By ARIELLE MITROPOULOS, ABC News

(NEW YORK) — Even with the race to vaccinate as many Americans as possible accelerating, some experts are warning that if Americans don’t also follow proper safety and mitigation measures the U.S. could see a COVID-19 resurgence.

Although the country’s national daily case average continues to fall — about 32.5% over the last month — nearly a third of all states have seen their average number of cases rise at least 10%.

Those 15 states are: Alaska, Connecticut, Delaware, Idaho, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Nevada, New Hampshire, New Jersey, Oregon and West Virginia, according to an ABC News analysis of state data compiled by the Centers for Disease Control and Prevention.

Several states — Delaware, Idaho, Minnesota, Nevada, Oregon and West Virginia — have seen their case averages increase by at least 25% in the last week.

“Minnesota, I think, is really a harbinger of things to come. We’re loosening up all the restrictions we’ve had, and in some ways creating a perfect storm moment for virus transmission, whether it’s a variant or not,” Michael Osterholm, an epidemiologist and head of the University of Minnesota’s Center for Infectious Disease Research and Policy, told ABC News’ World News Tonight on Wednesday.

Dr. Tara Smith, a professor of epidemiology at the Kent State University College of Public Health, told ABC News that her “biggest concern” is a virus resurgence: “I don’t expect to see something as bad as we just recovered from, but if we’re not careful, cases can rise again if we let our guard down too soon.”

There also are 18 states — Colorado, Hawaii, Illinois, Indiana, Iowa, Kentucky, Louisiana, Montana, New York, North Carolina, North Dakota, Ohio, Pennsylvania, Rhode Island, South Dakota, Tennessee, Utah and Wyoming — that are experiencing a plateau, not a decline, in daily case averages.

Metropolitan areas, in particular, continue to struggle with high case rates. For weeks, New York City has led the nation in cases per 100,000 residents, and last week, New York mayoral adviser Dr. Jay Varma said two newly emerged COVID-19 variants — the U.K. and the N.Y. variant — may now account for a majority of the city’s cases.

However, it’s still unclear whether these trends are blips or lingering risks with longterm consequences.

Smith believes a “confluence” of factors may be to blame, including the spread of COVID-19 variants and the loosening of health-related restrictions.

“The combination of rapid reopening, increasing mobility and spread of new COVID variants is likely driving the upward trend in cases in many parts of the country,” said Dr. John Brownstein, chief innovation officer at Boston Children’s Hospital and an ABC News contributor. “While we should feel optimistic with the vaccine rollout, the new surge, even if short-lived, will likely result in avoidable hospitalizations and deaths.”

According to data compiled by ABC News, in the last three weeks, over two dozen states are reopening or planning to do so, dismantling restrictions imposed earlier in the pandemic. Governors across the U.S. are relaxing rules on businesses and public gatherings.

Arizona and West Virginia, for instance, are permitting restaurants and bars to fully reopen. In Texas, Gov. Greg Abbott has issued several executive orders detailing plans to re-open businesses and activities in Texas, tweeting, “I just announced Texas is OPEN 100%.”

At least 17 states no longer have a mask mandate, and even states like Connecticut, Maryland, Massachusetts and New York that previously were among the most cautious are easing restrictions. On Friday, Connecticut plans to eliminate capacity limits for many businesses, such as restaurants, retail stores and fitness centers in addition to places of worship.

In the coming weeks, New York will be increasing indoor and outdoor gathering capacities, and Maryland Gov. Larry Hogan has announced that capacity restrictions will be lifted everywhere with the exception of large venues.

“Cases are lower than they were in our big fall, winter spike, and vaccinations are increasing, but we all need to still be careful and realize we’re not yet out of the woods,” Smith said.

Other experts, however, said they’re not as concerned yet.

Jennifer Nuzzo, a public health researcher at Johns Hopkins University, told ABC News that although there’s always the risk of a resurgence, particularly in states that lifted restrictions and mask mandates, “I usually don’t get worried until I see a two-week trend in the uptick in cases.”

Until the U.S. begins broadening vaccinations to include other age and social groups, Nuzzo said she expects to see fluctuations in the data. But, she added, if these trends continue, a sustained increase in the daily case average would be cause for concern as many states are scaling back on testing.

The U.S. is now averaging just over one million tests a day, down from 1.8 million daily in January.

“I do worry about the possibility that we’re missing some spread that could then come and surprise us in a month or so, when suddenly it becomes very obvious,” Nuzzo added.

“We’ve seen throughout the pandemic, decreases in testing capacity creates unnecessary blindspots in our surveillance and likely results in delayed public health response,” Brownstein added.

The surest way forward, experts agree, is vaccinations and more vaccinations.

“The best way to circumvent the variants is to continue to vaccinate as many people as quickly and as efficiently as possible, at the same time as maintaining your public health measures,” Dr. Anthony Fauci, the nation’s top disease expert, told ABC News on Tuesday.

To date, 12% of the total U.S. population has been fully vaccinated and 22.2% have received at least one dose.

With approximately 500 million COVID-19 vaccine doses expected to be in the pipeline by the end of May, more states are moving to open up eligibility requirements to allow anyone 16 or older access to them.

“Given that the vaccines have been shown to provide broad protection against a range of variants,” Brownstein said. “These early warning signs reaffirm the need to sustain and even increase the pace of vaccination across the country.”

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