Over the month of December, the percentage of new COVID-19 infections in the country caused by XBB.1.5 rose from an estimated 4 per cent to 41 per cent.
"That's a stunning increase," Dr Ashish Jha, the White House COVID-19 response coordinator, wrote in a Twitter thread.
Officials at the World Health Organisation shared similar thoughts Wednesday.
"We are concerned about its growth advantage," Maria Van Kerkhove, an epidemiologist who is the WHO's technical lead on COVID-19, said.
Van Kerkhove noted that XBB.1.5, which was first detected in the US, has spread to at least 29 countries and "is the most transmissible form of Omicron to date".
"We do expect further waves of infection around the world, but that doesn't have to translate into further waves of death because our countermeasures continue to work," she said.
Jha noted that effective tools to avoid severe COVID-19 infections include rapid tests, high-quality masks, ventilation and filtration of indoor air, oral antiviral pills and updated vaccines.
"We will soon have more data on how well vaccines neutralize XBB.1.5," Jha said, suggesting that research to determine vaccine effectiveness against the new sublineage is underway.
Jha said XBB.1.5 is probably more able to slip past our immune defences and may be more contagious.
But he said it's still not clear whether it causes more severe disease, something that was also stressed by Van Kerkhove.
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She said the WHO is working on a risk assessment for this sublineage and hopes to publish it within the next few days.
The group's technical advisers are looking at both real-world data on hospitalisations and lab studies to assess severity.
Jha said that although he is concerned about XBB.1.5, he doesn't think it represents a huge setback in the fight against COVID-19.
"And if we all do our part," he wrote, "We can reduce the impact it will have on our lives."