The World Health Organization warned Wednesday that new coronavirus variants could emerge during the pandemic that render the current vaccines useless.

“As this pandemic drags on, it’s possible that new variants could evade our countermeasures and become fully resistant to current vaccines or past infection, necessitating vaccine adaptations,” WHO Director-General Tedros Adhanom Ghebreyesus said at a press conference in Geneva.

Tedros sounded that alarm as Covid cases hit record highs in many nations, including in the U.S. The latest surge, coming more than 21 months after WHO first declared Covid a pandemic, is fueled by the spread of the new and highly transmissible omicron variant.

Tedros reiterated his frequent calls for nations to work together to improve global supplies and access to Covid vaccines and other crucial health equipment.