Authors: Naomi Kresge
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Covid-19 vaccines are less effective against the delta variant, according to results in the U.K. from one of the largest real-world studies into the efficacy of the shots. Pfizer Inc. and BioNTech SE’s messenger RNA vaccine lost effectiveness in the first 90 days after full vaccination, though that shot and the one made by AstraZeneca Plc still staved off a majority of Covid infections. When vaccinated people did get infected with delta, they were shown to have similar levels of virus in their bodies as those who hadn’t had shots. This suggests that vaccinating large portions of a population might not protect those who don’t get inoculated, casting doubt on the idea of achieving herd immunity.   There isn’t yet data to show how much the vaccines continue to protect against hospitalization and severe cases of Covid over time.   These latest results are likely to fuel calls to give booster shots to the fully vaccinated even as countries around the world still lack enough supply for first immunizations. The U.S. plans to start offering booster shots on Sept. 20 to all vaccinated U.S. adults. U.K. authorities are still deciding how broadly boosters should be given. In Israel, which started giving third doses of Pfizer-BioNTech this month, initial results show they have been 86% effective for people over the age of 60. 

The U.K. survey, run by the University of Oxford and the Office for National Statistics and published Thursday in a preprint, analyzed more than 3 million PCR tests from a random sample of people for a detailed picture of infection patterns as delta became the dominant variant this year. “We’re seeing here the real-world data of how two vaccines are performing, rather than clinical trial data, and the data sets all show how the delta variant has blunted the effectiveness of both the Pfizer and AstraZeneca jabs,” said Simon Clarke, an associate professor in cellular microbiology at the University of Reading. 

By roughly four and a half months after the second dose, Pfizer’s shot will probably be about on par with Astra’s at preventing infections with a high viral burden, said Koen Pouwels, an Oxford senior researcher who helped lead the study. There wasn’t a statistically significant difference in the Astra shot’s effectiveness over time.  The study results may bring further unease as infections start

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