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  still staved off a majority of Covid infections.

 When vaccinated people did get infected with delta, they had similar levels of virus in their bodies as those who hadn’t their shots, backing up a recent assessment by the Centers for Disease Control and Prevention.

 The U.S. on Wednesday said Americans who got both doses of the Pfizer-BioNTech or Moderna Inc. mRNA vaccine will be able to get a third one after eight months. 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, the University of Oxford and the Office for National Statistics and published Thursday , 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.

 Four and a half months after the second dose, Pfizer’s shot will probably be about  with Astra’s at preventing infections with a high viral burden, said Koen Pouwels, an Oxford senior researcher who helped lead the study.

The results  possibility of achieving herd immunity via vaccination, said Sarah Walker, a professor of medical statistics and epidemiology at Oxford, who helped lead .

 “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.”

  The findings may also support giving a booster dose of mRNA vaccine to people who got the Astra shot, which uses a different technology, Ward said in a statement.

Ward said ,”No vaccine is completely protective against infection with the delta variant”. “The low incidence of hospitalization seen to date suggests that in this respect, at least, the vaccines are protecting individuals from developing severe Covid.”

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