United States coronavirus cases have increased again after hitting a low point late last month, and some of the states driving the upward trend have also been hit hardest by variants, according to an analysis of data from Helix, a lab testing company.

The country’s vaccine rollout has sped up since the first doses were administered in December, recently reaching a rolling average of more than 3 million doses per day. And new U.S. cases trended steeply downward in the first quarter of the year, falling by almost 80% from mid-January through the end of March.

But during that period, states also rolled back virus control measures, and now mobility data shows a rise in people socializing and traveling. Amid all this, more-contagious variants have been gaining a foothold, and new cases are almost 20% higher than they were at the lowest point in March.

“It is a pretty complex situation, because behavior is changing, but you’ve also got this change in the virus itself at the same time,” said Emily Martin, an epidemiologist at the University of Michigan School of Public Health.

Cases are up in states with strong variant presence.

Michigan has seen the sharpest rise in cases in the last few weeks. B.1.1.7 — the more transmissible and more deadly variant of the coronavirus that was first discovered in the United Kingdom — may now make up around 70% of all of the state’s new cases, according to the Helix data.

The vaccines authorized in the United States are very effective against the B.1.1.7 variant and will significantly slow virus spread once a large share of the population is vaccinated. Some experts estimate 70% to 90% of the population would need to acquire resistance before transmission would substantially slow. As of April 5, less than 20% of the U.S. population has been fully vaccinated against the virus.

Regional variants have also fueled spikes.

Several states in the Northeast also have among the country’s worst outbreaks now. Connecticut, New Jersey and Pennsylvania, among others, are all experiencing marked rises in case counts, and labs have identified both the B.1.1.7 variant and large shares of another variant, B.1.526.

The B.1.526 variant, which first appeared in New York City in samples from November, appears in two forms: one with a mutation that may help the virus evade antibodies and another that may help it bind more tightly to human cells.

The rapid spread of the B.1.526 variant, which made up more than 40% of sequenced cases in New York City as of mid-March, has prompted officials to say they believe it also could be a more infectious strain, though they say it is too early to tell whether it results in a more severe illness.

There is not enough genomic sequencing, the resource-intensive process required to discover that a case has been caused by a variant, to be certain how exactly much B.1.526 is spreading in the Northeast, but the available data indicates it is likely widespread.

The outbreak in the Northeast is currently much worse than it is in California, but California faces a variant of its own that makes up a large share of cases.

Studies have indicated the variant first discovered in California, B.1.427/B.1.429, may also be more transmissible than earlier forms of the virus, but it does not appear to spread as quickly as B.1.1.7. It was discovered in more than half of samples tested in Los Angeles in mid-January, suggesting that it may have helped fuel the state’s enormous winter surge, which hit Southern California hardest.

Like the variant first discovered in New York City, the B.1.427/B.1.429 variant has also been seen in high levels in neighboring states, including Arizona, but does not yet make up a significant number of cases outside the region.

After months of progress, cases have risen again in most states.

Nationwide, the uptick in the number of hospitalized COVID-19 patients is more subtle, but with a large divergence by age group: There is a clear recent increase in coronavirus hospital admissions among adults under 50. But among those in the oldest age groups, who are most likely to have been vaccinated already, admissions continue to decline.

The vaccine rollout continues to speed up, and recent studies confirm that vaccines are effective against the coronavirus in the real world, giving experts hope that an end may be in sight. But with increased transmission, they say, comes a renewed need for caution in the immediate term.

“I think we’ve got to hang on just a little bit longer, being conservative and getting more people vaccinated,” Martin said. “I’d hate to see us having another hospital surge when we’re getting so close to being done with this. I’m definitely worried about it.”

© 2021 The New York Times Company