Pfizer Inc. plans to request U.S. emergency authorization in August for a third booster dose of its Covid-19 vaccine, based on early data showing that it can sharply increase immune protection against the coronavirus.

At the same time, however, federal health officials signaled that they would take a cautious approach to potential booster shots, and underlined that the currently available vaccines are effective at keeping people from being sickened by the coronavirus.

Pfizer has received initial data from an early human study showing that a third dose of its existing coronavirus vaccine is safe and can raise neutralizing antibody levels by fivefold to tenfold compared with the original vaccine, the company’s research head, Mikael Dolsten, said in an interview.