Pfizer and AstraZeneca’s COVID-19 vaccines are less effective against the delta variant, a large-scale UK study showed in results that may drive booster shots for fully vaccinated people.
The Pfizer and BioNTech messenger RNA vaccine lost effectiveness in the first 90 days after full vaccination, although that injection and the one by AstraZeneca were still able to prevent most coronavirus infections. When vaccinated people were infected with delta, however, they had similar levels of the virus in their bodies as those who had not received their injections, supporting a recent evaluation by the Centers for Disease Control and Prevention.
The results are likely to fuel calls for booster shots to those who are fully vaccinated, even as countries around the world still lack sufficient supplies to inject the first few doses. The United States noted Wednesday that Americans who received both doses of the mRNA vaccine from Pfizer-BioNTech or Moderna will be able to receive a third after eight months. UK authorities are still deciding to what extent the reinforcements should be administered. In Israel, which began administering third doses of Pfizer-BioNTech this month, initial results show that they have been 86 percent effective for people over the age of 60.
The UK survey, conducted by the University of Oxford and the Office for National Statistics and published in a preprint on Thursday, analyzed more than 3 million PCR tests of a random sample of people to get a detailed picture of the patterns of infection, as delta became the dominant variant this year.
“We’re looking at real-world data on the performance of two vaccines here, rather than data from clinical trials, and all the data sets show how the delta variant has mitigated the efficacy of Pfizer and AstraZeneca injections.” said Simon Clarke, associate professor of cell microbiology at the University of Reading.
About four and a half months after the second dose, Pfizer’s injection will likely be on par with Astra’s in preventing infections with a high viral load, said Koen Pouwels, the Oxford principal investigator who helped lead the study. There was no statistically significant difference in the effectiveness of the Astra shot over time.
These results cast further doubt on the possibility of achieving herd immunity through vaccination, said Sarah Walker, a professor of medical statistics and epidemiology at Oxford, who helped lead the study.
“The hope was that unvaccinated people could protect themselves by vaccinating many people,” Walker said. “The higher levels of virus that we’re seeing in these infections in vaccinated people are consistent with the fact that unvaccinated people are just going to be at higher risk, I’m afraid.”
One important piece of the puzzle still missing is data showing how much vaccines continue to protect against hospitalizations and severe cases of COVID over time, said Penny Ward, visiting professor of pharmaceutical medicine at King’s College London, that did not participate in the study. The findings may also support giving a booster dose of the mRNA vaccine to people who received the Astra vaccine, which uses a different technology, Ward said in a statement. They also exhibit the need for better COVID treatments, he said.
“No vaccine is completely protective against infection with the delta variant,” Ward said. “The low incidence of hospitalization observed to date suggests that, in this sense, at least, vaccines are protecting people from developing severe COVID.”
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