Clinical trial data on two Covid-19 vaccines show that a coronavirus variant first identified in SA is lessening their ability to protect against the illness, underscoring the need to vaccinate vast numbers of people as quickly as possible, scientists said.

The vaccines from Novavax Inc and Johnson & Johnson were welcomed as important future weapons in curbing deaths and hospitalizations in a pandemic that has infected more than 101 million people and claimed over 2 million lives worldwide.

But they were significantly less effective at preventing Covid-19 in trial participants in SA, where the potent new variant is widespread, compared with countries in which this mutation is still rare, according to preliminary data released by the companies.

"Clearly, the mutants have a diminishing effect on the efficacy of the vaccines," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, said in a briefing. "We can see that we are going to be challenged."

Novavax reported midstage trial results on Thursday that showed its vaccine was 50% effective overall at preventing Covid-19 among people in SA.

That compared with late-stage results from the United Kingdom, in which the vaccine was up to 89.3% effective at preventingCovid-19.

On Friday, J&J said a single shot of its coronavirus vaccine was 66% effective overall in a massive trial across three continents.

But there were wide differences by region. In the United States, where the South African variant was first reported this week, efficacy reached 72%, compared with just 57% in SA, where the new variant, known as B 1.351, made up 95% of the Covid-19 cases reported in the trial.

Another highly transmissible variant first discovered in the UK and now in more than half of U.S. states has been less able to evade vaccine efficacy than its SA counterpart.

The new findings, however, raise questions about how highly-effective vaccines from Pfizer Inc with partner BioNTech, and Moderna Inc will fare against new variants. The two vaccines showed an efficacy of around 95% in trials conducted primarily in the United States before the new virus versions were identified in other countries.

"It's a different pandemic now," said Dr. Dan Barouch, a researcher at Harvard University Medical School's Beth Israel Deaconess Medical Center in Boston who helped develop the J&J vaccine.

Barouch said there are now a wide variety of new variants circulating, including in Brazil, SA and even the United States, that are substantially resistant to vaccine-induced antibodies.

Pfizer Chief Executive Albert Bourla said there was "a high possibility" that emerging variants may eventually render the company's vaccine ineffective.

"This is not the case yet ... but I think it's a very high likelihood that one day that will happen," Bourla said at the World Economic Forum. The drugmaker is considering whether its vaccine needs to be altered to defend against the SA variant.

'STOP HOSPITALS FROM GOING INTO CRISIS'

Experts said that all four vaccines still have great value in their ability to reduce severe Covid-19.

"The end game is to stop death, to stop hospitals from going into crisis - and all of these vaccines, even including against the variant found in SA, seem to do that substantially," said Dr. Amesh Adalja, in infectious disease expert at the Johns Hopkins Center for Health Security

For example, J&J's vaccine was 89% effective at preventing severe disease in SA.

J&J Chief Scientific Officer Dr. Paul Stoffels said he suspects a type of immune system reaction called a T-cell response is playing a protective role and may be helping to prevent severe disease.

"We knew that to a certain extent, but it's also better and very confirming that we can see that now in the clinic," Stoffels said in an interview.

Nevertheless, Fauci said the decreased efficacy rates underscore the need to follow variants closely, and to accelerate vaccination efforts before new, and even more dangerous, mutations arise.

"The best way to prevent further evolution of a virus is to prevent it from replicating," Fauci said, "and you do that by vaccinating people as quickly as you possibly can."