White House health officials are cautioning against a rush to panic over the XBB.1.5 omicron subvariant, which has rapidly grown to account for 40 percent of U.S. COVID-19 cases, with officials stating it is not yet known if this version is more dangerous.

As of last week, the XBB.1.5 omicron subvariant is responsible for at least 40 percent of COVID-19 cases in the U.S. As a recombinant virus, the subvariant carries genetic data from two previous strains that descended from the BA.2 subvariant. Roughly 75 percent of cases in the Northeast are now attributed to this strain.

White House COVID-19 response coordinator Ashish Jha posted a lengthy Twitter thread on Wednesday addressing concerns over the sudden rise of XBB.1.5. He acknowledged that the subvariant going from 4 percent of cases to 40 percent in a matter of weeks was a “stunning increase.”

Jha said XBB.1.5 is likely more immune-evasive than other omicron subvariants and could likely be more contagious as well but said it is not yet known if it is more dangerous than previous mutations.

“If you had an infection before July or your last vaccine was before bivalent update in September, your protection against an XBB.1.5 infection is probably not that great,” Jha said.

COVID-19 antivirals such as Paxlovid and Molnupiravir, however, should still work in treating infections, Jha said. Paxlovid, a combination of two antiviral medications, functions by blocking an enzyme the SARS-CoV-2 virus needs to replicate.

Robert Califf, commissioner of the Food and Drug Administration (FDA), posted his own thread on XBB.1.5 in response to what he characterized as “misinformation/disinformation” in an editorial published by The Wall Street Journal earlier this week.

The opinion piece suggested that repeated vaccination could be making people more susceptible to infection due to XBB subvariants and fueling viral mutations.

“There is no clear evidence that repeated vaccination with COVID-19 vaccines makes people more susceptible to the XBB.1 or XBB.1.5 variants,” Califf tweeted in response to the op-ed. He further said that a Cleveland Clinic study cited in the article was “provocative,” but noted that it ultimately concluded that bivalent vaccines offered protection against infection.

“It is highly likely that the current bivalent vaccines provide some protection against XBB, especially in the prevention of serious illness and death,” said the FDA commissioner.

“Immune evasion by COVID-19 would happen even if no vaccine existed because of the high level of replication of SARS-CoV-2 and because many of the novel variants have been traced to continuous replication of the virus in immune compromised individuals,” added Califf.

Outside the U.S., health officials have noted XBB.1.5’s apparent enhanced ability to evade immune protection.

XBB.1.5 has been detected in 29 countries so far, according to Maria Van Kerkhove, technical lead of the COVID-19 response at the World Health Organization. In a briefing, Van Kerkhove said XBB.1.5 is “the most transmissible subvariant that has been detected yet.”

“The reason for this are the mutations that are within this recombinant, this subvariant of omicron, allowing this virus to adhere to the cell and replicate easily,” Van Kerkhove said. “We are concerned about its growth advantage, in particular in some countries in Europe and the Northeast part of the United States, where XBB.1.5 has rapidly replaced other circulating sub-variants.”

—Updated at 5:46 p.m.