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he United States is in for a celebratory summer, experts and political leaders have forecasted — when widespread availability of Covid-19 vaccines will allow the safe return of gatherings and activities shunned for the past year.

But epidemiologists have been warning it might not be a smooth road to that point, and now, data in some states are pointing to, if not just stalled progress, increased cases.

In New Jersey, average daily cases fell below 3,000 in late February and have recently been around 3,800. Michigan saw its average daily infections go from just over 1,000 to more than 2,000 over the same period, and hospitalizations have been increasing for three weeks. Other states, including Minnesota and Missouri, have also seen infections inch up in recent weeks, while many other states’ progress from earlier this year has flattened out, not budging from levels that would have once alarmed the public.

Experts point to at least two key reasons. Governors and mayors are tossing aside restrictions like capacity caps on businesses and, in some states, mask mandates, even as the level of virus in the country — about 50,000 new cases are being reported a day — is not all that far below where it was during parts of last summer, during what was then considered a surge.

The other is that the B.1.1.7 variant, a more transmissible version of the coronavirus first seen in the United Kingdom, is building up in the United States. It could be reaching the point where it’s gained enough steam in some places that it starts to drive outbreaks. Another variant common in the New York City area, called B.1.526, could be buoying cases in that region, though it doesn’t appear to be having national impact.

“This tension between the desire to start opening up and the risk associated with B.1.1.7 is placing us in a precarious position,” said infectious disease researcher Yonatan Grad of Harvard’s T.H. Chan School of Public Health. “It would be great if people could wait a little bit longer until we get higher levels of vaccine coverage.”

Some experts say that it’s too early to know if the current rise in cases in some places will stay on those trajectories; it’s possible that some of these are short-lived blips. But what’s occurring could be the early days of what infectious disease specialists had warned about: localized springtime outbreaks that, while not on the scale of the winter surges, will lead to more people getting sick. The potential of that scenario is adding pressure to accelerate vaccine campaigns.

An open question is whether or to what extent a corresponding rise in hospitalizations and deaths will follow an increase in cases — a feature of the first three waves of the U.S. epidemic. B.1.1.7 seems to be deadlier than other coronavirus strains, but older people and people with other health conditions — those most vulnerable to severe Covid-19 and death — are increasingly being vaccinated.

It’s possible then that the worst effects of potential B.1.1.7-driven outbreaks will be blunted somewhat, though surges could lead to higher hospitalization rates among younger populations still waiting for vaccines. Vaccine campaigns have also been slow to reach people of color, who have been disproportionately sickened and killed by the pandemic. And even with a vaccine rollout clicking into gear — about 40% of people 65 and older are fully vaccinated, according to federal data, and [the vast majority]( 11%2C376 (99.3%25) of,were vaccinated (Figure 1).) of long-term care residents are — about a third of people in that age group haven’t received any dose.

“That is going to be a group that could very, very adversely be impacted by this virus,” said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, who has been more dire than many experts in his warnings about B.1.1.7.

Whether local epidemics grow or shrink depends on what’s called the effective reproductive number — how many additional people each infected person infects on average. If the number is at 1, the curve stays flat; if it’s above 1, epidemics expand.

There are multiple factors right now that are acting as a drag on that number — that would help communities keep the number below 1 and shrink their epidemics. Millions of people are being vaccinated each day, and the global evidence increasingly suggests mass immunization not only protects people from Covid-19, but reduces viral spread. Warmer weather allows for more activities outside and appears to blunt some of the efficiency with which the virus circulates. The country is also nearing 30 million confirmed Covid-19 cases, but the true number could be more than 100 million people having some protection from a previous infection — a big wall of natural immunity.

Still, lots of people remain susceptible to the virus. And together, B.1.1.7 snowballing in prevalence in more places and political leaders tossing off mask mandates and other interventions that reduce contact among people are pulling that reproductive number back up. It’s also spring break.

Think of all those factors like being in a tug-of-war. A yank from one side or the other could bring the number above or below 1. It’s not clear what’s going to win out in every community, though all the counterforces could limit how precipitously cases rise or fall — and help explain why they’ve plateaued for the time being.

The country’s reproductive number as a whole is hovering right around 1, according to the latest estimates from the Children’s Hospital of Philadelphia’s PolicyLab. But the researchers there see ominous signs of growth or frustrating stalls in and around cities in the Midwest and East Coast.

“There is still a significant burden of virus in the community that we need to respect,” said Brian Fisher, one of the CHOP researchers. “Don’t think we’re past the entire risk period.”

There is some concern that what’s occurring in Europe — surges in infections driven in part by B.1.1.7 — is a preview of the U.S. epidemic. The variant’s course in those countries followed by a few weeks a similar pattern in the U.K., and once it took sufficient hold, cases started going up. Countries such as Italy and France have imposed new restrictions to slow the spread.

The fear is that the United States could just be a few weeks behind Europe. The Centers for Disease Control and Prevention has warned B.1.1.7 could become dominant domestically by the end of this month, at which point increases in cases become more likely.