Pfizer studying effects of 3rd vaccine dose against new Covid variants

Pfizer announced on Thursday that it has begun studying a third dose of its Covid-19 vaccine, part of a strategy to guard against mutated versions of the coronavirus.

Health authorities say first-generation Covid-19 vaccines still protect against variants that are emerging in different parts of the world.

But manufacturers are starting to prepare now in case a more vaccine-resistant mutation comes along.Pfizer said it will offer a third dose to 144 volunteers, drawing from people who participated in the vaccine's early-stage US testing last year. It wants to determine if an additional booster shot given six to 12