‘Highly contagious’ XBB variant becomes dominant COVID strain, as Boston-area virus wastewater levels keep rising

A new “highly contagious” COVID variant is on the rise across the region, as the Boston-area virus wastewater levels continue to predict a jump in local cases.

The XBB omicron subvariant has now become the dominant strain in New England, according to this week’s variant update from the CDC.

XBB now accounts for more than 52% of new virus cases in the region, a significant increase from 34% of new cases during the previous week.

“XBB is on the rise in the U.S. and especially in New England and New York,” said Davidson Hamer, a Boston University School of Public Health infectious diseases specialist.

“It is worrisome given the apparent capacity of this omicron subvariant to evade the immune response generated by past infection and/or vaccination,” he added.

Hamer called XBB a “unique subvariant” because it seems to have arisen through the recombination of two subvariants, rather than additional single point mutations.

“It is highly contagious but fortunately may be less likely to cause severe disease,” Hamer said.

As the variant spreads, the Boston-area COVID wastewater levels are surging. The virus wastewater data has helped predict COVID waves throughout the pandemic.

The seven-day average in the south-of-Boston region is up to 1,440 viral copies per milliliter, which is a jump from 1,331 viral copies in the previous report. The average on Thanksgiving was 721 viral copies.

The north-of-Boston region’s seven-day average is up to 1,254 viral copies per milliliter, an increase from 1,179 viral copies in the previous report. The average on Thanksgiving was 483 viral copies.

“Bad timing for this surge of XBB given holiday gatherings of family and friends,” Hamer said. “Mask use in public places — public transportation, crowded stores, restaurants or bars when not eating — would be a wise idea for the next few weeks.”