CNN  — 

The United States has reached another important milestone in the race to protect its population against the coronavirus, with more than a quarter of Americans now fully vaccinated against Covid-19.

More than 84 million people – roughly 25.4% of the population – have been fully vaccinated, according to Centers for Disease Control and Prevention data released Sunday.

The CDC says more than 129 million Americans have received at least one Covid-19 vaccine dose, or roughly 39.5% of the total US population.

As of Monday, all people in the US 16 years and older will be eligible to receive a vaccine.

Experts estimate somewhere between 70-85% of the country needs to be immune to the virus – either through inoculation or previous infection – to suppress its spread.

But as eligibility opens up, concerns around access and vaccine hesitancy remain.

“We’re reaching the point where we’re getting to the hard audiences,” said Lori Tremmel Freeman, CEO of the National Association of County and City Health Officials.

“The ones that either are unsure or on the fence about the vaccine, don’t have enough information or are just plain outright… not interested in the vaccine for other reasons,” she said.

Dr. Anthony Fauci, director of the National Institutes of Allergy and Infectious Diseases, told CNN Sunday it was quite frustrating that “a disturbingly large proportion of Republicans” are saying that they don’t want the vaccine.

The comments follow Fauci’s heated exchange Thursday with Ohio Congressman Jim Jordan during which the Republican said coronavirus precautions infringe on first amendment rights.

“They want to be able to say these restrictions that are put on by public health recommendations are things that they’re very concerned about, we’re all concerned about that, we share that concern,” Fauci told CNN’s Dana Bash on “State of the Union.” “But the way you get rid of those restrictions is to get as many people vaccinated as quickly and as expeditiously as possible.”

When this happens, “for absolutely certain” the level of virus in the community would go down to the point where the restrictions aren’t needed, he said.

“It’s almost paradoxical that on the one hand they want to be relieved of the restrictions, but on the other hand they don’t want to get vaccinated,” said Fauci. “It just almost doesn’t make any sense.”

J&J vaccine pause

Some experts also worry the recent Johnson & Johnson vaccine pause could fuel further vaccine hesitancy.

CDC and Food and Drug Administration officials made the recommendation over six reported US cases of a “rare and severe” type of blood clot, among more than 6.8 million Americans who took the shot.

Vaccine advisers to the CDC, after meeting last week without making a decision, have scheduled another meeting for April 23 to take up the question of whether the J&J vaccine causes blood clots and, if so, what to do about it. The group previously said they needed more information.

Fauci told CNN that he expected a decision to be made about the vaccine by Friday.

“I don’t want to get ahead of the CDC and the FDA and the advisory committee, but I would imagine that what we will say is that it would come back and it would come back in some sort of either warning or restriction,” he said.

Fauci said pausing was the prudent thing to do, saying “you want to make sure that you have all the information that you need.”

Speaking to CNN Saturday, emergency physician and CNN medical analyst Dr. Leana Wen also said she welcomed the pause.

“I am glad that the pause is happening because it really illustrates that our system is working, that our federal health officials are prioritizing – more than anything else – safety,” Wen said “If they’re willing to hit pause on something that’s less than one in a million, we should be really reassured about their commitment to safety.”

The other two Covid-19 vaccines that have also gotten the green light in the US – Pfizer and Moderna – are not implicated in the pause. And in the coming weeks, it will be key to continue the important messaging about their safety, Wen said, and why Covid-19 vaccinations remain critical.

“We’re doing this because we have a pandemic that’s claimed more than 500,000 lives here in the US,” Wen added.

Fauci warned that even vaccinated Americans needed to continue with other mitigation measures, such as mask wearing.

He told CNN it was possible that someone could be vaccinated but still be infected without any signs of clinical disease. They could then inadvertently transmit the virus to someone who has not been vaccinated.

Variants and breakthrough infections, although they are unusual, are another reason to continue wearing masks, he told CNN.

“We don’t want people to think that you don’t dramatically diminish your risk when you get vaccinated, you absolutely do, the risk is very low, and people will make decisions about what they want to do and it will be a relative risk,” he said.

Surge overwhelming Michigan hospitals

States across the country are reporting a rise in Covid-19 cases. But in one state – Michigan – another surge has overwhelmed hospitals and healthcare workers who were already exhausted, one local health leader says.

And there are several reasons behind the crisis.

“One, we believe the overall infection rate in Michigan was lower during the pandemic to date,” Beaumont Health CEO John Fox said on CNN Saturday. “Secondly, Michigan opened up recently … with various orders being relieved.”

Officials reported last week that Beaumont Health’s eight hospitals in two Detroit-area counties were 90%-95% full and the number of their Covid-19 patients jumped from 129 in late February to more than 800 patients in April.

“Unfortunately, I think people have dropped their infection control issues, they’re not wearing their masks as much as they should, (or) social distancing, hand hygiene,” Fox added.

Fauci told NBC’s “Meet the Press” Sunday that the most effective way for places like Michigan to deal with big Covid-19 outbreaks was to shut down, rather than to redeploy vaccines from other states.

“The best way when you’re in the middle of a real big outbreak and a big surge is really to shut down things much more so,” Fauci said. “If you take vaccines from other places and move it around, you make that place vulnerable to what’s going on in Michigan. That’s the reason why you’re not seeing a lot of remobilization of vaccines from one state to another.”

Fox said the highly contagious B.1.1.7 coronavirus variant is further complicating the situation in Michigan, spreading rapidly not just in the state, but across the country.

Cases of that variant, which was first detected in the UK, have now been reported in all 50 US states, according to CDC data. Florida leads the country with the highest number of cases of the B.1.1.7 strain, followed by Michigan and Minnesota, according to the CDC data.

Nurses file paperwork for patients at Beaumont Hospital in Grosse Pointe, Michigan, on April 16, 2021

And with a big part of the older population vaccinated against the virus, the variant has hit younger groups hard.

“It really is presenting in all of our ERs and frankly in our inpatient units,” Fox said. “We’re treating younger patients than we ever saw before.”

Dr. Wen says she’s worried about what this could mean for Michigan and the US.

“What’s happening in Michigan now could very well happen in other states,” she said on CNN Saturday. “Especially because we have this more contagious B.1.1.7 variant that is now dominant in the US.”

The encouraging news, Wen added, is that higher numbers of vaccinations could help blunt another potential violent surge in the country.

Looking ahead, Fauci told ABC’s “This Week” Sunday that it should be clear by fall whether people will need booster shots for Covid-19.

“I believe by the time we get to the end of the summer and the beginning of the fall, we’ll have a pretty good idea whether we definitely or not need to give people boosts and when we need to give it to them,” he told ABC’s Martha Raddatz.

Fauci said that determination would be based on the level of what would be considered a “correlative immunity,” like the level of antibody. “When the slope starts coming down, you could predict when you’re going to get below the safe level or you could start seeing breakthrough infections,” he said.

CNN’s Michael Nedelman, Jen Christensen, Maggie Fox, Virginia Langmaid and Lauren Mascarenhas contributed to this report.