Pfizer Inc. PFE 0.21% and BioNTech SE BNTX -1.61% have begun a study testing in people whether the companies’ Covid-19 shot can provide protection against emerging strains of the coronavirus. The companies said Thursday they have started the small study to see whether a third dose of their authorized Covid-19 vaccine would increase its effectiveness against new variants, such as the strain first identified in South Africa.

The approach differs from that of Moderna Inc., which said Wednesday it had made a new vaccine targeting the strain found in South Africa and shipped doses to U.S. government researchers for human testing. Pfizer and BioNTech said they are also in discussions with U.S., European and other health regulators about studying a tweaked version of their vaccine that researchers designed to protect against the variant found in South Africa.

“We are taking multiple steps to act decisively and be ready in case a strain becomes resistant to the protection afforded by the vaccine,” Pfizer Chief Executive Albert Bourla said in a statement.

Both the Pfizer-BioNTech and Moderna Covid-19 vaccines appeared to generate a weaker immune response to the strain identified in South Africa, as did other shots in the advanced stages of development.

Companies including Johnson & Johnson are also working on new vaccines. The U.S. Food and Drug Administration said this week it planned to evaluate any vaccine booster shots against variants quickly and wouldn’t require large efficacy trials for authorization.

The two-dose Pfizer-BioNTech vaccine was the first authorized for use in the U.S., in December, after a large global study in 44,000 people found the shot was safe and 95% effective at protecting against symptomatic Covid-19.