NEW YORK (NYTIMES) - A year ago, as millions of people in China were starting life under lockdown and the United States was recording its first coronavirus cases, much was unknown about the contagion that was already leaping across borders and oceans.

Experts have since made great strides in understanding the virus and how to protect the world against it, with several vaccines developed and authorised at record speed. But as the world approaches 100 million cases - including 25 million in the United States as of Saturday (Jan 23) - a new set of questions has been raised about variants of the virus that could slow or even reverse the progress that has been made toward ending the pandemic.

One of those questions is how effective the current vaccines will be against these altered versions of the virus, which initially appeared in Britain, South Africa, Brazil and the United States. Some appear to be more contagious than the original version, and all are little understood.

The variant first detected in Britain could become the dominant source of infection in the United States by March, the Centres for Disease Control and Prevention recently warned, and would very likely lead to further surges in cases and deaths.

That variant has been found in at least 22 states. On Saturday night, the University of Michigan said that it had suspended all athletic activities after several cases of the variant were found among people linked to the athletic department.

On Friday, Prime Minister Boris Johnson of Britain ignited concern when he said at a news conference that the variant first found in his country might also be associated with a slightly higher chance of death, even as he acknowledged it was too soon to be sure. His own scientific advisers urged restraint in interpreting preliminary evidence.

Dr Francis Collins, director of the US National Institutes of Health, said on MSNBC on Saturday that the variant could be increasing deaths in Britain simply because it is more contagious, spreading so rapidly that it is overwhelming the British health system.

Dr Collins pointed to a study that indicates the Pfizer and Moderna vaccines hold up against the British-affiliated variant, but he said the variant first found in South Africa, called B.1.351, is "more of a concern."

That variant "seems to have a somewhat more significant effect on the vaccine response, although it still looks like it would be protected," he said.

"Fortunately we have not yet seen that variant in the US, but it would not surprise me if it appears."

Studies published last week show that the variant identified in South Africa is less susceptible to the antibodies created by natural infection and by the vaccines made by Pfizer-BioNTech and Moderna. Those vaccines can be altered in a matter of weeks, but experts warn that it would be difficult to update them constantly.

Vaccine trials being conducted in South Africa by Novavax and Johnson & Johnson will provide more real-world data on how the vaccines perform against the new variant there. Those results are expected within the next few weeks.