(CNN)Just five states have accounted for about 43% of new coronavirus cases over the last week, according to data from Johns Hopkins University.
New York, Michigan, Florida, Pennsylvania and New Jersey had more than 196,400 of the country's 453,360 cases reported in the last week, according to data available Wednesday morning.
Those states are home to just 22% of the US population, according to estimates from the US Census Bureau.
Case rates have risen especially in Michigan lately, averaging more than 6,600 cases a day over a week now against 1,350 daily cases five weeks ago. And elected officials and health experts have said highly contagious variants such as B.1.1.7 have helped spur increases there and in other parts of the country.
B.1.1.7, first identified in the UK, is now the most common strain of coronavirus in the United States, Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said Wednesday.
"Based on our most recent estimates from CDC surveillance, the B.1.1.7 variant is now the most common lineage circulating in the United States," Walensky said at the White House Covid-19 response team briefing.
With more-transmissible variants adding up, surges like Michigan's may soon be seen more widely, even though vaccination rates have increased nationally, epidemiologist Dr. Michael Osterholm said.
"I wish we had another three or four months before this B.1.1.7 variant surge started to occur," Osterholm, director for the Center for Infectious Disease Research and Policy at the University of Minnesota, said Tuesday.
The country's daily rate of new coronavirus cases rose over most of the last four weeks. Part of that is due to the spread of B.1.1.7 and other concerning variants, Walensky said earlier this week.
The US has averaged more than 64,760 new coronavirus cases a day over the past week -- slightly lower than the week prior, but still about 21% higher than two weeks ago, and more than 12% higher than four weeks ago, according to Johns Hopkins.
The surge of variants in some states like Michigan should make the federal government think about shifting how it allocates vaccines, Dr. Ashish Jha, dean of the Brown School of Public Health, told CNN on Wednesday.
"In some states ... case numbers are actually declining -- California is one of them. Other states like Michigan, they're actually doing a perfectly good job on vaccinations, but they have a huge surge," Jha said.
"So the federal strategy has to be to shift more vaccines to places like Michigan that are surging so they can use more vaccinations to stop that surge and save lives," he said.
The CDC says more than 16,200 cases of the B.1.1.7 variant have been confirmed in surveillance testing in the US, and B.1.1.7 has been found in every state. This number does not represent the total number of such cases circulating in the United States, but rather only those found by analyzing samples.
CDC director: Decreasing number of daily deaths may be 'an impact of vaccination'
While cases have been recently rising or plateauing, the number of daily Covid-19 deaths has been dropping.
The US has averaged 774 Covid-19 deaths a day over the last week -- down 22% from the average the week prior, according to Johns Hopkins data.
It's down even further from the first day of March, when the average was about 2,000 deaths a day.
At Wednesday's briefing, Walensky attributed these dropping numbers to vaccinations.
"I'm really encouraged about these decreased numbers of deaths that I believe to be an impact of vaccination, especially the vaccination of our elderly communities," Walensky said.
But, she added, current case counts are "way too high to be thinking that we've won this race."
According to the latest CDC data, 19.4% of the total US population was fully vaccinated as of Wednesday morning; and 57.4% of people age 65 and older are fully vaccinated.
Half of US adults may have had their first shot by the weekend, White House says
By the end of the weekend, the US will be approaching nearly half of all adults having their first shot of a Covid-19 vaccine, a White House official said.
But progress on vaccines does not mean the country is finished with the pandemic, a Biden administration adviser said Tuesday.
"We do have to remember that there are 100 million-plus adults that still haven't been vaccinated," Andy Slavitt, White House senior adviser for Covid-19 response, told CNN's Chris Cuomo. "They're not there yet, and you don't win the war until you bring everybody over with you."
As of early Wednesday, the US had more than 109.4 million people ages 18 and older with at least one dose of a Covid-19 vaccine. That's about 42.4% of the adults in the US and a .7 percentage points increase in the past day, according to CDC data.
President Joe Biden has set a goal for every state to open vaccination to all adults who want them by April 19. That task is becoming easier as the supply and accessibility of vaccines increases, Slavitt said.
But the US has a long way to go before reaching herd immunity. Dr. Anthony Fauci has estimated 70%-85% of the population needs to become immune. And the pace for vaccinating all willing adults varies greatly among states, according to a CNN analysis of federal data.
While states such as New York and North Dakota might vaccinate all willing adults by June, it could take some states until the end of 2021. And in a race against both spreading variants of concern and increasing Covid-19 fatigue, experts say it is imperative to vaccinate Americans quickly.
Some states are pulling back mask mandates and opening capacity for businesses. But Fauci said Tuesday that foregoing preventative measures while the majority of Americans are still unvaccinated is a mistake.
"It is premature to declare a victory," said Fauci, director of the National Institute of Allergy and Infectious Diseases. "We've got to continue and hang in there a bit longer by continuing with the public health measures."
The US still has tools to stay on course even if the public is fatigued, Slavitt said. Opening vaccine accessibility and focusing on some groups, as the US did to give 80% of teachers and school staff at least one shot amid calls for school to reopen, can help the nation continue to fight Covid-19, Slavitt said.
Correction: An earlier version of this story and the story's headline overstated the percentage of US adults who will be vaccinated by the end of the weekend. The White House clarified that the US will be approaching having nearly half of all adults with their first shot of the coronavirus vaccine by this weekend.
