The virus is again surging in parts of the United States, but it’s a picture with dividing lines: ominous figures in the Northeast and Upper Midwest, but largely not in the South.
Experts are unsure what explains the split, which doesn’t correspond to vaccination levels. Some point to warmer weather in the Sun Belt, while others suspect that decreased testing is muddying the virus’s true footprint.
The contours of where the virus is resurgent can be drawn around one figure: states that are averaging about 15 new cases a day for every 100,000 people. The 23 states — including Alabama, Mississippi and Arkansas — that have averaged that or fewer over the past week seem to be keeping cases relatively low, according to a New York Times database.
In the 27 states above that line, though, things have been trending for the worse. Michigan has the highest surge of all, reporting the most drastic increase in cases and hospitalizations in recent weeks. Illinois, Minnesota and others have also reported worrisome increases.
Nationally, reported cases in the United States are growing again after a steep fall from the post-holiday peak in January. In the past two weeks, new confirmed cases have jumped about 11 percent, even though vaccinations picked up considerably, with an average of over 3.3 million doses given daily.
Hospitalizations are rising in some states, most notably in Michigan.
Some Southern states, like Alabama and Mississippi, are lagging in vaccinations. Only about 28 percent of people in each state have received at least one shot, according to a New York Times vaccine tracker. Still, case counts continue to drop in both states.
Health experts say cases are rising in the Northeast and Upper Midwest for several reasons, including pandemic fatigue, the reopening of schools and the resumption of youth sports.
Officials are also concerned about the spread of more contagious virus variants, especially B.1.1.7, first identified in Britain. The variant is now the leading source of new coronavirus infections in the United States, the director of the Centers for Disease Control and Prevention said last week.
Just why those factors might affect some states more than others is hard to pinpoint, experts say.
Dr. David Rubin, the director of PolicyLab at the Children’s Hospital of Philadelphia, said warmer weather in Southern states and California was probably playing a role, because it allows people to gather outdoors, with less risk of transmission.
Some experts say, though, that reduced testing in some states could be obscuring the true picture. Testing in Alabama, for instance, has started to dip, but the share of tests that come back positive has remained high, at 11.1 percent, compared with a nationwide average of 5.1 percent, according to data compiled by Johns Hopkins University.
“People who are symptomatic and go to their provider are going to get a test,” said Dr. Michael Saag, the associate dean for global health at the University of Alabama at Birmingham, but “the desire for people to go get tested just because they want to know what their status is has dropped off dramatically.”
Still, Dr. Saag said, there is probably not a hidden spike in cases in Alabama right now, since hospitalizations in the state remain low.