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 minus 121 percent effectiveness on day 84 and minus 85 percent effectiveness on day 98. A second Pfizer dose held up better, but still dipped below 50 percent at day 98, researchers concluded.
Negative effectiveness means that a vaccinated person is more likely to experience a condition than an unvaccinated person.
Pfizer and AstraZeneca didn't respond to requests for comment.
Researchers reached the estimates by analyzing health records from roughly 12.9 million people in England, Scotland, Wales, and Northern Ireland. The study period was from Dec. 8, 2020, to June 30, 2021—one of the reasons that boosters, which weren't available until later in 2021, weren't 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 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.