Pfizer Inc. and partner BioNTech SE have begun a clinical study to see if a third shot of their existing Covid-19 vaccine can stimulate stronger immune responses against new variants.

The clinical trial will examine safety and immune responses of a third dose of the vaccine in as many as 144 people who participated in the companies’ Phase 1 study. The dose is being given six to 12 months after the initial two-dose regimen, and researchers will test blood from the participants to see if it enhances levels of antibodies produced against new strains from South Africa and elsewhere.

In addition, the companies plan to begin a human study of a new vaccine that is specific to the newly circulating variant that emerged in South Africa, the companies said in a release. That trial could begin in about a month, depending on negotiations with regulators, said Philip Dormitzer, Pfizer’s chief scientific officer for viral vaccines.

Researchers around the world are monitoring whether new strains emerging from South Africa, Brazil and elsewhere could reduce the real-world efficacy of vaccines. While Pfizer says the existing vaccine is potent enough to be effective against the new strains, in the lab it produces lower levels of neutralizing antibodies against the South Africa strain — the one scientists are most worried about so far.

Vaccines from Johnson & Johnson and Novavax Inc., which were tested in human trials in South Africa at a time when the new strain was present, showed somewhat lower efficacy against it.

“While we have not seen any evidence that the circulating variants result in a loss of protection provided by our vaccine, we are taking multiple steps to act decisively and be ready in case a strain becomes resistant to the protection afforded by the vaccine,” Albert Bourla, Pfizer’s Chief Executive Officer, said in a statement.

The new Pfizer trials could produce results in several months and provide crucial data that would allow Pfizer and BioNTech to rapidly update the vaccine or roll out booster shots if needed, Dormitzer said.

Based on data so far, with the new strains “it seems unlikely that the virus would escape protection,” Dormitzer said. But he acknowledged that it is still early and Pfizer wants to be ready if a booster shot or strainspecific vaccine is suddenly needed.

“There is a possibility there some day will be a variant that really does escape protection, so we are making preparations,” Dormitzer said.