"In the Upper Midwest, we're starting to see lots of younger adults getting sick and going to the hospital from Covid because of the B.1.1.7 variant," said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. "So remember that the B.1.1.7 variant is different from past types of Covid infections that we've seen -- more serious and possibly more severe disease among younger people."
Where B.1.1.7 is thriving
The US Centers for Disease Control and Prevention has reported more than 11,500 cases of B.1.1.7 infections. But those were just the ones caught by genomic sequencing.
The actual number may be much higher, since not every Covid-19 case is sequenced.
The highest B.1.1.7 case counts are in Florida, which has welcomed packed crowds of spring breakers -- many of whom are unvaccinated and maskless.
Michigan has the next highest number of B.1.1.7 cases. Officials there say another surge is well on its way.
"We've got a high proportion of variants, and that means coronavirus spreads faster," Michigan Gov. Gretchen Whitmer said. "These are much more contagious, and we're seeing that whether it is at youth sports or it is the re-engagement of some of our restaurants."
Whitmer said the disturbing trend is fueled by new variants, pandemic fatigue and more travel.
March was the busiest month for air travel since the pandemic started. More than 38 million people were screened at US airports last month, the Transportation Security Administration said.
New Jersey officials also cited the B.1.1.7 variant as they reported increases in cases and hospitalizations. The numbers could stay high into the summer, officials said.
"It is believed that the uptick in cases is due primarily to more contagious variants, for example B.1.1.7, the UK variant, coupled with less cautious behaviors," state health commissioner Judy Persichilli said Wednesday.
New hospitalizations have increased 28% over the past two weeks, Persichilli said.
Young people are getting hospitalized
In March, New Jersey suffered a 31% jump in Covid-19 hospitalizations among young adults ages 20 to 29, the state health commissioner said. And the 40-49 age group saw a 48% increase in Covid-19 hospitalizations.
But older residents, who are more likely to be vaccinated, have only seen single-digit percentage increases, Persichilli said.
In Pennsylvania, the University of Pittsburgh has ordered a shelter-in-place period due to a rise in cases among students and concerns about the B.1.1.7 variant.
"Of significant concern is that the increase in positive cases since the end of last week is now among our residence hall students," an official university email said.
Fauci: 'Hang in there a bit longer'
The good news: The vaccines that are being administered across the country appear to protect people well against the B.1.1.7 variant.
But only about 17% of Americans have been fully vaccinated, according to CDC data. And a large part of the US population is still vulnerable to the virus and variant strains.
"We're vaccinating about 3 million people a day. Every day that goes by we get closer and closer to a greater degree of protection," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "So now is just not the time to pull back and declare premature victory."
The exact same safety measures that work against the original novel coronavirus also work against variant strains -- especially mask wearing and physical distancing.
"Hang in there a bit longer," Fauci said. "Just hang on, continue to do the public health measures, and then we can pull back later -- when we get a greater degree of protection from the vaccines."
But some governors or local officials have recently lifted mask mandates.
Arkansas Gov. Asa Hutchinson announced Tuesday he was lifting a statewide mask mandate.
Alabama Gov. Kay Ivey plans to end her state's mask mandate next week, a spokesperson for the governor said.
New Orleans leaders announced Wednesday they were relaxing restrictions on capacity limits.
But while the city is seeing improvements in several Covid-19 categories, "concerns remain, especially considering the reversal of trends in several states, as well as the spread of stronger variants that are being detected in several cities in Louisiana."
Millions more Americans can get vaccines
To try to get more shots into arms faster, every US state has now expanded or announced plans to expand vaccine eligibility to everyone ages 16 and up.
Pfizer's Covid-19 vaccine is the only one available for use for people age 16 and older. The Moderna and Johnson & Johnson vaccines are authorized for adults, 18 and older.
Kentucky Gov. Andy Beshear said everyone 16 and older will be eligible for a vaccine starting Monday.
"We want to get ahead of the more aggressive COVID-19 variants and make sure that we fill every available appointment."
In Indiana, which has expanded eligibility to anyone 16 and up, state officials said they're dropping the proof of residency requirement for vaccines.
The change was made to comply with FEMA vaccination site rules, as well as to accommodate college students who don't live in the state and people who live with multiple others that might not have proof of residence, state health commissioner Dr. Kristina Box said.
"It's our preference that individuals who live in Indiana, work in Indiana, are the ones that come over and get vaccinated here because we do get allocated vaccines based on our population," Box said, "but what we want to do is remove any barriers."
Nursing homes see 96% decline in new cases
If you want real-life clues that vaccines work, take a look at what's happening in nursing homes.
Since vaccines started rolling out in December, nursing homes saw a 96% decline in new Covid-19 cases, according to an analysis from the American Health Care Association and the National Center for Assisted Living (AHCA/NCAL).
By March 7, the country's nursing homes saw the lowest number of weekly Covid-19 cases and deaths since Centers for Medicare & Medicaid Services started tracking them, according to the report.
And by that same day, virus-related deaths in nursing homes were down 91% since December.
"We are not out of the woods yet, but these numbers are incredibly encouraging and a major morale booster for frontline caregivers who have been working tirelessly for more than a year to protect our residents," said Mark Parkinson, president and CEO of AHCA/NCA..
"This trend shows that when long-term care is prioritized, as with the national vaccine rollout, we can protect our vulnerable elderly population."
CNN's Naomi Thomas, Jen Christensen, Greg Wallace, Anjali Huynh, Alec Snyder, Rebekah Riess, Ganesh Setty and Jacqueline Howard contributed to this report.