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The head of the Centers for Disease Control and Prevention is sounding the alarm that recent gains against the coronavirus may be stalling.

Dr. Rochelle Walensky says the CDC is looking at data that COVID-19 cases have been increasing the past three days, but more time is needed to see if that is a blip or the start of a trend.

Walensky spoke at the White House coronavirus briefing Friday, noting virus mutations spreading in the U.S. are among the CDC’s biggest concerns. Along with a more transmissible strain first detected in Britain, scientists here are tracking variants in New York and California, which also appear to spread more easily.

“We may be done with the virus, but clearly the virus is not done with us,” says Walensky, stressing now is not the time to relax protective measures like wearing masks and avoiding gatherings.

Viruses constantly mutate as they spread, and most changes aren’t significant. First-generation COVID-19 vaccines appear to be working against today’s variants, but makers already are taking steps to update their recipes if health authorities decide that’s needed.

COVID-19 vaccines by Pfizer and Moderna are made with new technology that’s easy to update. The so-called mRNA vaccines use a piece of genetic code for the spike protein that coats the coronavirus, so your immune system can learn to recognize and fight the real thing.

Cases and hospitalizations have fallen dramatically since the January peak that followed the winter holidays. Deaths have also declined. But Walensky says those gains could be in jeopardy because the background level of cases is still too high.