Pfizer and BioNTech have announced that they were developing a Covid-19 booster shot intended to target the highly transmissible Delta variant, according to US media reports.Concerns continue to rise as the Delta strain has already become the dominant variant in the US, causing increasing infections, Xinhua news agency quoted the reports as saying on Thursday.The two companies said they believe a third shot of their current two-dose vaccine has the potential to preserve the "highest levels" of protection against all currently known variants, including Delta, but they are "remaining vigilant" and developing an updated version of the vaccine, a CNBC News report said."These findings are consistent with an ongoing analysis from the companies' Phase 3 study," the companies said in a statement issued on Thursday."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 six to 12 months after full vaccination."Clinical studies could begin as early as August, subject to regulatory approvals, the companies said.Executives from Pfizer and BioNTech have said people will likely need a booster shot, or third dose, within 12 months of getting fully vaccinated because they expect vaccine-induced immunity to wane over time.