(Reuters) – The United States reported a 25% drop in new cases of COVID-19 to about 825,000 last week, the biggest fall since the pandemic started, although health officials said they were worried new variants of the virus could slow or reverse this progress.

New cases of the virus have now fallen for four weeks in a row to the lowest level since early November, according to a Reuters analysis of state and county reports. The steepest drop was in California, where cases in the week ended Feb. 7 fell 48%. Only Oregon, Puerto Rico, Arkansas and Vermont saw cases rise.

At least three new variants of the novel coronavirus are circulating in the United States, including the UK variant B.1.1.7 that is 30% to 40% more contagious, according to researchers.

“I’m asking everyone to please keep your guard up,” Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said on Monday. “The continued proliferation of variants remains a great concern and is a threat that could reverse the recent positive trends we are seeing.”

The average number of COVID-19 patients in hospitals fell by 15% to 88,000 last week, also a record percentage drop, according to a Reuters analysis of data from the volunteer-run COVID Tracking Project. It was the lowest average number in hospitals since late November.

Death fell 2.5% last week to 22,193. Excluding a backlog of deaths reported by Indiana, fatalities were down 9.5% last week. Deaths are a lagging indicator and usually fall several weeks after cases and hospitalizations drop.

Cumulatively, nearly 464,000 people have died from the virus in the United States, or one in every 704 residents.

Nationally, 7.3% of tests of tests came back positive for the virus, down from 8.5% the prior week, according to data from the COVID Tracking Project.

(Graphic by Chris Canipe, writing by Lisa Shumaker, editing by Tiffany Wu)