The drugmakers are seeking regulatory approval in the US to begin clinical trials on a Delta booster shot as early as August.

Pfizer and BioNTech announced they are developing an update version of their Covid-19 vaccine that will target the more transmissable Delta variant of the virus, which is responsible for the vast majority of new cases in the UK.

A third dose of the vaccine has the potential to “preserve the highest levels” of protection against all current known variants of the disease, the companies said, as it specifically targets the structure of the Delta variant.

The drugmakers are in discussions with the US medicines regulator and anticipate starting clinical trials in August, subject to their approval.

Pfizer/BioNTech said they were “remaining vigilant” by developing an updated version of their vaccine, amid concerns that the Delta strain may be resistant to existing jabs.

Data from Israel, where the mass vaccination drive took the global lead at the start of the year, shows that the vaccine’s efficacy in preventing both infection and symptomatic disease has declined six months after vaccination, the companies said.

Similar to what current data from the UK suggests, the drugmakers cited a study by Israel’s health ministry that suggested the Pfizer/BioNTech vaccine was 93 per cent effective at preventing serious illness from the Delta variant, but only 64 per cent effective at preventing infection with the variant.

A study by Israel’s health ministry this week indicated that the BioNTech/Pfizer jab was 93 per cent effective at preventing severe illness and hospitalisation from Delta, but only 64 per cent effective at protecting against infection from the variant. 

“While protection against severe disease remained high across the full 6 months, a decline in efficacy against symptomatic disease over time and the continued emergence of variants are expected,” Pfizer/BioNTech said.

“That is why we have said, and we continue to believe that it is likely, based on the totality of the data we have to date, that a third dose may be needed within 6 to 12 months after full vaccination.”