Encouraging trends emerge in Mass. coronavirus data. But experts say it’s no time to celebrate.

Experts say the way out of the pandemic is to get vaccinated.Lane Turner/Globe Staff

Encouraging trends have emerged in Massachusetts coronavirus data, including a decrease in cases and a downtick in hospitalizations. Will the unpredictable virus, which has killed more than 18,000 people in the state so far, continue to wane or make a deadly comeback this fall and winter?

The declines in coronavirus metrics here align with a national trend. “We certainly are turning the corner on this particular surge,” Dr. Anthony Fauci, President Biden’s chief medical adviser said Sunday on ABC-TV’s “This Week.”

But experts say it’s too soon to pop open the champagne. “I’ve learned not to declare victory over this tricky virus ... it has surged back several times before,” Dr. Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital, said in an e-mail.

“We’re on track, but let’s not let our guard down right now,” said Dr. Sabrina Assoumou, an infectious disease physician at Boston Medical Center and professor at Boston University School of Medicine.

Both emphasized the importance of persuading more people to get their coronavirus shots in a state that is already a national leader in vaccinations. “This is when we should vaccinate, vaccinate, vaccinate,” said Assoumou.

Some pandemic models are suggesting cases will rise again dramatically as winter approaches and the highly contagious Delta variant attacks those who are unvaccinated, people gather inside in cooler weather, and immunity wanes among those who were vaccinated months ago. The good news, the Globe reported last week, is that hospitals are not expected to be overwhelmed, due to the state’s high vaccination rate.

Here’s a look at how we’re doing right now:

Cases

With the arrival of the Delta variant, cases began to rise from rock-bottom lows in July, reaching a seven-day average of 1,849 a day on Sept. 17. As of Monday afternoon, the number had fallen to 1,306 a day.

Cases overall

Zoom out and you can see that the recent case increases did not reach the level of the previous two major surges or a smaller rise in cases in March and April that some worried might become a third surge.

Hospitalizations

Hospitalizations also saw a peak around mid-September. As of Sept. 13, 707 people were in the hospital with COVID-19. On Monday, the number was 581.

Hospitalizations overall

Here’s a look at hospitalizations since the beginning of the pandemic. Note the two surges, the bump this spring before the summer lull, and the current increases, which appear to be edging down, for the moment at least.

Deaths

The seven-day average of coronavirus deaths reached rock-bottom lows in July when, on some days, zero deaths were reported. But the Delta variant wasn’t done with its deadly work. On Monday, the seven-day average was 15 a day. Note that death numbers tend to lag behind case and hospitalization numbers so decreases in those metrics may not be reflected in this chart yet.

Test positivity

The percentage of coronavirus tests that come back positive has also been dropping. The chart below shows the positivity percentage since the summer of 2020 with the effect of college testing programs factored out. College testing programs dilute the data because they repeatedly test large numbers of asymptomatic people in an effort to rapidly identify new cases.

Test positivity climbed to 4.01 percent on Sept. 9. As of Monday’s report it was 2.74 percent.

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Experts say that the coming months in Massachusetts will be a test of whether a highly vaccinated state can survive a winter without a deadly surge like the one the state suffered last winter.

More than 5.1 million of the state’s approximately 7 million residents have gotten at least one shot of the two-shot vaccines from Pfizer and Moderna or of the one-shot vaccine from Johnson & Johnson. Pfizer booster shots are being administered to ensure people’s immunity remains strong, while Moderna and Johnson & Johnson are in the process of seeking booster approvals. Pfizer is also hoping for approval of a vaccine for children 5 to 11, a large group currently ineligible for shots.

But “we know that this particular time is a critical time where cases tend to go up,” Assoumou said.

With the colder weather arriving, “We’re not going to be able to spend as much time outside, and we know that spending time outdoors is a great way to reduce transmission,” she said.

The “back-to-back” holidays in the fall and winter also pose a threat of increasing virus transmission as people gather to celebrate, she said.

In addition to getting their shots, people should practice mitigation measures such as masking in indoor public spaces, improving ventilation, and practicing physical distancing, she said.

“These trends are encouraging but we have seen this movie before,” Dr. Howard Koh, a former high-ranking federal and state public health official who is now a professor at the Harvard T.H. Chan School of Public Health, said in an e-mail. “Letting down our guard now will only allow the virus to mutate and live on yet again.”

Dr. Shira Doron, an infectious disease physician who is the hospital epidemiologist at Tufts Medical Center, recalled the hopes that sprang up as the virus subsided in the summer of 2020 and then again in the summer of 2021. “I’ve learned my lesson,” she said. “We never know what the virus is going to bring us next.”

She said she was less worried about people gathering for the holidays, pointing out that with restrictions lifted, people have already been gathering, anyway. She was more worried, she said, about the fact that the seasonality of the virus is not well understood, making it possible that some unknown factor could cause it to rear its head again at the same time that immunity from vaccines and natural infections are waning.

The upshot is “total unpredictability,” she said.

In the months ahead, “Obviously, everyone should be vaccinated. No question about that,” she said.

“If you’re recommended to get a booster by current CDC guidelines, you should. If you’re permitted by CDC guidelines, you should consider it, because it’s really safe,” she said.

“If you’re not vaccinated,” she said, “you should be taking a lot of precautions - because this virus is going to find you.”

Felice Freyer of the Globe staff contributed to this report.