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

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.

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.

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.

Such concerns still affect just a small portion of the world as only 13% of people are fully vaccinated globally, most of them in the developed world, the authors noted.