• Daily Covid-19 cases are increasing once again after months of declines, a troubling sign as more Americans let down their guard, experts say.
• More transmissible virus variants could lead to more cases, hospitalizations and deaths among those without access to a vaccine, they warn.

With the possibility of summer barbeques just a few months away, along with the promise of widespread Covid-19 vaccine supply in the U.S. by the end of May, many Americans may be feeling as though the nation has finally turned the corner on the pandemic.

But to leading infectious disease experts, the country isn't there yet.

"When I'm often asked, 'Are we turning the corner?' My response is really more like, 'We're at the corner,'" White House Chief Medical Advisor Dr. Anthony Fauci said during a press briefing on Wednesday.

Before the U.S. can reach its long awaited destination — some semblance of pre-pandemic normality — it needs to get more vaccines into arms, infectious disease experts tell CNBC. But while the U.S. continues to report fresh daily vaccination records, the number of new cases is simultaneously growing once again.

The U.S. is recording a weekly average of 61,821 new Covid-19 cases per day, a 12% increase compared with a week ago, according to a CNBC analysis of data compiled by Johns Hopkins University. Daily cases are now growing by at least 5% in 27 states and D.C.

Coronavirus hospitalizations are also beginning to make a rebound. The U.S. reported a seven-day average of 4,790 Covid-19 hospital admissions on Thursday, a 2.6% increase compared with the week prior, according to Centers for Disease Control and Prevention data.

"We're in a delicate and tenuous period of transition," Dr. William Schaffner, an epidemiologist and professor of preventive medicine at Vanderbilt University, told CNBC. "We're doing well, but we're not there yet."

Don't 'fumble the ball'

The rise in infections coincides with an accelerated vaccine campaign that's beginning to reach more people.

The U.S. is now administering an average of 2.6 million shots per day and more than a third of adult Americans have received at least dose, according to the latest figures from the Centers for Disease Control and Prevention.

Nearly half of people ages 65 and older have completed all of their necessary shots, CDC data shows. However, just 19.4% of the adult population is considered fully vaccinated, which is required to achieve the high level of protection provided by the currently deployed vaccines from Pfizer, Moderna and Johnson & Johnson.

While most states have issued plans to open vaccine eligibility to all adults before President Joe Biden's May 1 deadline, just six have moved to widely offer the shots so far, according to recent data tracked by The New York Times.

"We're on the proverbial 10-yard line," Schaffner said. "We'll get the ball across and have a touchdown but don't fumble the ball on the 10-yard line."

Some states are widely reopening their economies while dropping mask mandates too soon, Schaffner added. The return of spring break travelers taking advantage of cheap flights and hotels has further exacerbated the risk of more infections.

"All of those things could conspire to create another surge in cases before the vaccinations start to really take hold in reducing transmission," Schaffner said. "We have the danger — and I do mean danger — of having another surge within the next two months."

Variants loom

Another concern is the spread of highly infectious coronavirus variants, particularly the one first identified in the U.K. dubbed B.1.1.7., infectious disease experts tell CNBC. The CDC is carefully following another variant found in New York City, called B.1.526, which is also thought to be more transmissible compared with previous strains, the agency's Director Dr. Rochelle Walensky said on Wednesday.

A more transmissible virus could lead to more infections and inevitably hospitalizations and deaths even as the most vulnerable are vaccinated against the disease, experts warn, making the race to inoculate more people crucial.

"The variants really throw quite a wrench into the response," said Dr. Angela Hewlett, a professor of infectious diseases at the University of Nebraska Medical Center, noting that the vaccines should still provide protection.

"We just have to vaccinate more of our population in order to really stamp this thing out," Hewlett said.

Increased travel could bolster B.1.1.7's spread, which is a particular concern in Florida where out-of-state spring break visitors could take the virus back to their local communities, said Cindy Prins, an epidemiologist at the University of Florida.

Florida has identified more than 1,000 coronavirus cases with the B.1.1.7 variant, the most of any state so far, according to the most recent CDC data.

"There's no doubt that there are lots of people who have come in from out of state. That happens every year for spring break," Prins said. "And then the concern is what's being brought back to their own state. Are they going to bring back the variant?"

— CNBC's Hannah Miao contributed to this report.