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COVID-19 vaccinations are on the rise in the U.S. � and so are coronavirus cases.

After a plateau lasting several weeks, the number of cases is once again on the increase in parts of the country.

New cases, test positivity rates and hospital admissions are creeping upward. An increase in daily COVID-19 deaths is likely to follow, health officials say.

Calling it a race between vaccinations and variants, Ashish Jha, a public health policy researcher and dean of Brown University's School of Public Health, : "Well, despite phenomenal vaccination rates, variants pulled ahead this week."

According to from the White House COVID-19 team, more than 410,000 people tested positive for the coronavirus in the U.S. this past week � a 9% increase from the previous week. More than 33,000 people were admitted to the hospital with COVID-19 this past week, representing a 2% increase.

The number of new cases and hospitalizations per day is still far lower than an all-time peak in mid-January, when the U.S. recorded around 250,000 new daily cases. Still, the current trend in cases raises concerns among health officials that the U.S. could see a fourth surge in coronavirus infections as and some members of the pandemic-fatigued public .

"There is a case for optimism, but there is not a case for relaxation," said Jeff Zients, the White House coronavirus coordinator, at a . "This is not the time to let down our guard. We need to follow the public health guidance, wear a mask, socially distance, and get a vaccine when it's your turn."

Cases are rising in a number of pockets around the country. In Michigan, the increase in coronavirus cases is landing people in their 30s and 40s in the hospital at rates similar to the winter surge � around 60 people each day � according to reporting from . Detroit, New York and Philadelphia were among several cities that saw this past week.

Health officials warn that the country is now facing a more difficult foe than in earlier stages of the pandemic � have mutated to spread more easily and to possibly cause more severe disease. According to a tally maintained , these so-called "variants of concern" have shown up in every state and are in the places where they have emerged.

"We're watching it very closely," said Dr. Rochelle Walensky, CDC director, , "We're worried about the increase."

The rise in cases and variants is happening at a time when the COVID-19 vaccination campaign is also reaching new heights. A third of the has received COVID-19 vaccines, and President Biden set a new goal of getting at least 200 million jabs into arms in his first hundred days. Health officials say that people who are � meaning at least two weeks have passed since they received the last shot � are largely protected against severe illness and less likely to spread the virus to others.

Until of the population has been fully vaccinated, however, health experts say the public should adhere to mitigation measures � such as masking, physical distancing and improving indoor ventilation � to protect their own lives and those of their companions.

"We're so close to vaccinating so many more people," Walensky said at a , referring to an anticipated surge in vaccine availability. She sees the situation in Europe, where again and reintroducing restrictions in response to , as a warning.

"We just don't want to be at this rapid uptick of cases again, and that is very possible that that could happen," she said.

Copyright 2021 NPR. To see more, visit https://www.npr.org.