After a post-holiday spike, Massachusetts COVID-19 numbers plummeted at the start of the year, to the relief of public health experts and basically everyone else in the state. But over the last month, that progress has stalled.

Even as the state has ramped up its vaccination effort, the pandemic seems to be showing off its tenacity. Local public health and infectious disease experts say a few key factors have created a plateau in the state’s numbers, and they’re not sure what’s going to happen next.

“It was coming down really nicely, and then over the past couple of weeks or so has kind of stalled, in terms of both daily new cases reported, as well as the percent positivity,” said Dr. Erica Shenoy, associate chief of infection control at Massachusetts General Hospital. “And we just don't know, is the next direction going back up a little bit or will we start to see it leveling and then fall again?”

The state is currently averaging about 1,500 new casesa day, roughly the same as it was in mid-February. And the percentage of COVID tests showing a positive result recently crept back over two percent again, after having dipped below two percent on Feb. 21.

The plateau in COVID numbers coincides with a relaxing of restrictions by the state, including the resumption this week of weddings and other indoor gatherings of 100 people.

Dr. Cassandra Pierre, acting hospital epidemiologist at Boston Medical Center, said the lifting of those restrictions, and more importantly the sense of confidence that gives people, is partly to blame for the leveling off of the downward curve in new cases.

“It's the thought that if something is open, then it must be safe,” she said.

Even if a restaurant is strictly following masking and social distancing regulations, Pierre said, that perception of safety can be dangerous.

“You’re going to have people mixing households at those tables that are in close proximity who are unvaccinated,” she said. “You're going to have people then going to the bar afterwards. You have this sense of safety.”

And that relaxed attitude could lead to another surge, said Sam Scarpino, who directs the Emergent Epidemics Lab at Northeastern University.

“We are so close,” he said. “We're under a month away from the general availability of vaccines for adults. We're a month away from consistently good weather … so that we can have windows open, we can have more of our gatherings outside, we can have outdoor dining. And if we enter into another surge now, it will be completely unacceptable.”

By being too aggressive now about reopening, he said, Massachusetts could wind up just prolonging the pandemic.

The easing of restrictions may not be the only reason COVID numbers have stopped falling, according to experts.

“We're also worried about the rise of more transmissible variants [of the virus],” Pierre said.

So far, the CDC reports Massachusetts has had just over 440 cases of the B.1.1.7 variant of COVID-19, which was first detected in the U.K. But experts say the real number could actually be much higher.

All of the now-familiar prevention techniques — masks, social distancing, etc. — work against the new variants, said Andrew Lover, an assistant professor of epidemiology at UMass Amherst.

“It's just, your margin for error gets smaller and smaller,” he said. “You have to be a lot more rigorous about it.”

For example, CDC guidance for COVID-19 has been that 15 minutes of close exposure is likely to lead to infection, but Lover said with the new variant, that may drop to 10 minutes.

The good news is that while new cases and positivity rates have leveled off, deaths and hospitalizations have continued to decline.

On Tuesday, the average number of deaths was 26 a day, down from 76 in late January. There are about 600 people hospitalized at the moment, down from over 2,400 in the post-holiday surge.

Experts say increasing vaccination is playing a crucial role in keeping those numbers down.

“[For] priority groups, the older age groups, those in skilled nursing facilities who got vaccinated early on … we're reaping some of those benefits right now, where we don't see as much COVID in that population,” Shenoy said.

Younger people are making up an increasingly large portion of the state’s new COVID cases.

“You can see that in the numbers in terms of the age of individuals who are getting hospitalized, which is lower now,” Shenoy said. “It used to be greater than 70 was the average age of individuals getting hospitalized. Now it’s 60 or 61.”

It could be quite some time before vaccinations lead to a significant drop in new cases, experts say. About 15 percent of the state's nearly 7 million residents are fully inoculated, and just under 30 percent of people in the state have received at least one dose of the two-dose vaccines.

“That will not induce community immunity,” Pierre said. “We know that we need much higher levels for that.”

Lover said Israel's experience, which has seen the world’s most aggressive vaccination campaign, shows that it will take time for vaccinations to cut down infection numbers.

“Dec. 19 was their first set of vaccinations, and it's taken just until the last week or two to see a really dramatic drop off in caseloads,” Lover said.

Even as Massachusetts accelerates its vaccination program and makes the vaccine available to the general public beginning in April, it will be several months, at least, before Massachusetts sees that kind of steep drop-off, Lover said.

Still, the vaccination effort, along with the continued vigilance on the part of Massachusetts residents, could be enough to avoid another surge, said Shenoy.

“I think we are on our way out of this. It may not just be as fast as we want it to,” Shenoy said. “Certainly we would have been cheering if this curve looked like [it went] straight down. That was what we are hoping for, just like we had last summer,” she said. “But it just hasn't done that yet.”