By Christina Maxouris, Lauren Mascarenhas and Eric Levenson, CNN

(CNN) — Mass vaccinations and natural protection from those already infected are likely to prevent a fourth wave of Covid-19 in the United States, Dr. Scott Gottlieb, former commissioner of the Food and Drug Administration, said Sunday.

“We’re talking about some form of protective immunity in about 55% of the population,” Gottlieb said on CBS’s Face the Nation. “There’s enough of a backstop here that I don’t think you’re going to see a fourth surge.”

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About 81 million people have had at least one dose of a vaccine, a number that is going up significantly every day. In addition, about 29 million people have tested positive for the virus and recovered, and tens of millions more have had Covid-19 without a positive test and have some natural immunity.

Still, the US will continue to see Covid-19 cases and deaths, Gottlieb cautioned, particularly as a dangerous variant first identified in the United Kingdom spreads.

“I think what you could see is a plateauing for a period of time before we continue on a downward decline — in large part because (the UK variant) is becoming more prevalent, in large part because we’re pulling back too quickly, with respect to taking off our masks and lifting the mitigation,” he said.

Gottlieb also warned that the emergence of virus variants could change the nation’s trajectory.

“The only thing that can be a real game changer here is if you have a variant that pierces prior immunity, meaning it reinfects people who’ve either already been infected or who have been vaccinated,” he said.

Gottlieb’s comments come as the number of US Covid-19 cases has plateaued at about 50,000 new cases per day over the last seven days. Several experts have warned of another surge as the US races to vaccinate and stay ahead of the variant.

“This is crunch time,” Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, told CNN on Saturday. “This is going to be our most difficult period right now in terms of seeing who wins out.”

With about a quarter of all Americans having received at least one dose of a Covid-19 vaccine — and about 13% fully vaccinated — doubling down on safety measures now is what could help curb another surge, experts have repeatedly stressed.

“If we can hang on another month, another six weeks, that’s going to make a huge difference,” Hotez added.

However, air travel is hitting pandemic-era records and spring break crowds are swelling. In Miami Beach, officials declared a state of emergency Saturday in response to crowds the mayor says have been “more than we can handle.”

And at least a dozen governors and multiple local leaders have eased restrictions this month, while several have done away with mask mandates completely.

Michigan cases and hospitalizations climbing

In Michigan, where the governor announced a series of eased restrictions earlier this month, officials now say the state could potentially be at the start of another surge.

“Our progress with Covid-19 is fragile,” Dr. Joneigh Khaldun, the chief medical executive for the state of Michigan, said in a news conference Friday. “While we’re making great progress with our vaccination efforts and many people are doing the right thing by wearing masks and not gathering in large groups, what we are seeing now is very concerning data that shows that we are going in the wrong direction.”

Case rates have been increasing for the past month, Khaldun said, and increased 77% since mid-February.

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The state’s percent of Covid-19 tests that are positive have also jumped 177% since mid-February, Khaldun said. And hospitalization rates have also been climbing for the past two weeks, Khaldun added.

Michigan has also reported the country’s second-highest number of cases of the B.1.1.7 variant, after Florida, according to CDC data.

“It’s immensely concerning,” Dr. Rob Davidson, an emergency room physician in the state, told CNN Saturday. “We know in the past, cases went up, then hospitalizations, then deaths followed.”

Davidson told CNN he’s even more worried now that variants are circulating, and hopes the state can make enough headway to protect residents.

“It remains to be seen,” Davidson said. “We just would rather not wait and find out. We’d rather get people to mask up, keep distancing and get those numbers down.”

Political divide in vaccinations

More than 44 million Americans are fully vaccinated, according to CDC data.

The growing numbers are encouraging, but experts say the country still has a long way to go to reach the levels needed to suppress spread of the virus — and must address the vaccine hesitancy as well as political divisions that stand in the way.

In Missouri, Gov. Mike Parson said Friday that while he encouraged everyone to get vaccinated and thinks it’s the right thing to do, he knows there will be a “certain amount of people” who are not going to take the vaccine, “and they have every right to do that.”

“We got to do a better job of making sure everybody understands the importance of the vaccine, and yet maintain the respect of people that don’t want to take a vaccine, and it is going to be a challenge to see how many people we can get done, but we’re going to do everything we can,” the governor said.

A recent CNN poll conducted by SSRS, showed that while 92% of Democrats say they have gotten a dose of the vaccine or plan to get one, that falls to 50% among Republicans.

The former head of Operation Warp Speed, Moncef Slaoui, said he is very concerned by vaccine hesitancy fueled by politics.

“I’m very concerned that, for political motivation, people decide to actually place themselves and the people around them in harm’s way by refusing to be vaccinated,” Slaoui said in an interview broadcast Sunday on Face the Nation.

Slaoui also pushed back against President Joe Biden’s criticism of the Trump Administration’s vaccine plans, defending Operation Warp Speed’s efforts last year.

“I do think that we had plans, and in fact, 90% of what’s happening now is the plan that we had,” Slaoui said.

“We contracted specifically 100 million doses of vaccine, but also built into the contract options to acquire more vaccines, once we knew they are effective. The plan was to order more vaccines when we knew they are more effective,” he said. “What’s happening is, frankly, what was the plan — substantially, what was the plan.”

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