Harvard pandemic experts monitoring the global spread of the Omicron subvariant BA.2 say that early tracking in the U.S. suggests a milder impact than the dramatic case surge some nations have experienced.

Jacob Lemieux, an instructor in medicine at Harvard Medical School and infectious disease specialist at Massachusetts General Hospital, said that many regions of the world have seen large BA.2 outbreaks, but that the effects vary widely. In Hong Kong, for example, BA.2 led to a major surge in cases and what Lemieux described as a “complete loss of control over the epidemic.” In South Africa — which suffered a severe wave of Omicron, technically BA.1 — BA.2 almost completely replaced the COVID variant, but without cases rising substantially.

One possibility for BA.2 in the U.S. “is that we see almost nothing at all associated with the transition to BA.2 prevalence,” said Lemieux, who is the co-lead of the viral variants program for the Massachusetts Consortium on Pathogen Readiness, which held a media briefing on Monday. “That’s what happened in South Africa, where there was a severe BA.1 wave. BA.2 took over and there continued to be a low level of transmission, but there was no surge.”

Locally, BA.2’s replacement of BA.1 is already advanced, with no increase in cases or deaths. Bronwyn MacInnis, director of pathogen genomic surveillance at the Broad Institute of MIT and Harvard, said that the most recent genomic data, from March 8, showed that 45 percent of Massachusetts cases were BA.2. With a doubling time of 6.8 days, BA.2 likely makes up more than half of current infections, with no signs of a surge. New England data is about the same, she said, though she emphasized that the numbers may be affected by several factors, including at-home results that are not shared with public health experts. In any case, she said, “There’s really no indication of an increase in cases or deaths in the region that corresponds to this increase in BA.2 infections that we’re seeing.”

Nationally, BA.2 makes up about a third of cases, according to the Centers for Disease Control and Prevention, and COVID cases have continued the downward pattern that marked the end of Omicron’s BA.1 surge. On Sunday, the seven-day moving average of new cases was 27,786, the lowest level since mid-July. New hospital admissions, 2,121 on March 17, are also the lowest since July. Deaths have followed a similar trend.

After declining since late January, cases worldwide have started to rise, increasing 8 percent the week of March 7-13, according to the World Health Organization. Cases are up in Africa, Europe, and the Western Pacific, but have continued to fall in the Eastern Mediterranean, Southeast Asia, and the Americas. There have been 455 million global cases and more than 6 million deaths since the start of the pandemic.