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Vaccines targeting the highly transmissible delta variant may now be needed, given its ability to infect people with fading immunity and potentially increased severity, Bloomberg reported, citing researchers leading a large English study of Covid shots.

A third wave of English cases has been driven by the delta strain among both unvaccinated people -- especially those aged 12 to 24 -- as well as some people who had received Covid shots, according to authors of a study of samples from about 98,000 people in England.

The effectiveness of vaccines at stopping infection during the study period fell to 49%, the researchers estimated, down from 64% in a month earlier. Vaccines’ protection against development of Covid symptoms was 59%, down from 83%.

“Development of vaccines against delta may be warranted,” in the light of evidence that the strain’s spike protein has mutated to a point where antibodies raised by current shots are becoming less effective, the researchers said.

The U.S., U.K. and Israel are among the most fully vaccinated countries in the world, yet all have seen surges in Covid cases and hospitalizations linked to the delta strain. Health officials in the U.S. have said they’re pleading with the hesitant to get immunized to try to control the spread of the virus, which has potential to lead to further, more dangerous mutations.