A highly mutated descendant of the Omicron variant of the SARS-CoV-2 virus XBB has been described as the worst form of the virus so far. It’s more contagious than any previous variant or sub variant. It also evades the antibodies from monoclonal therapies, potentially rendering a whole category of drugs ineffective as COVID treatments.

“It is likely the most immune-evasive and poses problems for current monoclonal antibody-based treatments and prevention strategy.”

According to Amesh Adalja, a public-health expert at the Johns Hopkins Center for Health Security, the new sub variant of the novel-coronavirus was detected earlier this week, in Singapore where new rates of infection has doubled in a day, from 4,700 on Monday to 11,700 on Tuesday—and XBB is almost certainly why. The XBB cases has also been found in Hong Kong.

However, the new “bivalent” vaccine boosters from Pfizer and Moderna seem to work just fine against XBB, even though the original vaccines are less effective. They won’t prevent all infections and reinfections, but they should significantly reduce the chance of severe infection potentially leading to hospitalization or death. “Even with immune-evasive variants, vaccine protection against what matters most—severe disease—remains intact,” Adalja said.