Public-health experts are sounding the alarm about a new Omicron variant dubbed XBB. It isn't clear that XBB is any more lethal than other variants, but its mutations enable it to evade antibodies from prior infection and vaccines. Growing evidence also suggests that repeated vaccinations may make people more susceptible to XBB and could be fueling the virus's rapid evolution. Prior to Omicron's emergence in November 2021, there were only four variants of concern: Alpha, Beta, Delta and Gamma. Only Alpha and Delta caused surges of infections globally. But Omicron has begotten numerous descendents. "Such rapid and simultaneous emergence of multiple variants with enormous growth advantages is unprecedented," a Dec. 19 study in the journal Nature notes. The same study posits that immune imprinting may be contributing to the viral evolution. Vaccines do a good job of training the immune system to remember and knock out the original Wuhan variant. But when new and markedly different strains come along, the immune system responds less effectively. A Cleveland Clinic study that tracked its healthcare workers found that ... workers who had received more [vaccine] doses were at higher risk of getting sick. Those who received three more doses were 3.4 times as likely to get infected as the unvaccinated, while those who received two were only 2.6 times as likely. "This is not the only study to find a possible association with more prior vaccine doses and higher risk of COVID-19," the authors noted.