Pfizer and BioNTech announced Thursday they are studying adding a third dose to their vaccine regime and testing a new version targeting the COVID-19 strain from South Africa.

As countries globally rush to inoculate people, concerns have risen that more transmissible variants such as the one first detected in South Africa or another in Britain are more resistant to existing vaccines.

In one study, the US and German pharmaceutical firms said they would look at what happens when people are given a third dose of their two-shot vaccine, six to 12 months after the booster.

They said in a statement they are also talking to regulators about testing a modified version of their original vaccine to address the South African variant known as B.1.351.

"We are taking multiple steps to act decisively and be ready in case a strain becomes resistant to the protection afforded by the vaccine," Dr. Albert Bourla, Pfizer's CEO, said in a statement.

The variant from South African is considered among the more dangerous of current mutations because it evades some of the blocking action of antibodies that target the older coronavirus strain.

That means people who were infected with the classic strain are more susceptible to reinfection, and research has also shown the variant has partly reduced the protection of the current generation of vaccines.

Moderna, the other company whose vaccine has been approved for emergency use in America, said Wednesday that doses of its new COVID-19 vaccine candidate aimed at the coronavirus strain from South Africa had been shipped to the US National Institutes of Health for testing.