(Bloomberg) -- It’s extremely unlikely that a person can be reinfected with omicron, at least in the short-term, President Joe Biden’s top medical adviser said, as the U.S. begins to see sharp declines in cases in places hit earliest by the variant.

“There are reinfections, but it is unlikely that -- if you mounted a good immune response -- at least over a period of several months, it is extremely unlikely that you will be reinfected with the same variant,” Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said in a press briefing on Friday. 

Cases of reinfection have typically been in people who fell ill with one variant and then later contracted a different variant, Fauci said.

But he said the U.S. must prepare for a “worst-case scenario” in which a new variant emerges that eludes protection conferred by both vaccines and prior infection.

“I hope that doesn’t happen,” he said. “I can’t give you the statistics of what the chance of that happens, but we have to be prepared for it.”

There have meanwhile been “steep declines” in coronavirus cases across the northeast, including in New York, Rhode Island and Connecticut, said Rochelle Walensky, director of the Centers for Disease Control and Prevention. 

The seven-day average of new U.S. cases is about 750,000 a day, down 5% from the week prior, Walensky said. Hospitalizations remain flat but near all-time highs.

Walensky, however, stressed that surges started at different times in different regions, and that the U.S. “may continue to see high case counts in some areas of the country in the days and weeks ahead.” She also warned cities and states against quickly relaxing measures like mask mandates.

“Cases are still well higher than the threshold to start thinking about taking off some of those measures,” she said. 

The U.S. this week opened a website for people to request deliveries of at-home tests purchased by the government. Deliveries began Friday, and “millions” of orders have already been fulfilled, White House Covid-19 response coordinator Jeff Zients said. The government has received tens of millions of test kits to ship out, and will further detail the figures next week, including how many people have ordered tests so far, he said.

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