A new Covid-19 variant, NB.1.8.1, informally dubbed Nimbus, is raising global concern due to its severe sore throat symptom, described as feeling like "razor blades." Monitored by WHO, Nimbus is spreading across Asia, the U.S., and Canada. Experts warn it may evade prior immunity, but vaccines remain effective. Symptoms include fatigue, cough, congestion, fever, and muscle aches.