By Christina Maxouris and Holly Yan, CNN

(CNN) -- States trying to fend off a new, dominant strain of coronavirus have a new challenge: having to stop Johnson & Johnson vaccinations for now.

The US Centers for Disease Control and Prevention and the US Food and Drug Administration are recommending the country pause the use of Johnson & Johnson's single-dose vaccine after six reported cases of a rare and severe type of blood clot.

The six cases were among about 6.8 million Americans who have received the single-shot Johnson & Johnson vaccine.

"I know there are people who have gotten the vaccine who are probably very concerned. For people who got the vaccine more than a month ago, the risk to them is very low at this time," said Dr. Anne Schuchat, principal deputy director of the CDC.

"For people who recently got the vaccine within the last couple of weeks, they should be aware to look for symptoms. If you received the vaccine and develop severe headaches, abdominal pain, leg pain or shortness of breath, you should contact your health care provider."

The other two vaccines used in the US, from Pfizer/BioNTech and Moderna, are extremely safe and effective and continue to be distributed widely. Those both come in two doses and were created very differently from the Johnson & Johnson vaccine.

Several states, including Texas, Maryland and Missouri, have directed vaccine providers to pause the use of Johnson & Johnson vaccines while also keeping those vaccines in the proper conditions.

In New York, everyone with Tuesday appointments for Johnson & Johnson vaccines a state mass vaccination sites will be given the Pfizer vaccine instead, the state health commissioner said.

The New Jersey health department will work with all vaccination sites to help reschedule appointments and make arrangements to give one of the other two vaccines, the state health commissioner said.

Both CVS and Walgreens will also stop giving the Johnson & Johnson vaccine for now, representatives for the pharmacy chains said.

The CDC said it will meet with the Advisory Committee on Immunization Practices (ACIP) on Wednesday to further review these cases and assess their potential significance.

Don't get lax -- hospitalizations keep rising

Nationally, Covid-19 cases and hospitalizations are on the rise, especially among young Americans who are less likely to be vaccinated, health officials say.

"Don't declare victory prematurely," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

"We see so many pulling back on some of the public health measures: the mask mandates, the restaurant opening, the bars. We can't be doing that. We've got to wait a bit longer until we get enough vaccine into people that we will clearly blunt any surge."

If the US keeps vaccinating quickly and doubles down on safety measures, it could soon see a "turnaround," Fauci said.

One of the biggest challenges is the B.1.1.7 strain, which is more contagious than the original strain of coronavirus and is now the dominant strain in the US.

B.1.1.7 is also hitting young people particularly hard.

To try to get out of this pandemic, "it's still about masks and physical distancing and ventilation and vaccines," said emergency physician Dr. Megan Ranney, director of the Brown-Lifespan Center for Digital Health in Rhode Island.

"The current vaccines work really well against B.1.1.7."

Coronavirus variants are behind alarming trends in several states, including Michigan, which local officials say is in the middle of another surge, with both case and hospitalization numbers quickly rising.

The state has the second-highest number of cases of the B.1.1.7 variant, according to data from the Centers for Disease Control and Prevention. The state with the highest number of cases of the variant is Florida.

In New Jersey, where there are more than 950 cases reported of concerning variants, patients on ventilators and virus-related deaths have increased, state health commissioner Judy Persichilli said Monday.

About 48% of the state's new hospitalizations were people under the age of 60, she added.

US is on track to hit a key vaccination milestone

More than 120 million Americans -- about 36.4% of the population -- have received at least one dose of a Covid-19 vaccine, according to CDC data. About 74 million people -- about 22.3% of the US population -- have been fully vaccinated.

The US is on track to reach half of the US adult population with at least one dose by the end of the week, according to a CNN analysis of CDC data.

At least 11 states have already reached that threshold: Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New Mexico, Rhode Island, South Dakota, Vermont and Wisconsin.

In Kentucky, Gov. Andy Beshear announced a new "Team Kentucky Vaccination Challenge," saying that when 2.5 million residents have received at least their first dose of a Covid-19 vaccine, the state will remove most of its capacity restrictions.

"With the vaccine supply we have, we could get there in as little as three-and-a-half weeks from now," Beshear said in a news release.

"That minimum time frame might not be realistic, but we should get there in four to six weeks if we are intentional."

More than 1.6 million residents have so far received at least their first dose of vaccine, the release said. Kentucky has a population of about 4.5 million.

"We have to try everything to reach this point as quickly as possible. That will help us have a more normal summer than any of us could have imagined this winter," the governor added.

What Fauci says about indoor and outdoor dining

Fauci, who has been fully vaccinated, was asked by CNN's Wolf Blitzer whether he would consider outdoor dining.

Fauci said that while he's been too busy to do so recently, "in an outdoor restaurant, right now, where we are, I would not hesitate to do that."

And what about indoor dining for Americans who are fully vaccinated?

"What you need to do is you need to look at the level of infection in the community," Fauci said.

"Being vaccinated, the risk for you is very low. It isn't like before, when you were not vaccinated and you had a lot of activity in the community and you went into an indoor restaurant where there was no (restrictions) ... your risk would be up there. Whereas now, the risk is not zero, but it's extremely low" if you're fully vaccinated.

More universities will require vaccine proof

To lower Covid-19 risks on their campuses, a growing number of colleges and universities have said they will require all students to be fully vaccinated against Covid-19 before returning to campus.

Johns Hopkins University, in Maryland, is the latest to join the list.

All students who plan to be on campus will be required to be vaccinated unless they have a medical or religious reason not to be and faculty are also strongly recommended to be vaccinated, although it will not be required for them, according to an announcement sent to the campus community on Friday.

"Our plans are predicated on continuing public health strategies to promote a safe campus and community," Johns Hopkins University President Ronald Daniels and other university leaders wrote.

"Ensuring that the overwhelming percentage of our community's population is vaccinated will greatly reduce the risk of the virus's spread on our campuses and will also protect our neighbors in Baltimore," they added.

By CNN's count, there are at least 18 colleges and universities in the US that will be requiring Covid-19 vaccines.