer and Moderna vaccines, estimated in June that the effectiveness turned negative over time.
And another paper, published in May by U.S. scientists, determined that the effectiveness of Pfizer’s vaccine turned negative for 12- to 15-year-olds after five months.
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