Pfizer-BioNTech’s vaccine gets slightly weaker over time, company data shows, but remains strong in preventing severe disease.
The effectiveness of the Pfizer-BioNTech vaccine declines slightly over time, according to recently released data from the companies, but remains strong in preventing serious illness. With coronavirus cases on the rise again in many states, the results may influence the Biden administration’s deliberations on administering a booster.
The vaccine had an extremely high efficacy rate of around 96% against symptomatic Covid-19 during the first two months, the study showed, but then declined by around 6% every two months after that, falling to 83.7% after six months. Against serious illnesses, its effectiveness has remained at around 97 percent. The data was posted online Wednesday and has not been published in a scientific journal.
Despite the decline, data confirms that the vaccine offers potent protection against Covid-19. Still, the study raises questions about the protection two doses will provide in the coming months. Added to these concerns is the rise of the Delta variant, which makes vaccines a little less effective against infection. The variant did not become dominant until after the study was completed. But recent studies have also shown that vaccines remain highly protective against the worst consequences of Covid-19 caused by the Delta variant.
The results come from 42,000 volunteers in six countries who participated in a clinical trial that Pfizer and BioNTech began last July. Half of the volunteers received the vaccine while the other half received a placebo. The two groups received two injections three weeks apart. The researchers compared the number of people in each group who developed symptoms of Covid-19, which were then confirmed by a PCR virus test.
When the companies announced their first batch of results, the vaccine showed 95% efficacy against symptomatic Covid-19. In other words, the risk of getting sick was reduced by 95% in the group that received the vaccine compared to the group that received the placebo.
This result – the first for any Covid-19 vaccine – brought an exhilarating dose of hope to the world in December as it weathered what had been the biggest wave of the pandemic. Since then, the Pfizer-BioNTech vaccine has made up the majority of injections Americans have received, with more than 191 million doses administered to date, according to the Centers for Disease Control.
After the first analysis, researchers from Pfizer and BioNTech continued to follow the volunteers. Research became more difficult over time, as volunteers who received the placebo could apply to receive the vaccine once it was cleared in their country.
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For the new study, the researchers followed the volunteers for six months after vaccination, until the deadline of March 13. Over this entire period, the researchers estimated the effectiveness of the vaccine at 91.5% against symptomatic Covid-19. (The study did not measure the rate of asymptomatic viral infections.)
But during this period, the efficiency gradually declined. Between one week and two months after the second dose, the efficacy was 96.2 percent. In the period between two and four months, the efficiency fell to 90.1 percent. And between four and six months, the efficiency reached 83.7%.
Each estimate had a margin of uncertainty. But over the six months of the trial, there was a marked decrease in effectiveness.
The new study follows data from Israel suggesting that Pfizer-BioNTech’s protection may decrease there. But experts fended off the rush to approve a recall there. The data has too many sources of uncertainty, they say, to make an accurate estimate of the decrease in efficiency. For example, the epidemic caused by the Delta first hit parts of the country with high vaccination rates and then affected other areas. “Such an analysis is still very uncertain,” said Doron Gazit, a physicist at Hebrew University who analyzes trends in Covid-19 for the Israeli government.
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