Updated: 12 APR 21 12:01 ET

By Christina Maxouris, CNN

(CNN) -- As the US races to vaccinate more Americans, Covid-19 cases and hospitalizations are rising, predominantly among younger people who haven't yet gotten a shot.

Some experts worry this might only be the start of what's to come in the next weeks. Michigan is already in the middle of a violent surge, and one epidemiologist says other states should be paying close attention.

"Michigan is really the bellwether for what it looks like when the B.1.1.7 variant ... spreads in the United States," Dr. Celine Gounder told CNN on Sunday. "It's causing a surge in cases and it's causing more severe disease, which means that even younger people, people in their 30s, 40s and 50s are getting very sick and being hospitalized from this."

The B.1.1.7 variant, first spotted in the UK, is now the dominant strain of the virus in the US. Experts say it's more contagious, may cause more severe disease and may potentially be more deadly. And it's rapidly spreading across the country.

Florida has the highest number of cases of the variant, followed by Michigan, Minnesota and Massachusetts, according to data from the US Centers for Disease Control and Prevention.

Michigan is 'on fire'

Michigan's surge is a combination of two factors, Gounder says: the spread of the B.1.1.7 variant combined with people relaxing on mitigation measures before enough residents are vaccinated.

And while some officials -- including Michigan Gov. Gretchen Whitmer -- are trying to make a case for the federal government to ramp up vaccines to the state, Gounder says that won't help in this case.

Here's why: It takes about two weeks after the Pfizer and Moderna second doses and about two weeks after the Johnson & Johnson vaccine before people are immune, she said. Meanwhile, Gounder added, "the incubation period, which is the time from when you are exposed to when you are infected with coronavirus, is four to five days."

"So there is no way that a surge in vaccination is going to help curb this when transmission is happening right now," she said. "The hard truth is that the only thing that will curb transmission right now are measures that take effect immediately." For example, masking up, not dining indoors, and socializing outdoors, she added.

Michigan is now reporting thousands of new Covid-19 cases daily, when just weeks ago, state data showed the daily reported case count was as low as 563 cases. In one part of the state, emergency room physician Dr. Rob Davidson told CNN hospitalizations have gone up "four fold in just the last two weeks."

"Hospitals are being inundated," Dr. Jonathan Reiner, a professor of medicine and surgery at George Washington University, told CNN. "Michigan needs to shut down."

But he added the government should send more vaccines to the state.

"Think about it this way: Every year during fire season, when forest fires get out of control, we don't just leave the states to manage as best they can. We surge firefighting forces into those states. So Michigan is on fire now. And we need to put that out," he said.

Other state leaders alarmed

But it's not just Michigan.

Minnesota health officials warned the state was seeing a "sharp increase" in Covid-19 cases, saying it's "more important than ever" to keep wearing a mask and physical distancing.

Ohio Gov. Mike DeWine said the state was seeing rising Covid-19 cases, hospitalizations and ICU admissions and urged residents to get vaccinated to avoid another surge.

"This is a race," he said. "We are in a race. And it's a life-and-death race."

In Pennsylvania, Gov. Tom Wolf warned last week the state was seeing a rise in cases and urged residents to "do their part and practice proven public health practices to keep each other safe."

Also last week, New Jersey Gov. Phil Murphy expressed concern over the state's consistently high Covid-19 case numbers and what could be ahead.

"It's why we are continuing to enforce things like our statewide mask mandate, especially for indoor activities where we know the transmission is more likely," Murphy said. "It's also why we moved up our vaccine eligibility timeframe aggressively."

Mass vaccination center opens in Oregon

The Biden administration outlined new steps Monday to continue to increase vaccinations.

The administration announced a new federally-supported mass vaccination center in Oregon, part of its continued efforts toward promoting vaccine equity. It also announced other steps toward getting more shots in arms.

"We're excited to announce today a new federally-run mass vaccination site in Central Point, Oregon," White House senior Covid-19 adviser Andy Slavitt said at Monday's Covid briefing, noting that President Joe Biden had set a goal on March 29 of adding a dozen such new sites by April 19.

"We are on track to meet the President's goal this week, ahead of schedule. And by next week, we will have opened a total of 36 mass vaccination sites with a combined capability of administering 124,000 shots per day," Slavitt said.

US needs to address vaccine hesitancy, expert says

Some experts say the US could beat another potential surge in the coming weeks if Americans hold on a little longer and continue to practice safety measures while more of the population is vaccinated against the virus.

So far, about 35.9% of all Americans received at least one Covid-19 vaccine dose, according to CDC data. And about 21.9% of the population is fully vaccinated.

Among US adults, nearly 46% have received at least one dose of vaccine and about 28% are fully vaccinated, the CDC said. If vaccinations continue at the current pace, the US will likely hit a milestone this week: vaccinating half of adults with at least one dose of vaccine.

At the record pace that the US is administering vaccines, the country could reach the vaccination numbers needed to control the virus over the next few months, Reiner said.

"The way to do that is to vaccinate like crazy," he said.

About 4.6 million doses were administered Saturday, a new record for the number of doses reported administered in one day. Over the past week, more than 3.1 million doses of vaccine have been administered each day on average, according to the CDC.

But there are challenges ahead.

"The last 20 to 30% are going to be the hardest because a lot of folks in this country are still hesitant to get the vaccine. We're seeing it all over the country," Reiner from George Washington University said. "We need to really get down on the grassroots level, talk to people about their hesitancy and get shots into arms."

"Because if we don't vaccinate that last 30% or so, we're still going to have to live with this virus for a very long time."