‘The pandemic is not over’: In a worrisome trend, COVID-19 cases are rising again in Mass.

Yeriza Rodriguez prepared a Pfizer pediatric COVID-19 vaccine for an Ector County ISD student at Zavala Elementary School in Odessa, Texas.Jacob Ford/Associated Press

Massachusetts coronavirus case numbers have lurched upward in recent days, in a worrisome development that experts say underlines the need for people to get vaccinated and to take precautions such as wearing masks in indoor public spaces.

Case numbers began rising from rock-bottom levels in early July, hit a peak in mid-September, and then declined until early this month. But in the past week or so, they began to rise again. The seven-day average of reported cases was 1,182 on Nov. 3. A week later, as of Nov. 10, it had jumped to 1,481.

With coronavirus cases nationally plateauing at a relatively high level, experts have warned of the possibility of a winter surge caused by multiple factors, including people gathering indoors because of colder weather, holdouts refusing to get vaccines, and the waning of immunity from the shots.

Coronavirus outbreaks in Massachusetts have recently made headlines, including one that forced the closure of Boston’s Curley K-8 School and one that hit the Essex County jail in Middleton.

The latest numbers are a reminder that “it’s not over till it’s over,” said Dr. Howard Koh, a former assistant US secretary of health and human services and Massachusetts public health commissioner.

“These state trends are disconcerting, but not surprising, as national declines in COVID cases have stalled in recent weeks. We need to be extra-vigilant and careful as the winter season approaches. We must push the state’s vaccination rates even higher, resist suggestions to drop mask requirements too early, and eliminate disparities,” said Koh, who is now a professor at the Harvard T.H. Chan School of Public Health, in an e-mail.

Dr. Sabrina Assoumou, an infectious disease physician at Boston Medical Center and an assistant professor at the Boston University School of Medicine, said, “We definitely need to pay attention” to the new numbers.

“We sort of knew this was going to happen,” as colder weather causes people to spend more time indoors, she said. “We should use it as a warning sign to double down on those measures we know have worked.”

“The best way we can address this,” she said, “is vaccinating as many people as possible so we can stop transmission in the community. Vaccination is still our best way out of this pandemic.”

She said the recent authorization of COVID-19 vaccines for children 5 to 11, a cohort that numbers 515,000 in Massachusetts, would be a major help in combating the pandemic. “We now have a new group of people who are eligible. Let’s use that, and let’s vaccinate them,” she said, adding that it is also important to push up the number of teenagers 12 to 17 who have been vaccinated.

She also said, “We need to decide whether this is a time to encourage more masking in indoor public spaces. It’s probably time to think about that.”

Other key measures to address the pandemic include improving ventilation and the use of rapid at-home testing, she said.

Society has tools to address the pandemic and “these numbers are reminding us that we really need to use them as best we can,” she said.

Dr. Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital, said, “We’re watching case numbers closely, as most northern states [like ours] are showing an increase. Much of this is due to cases rising rapidly in children, teens, and young adults, who both have lower rates of vaccination and are more likely to engage in activities that facilitate spread of the virus. This is especially true with colder weather bringing people indoors.”

“I do think this is a real increase, and we always worry about the vulnerable populations when cases start going up,” he said in an e-mail.

Dr. Gabriela Andujar Vazquez, an infectious disease physician and associate epidemiologist at Tufts Medical Center, said case counts might also be going up due to more people seeking tests now that the weather is colder and they’re getting symptoms of various respiratory diseases. She also suggested the numbers might be affected by people seeking out tests before traveling for the holidays. “Every time you have a bigger number of tests, you’re also able to detect more,” she said.

She emphasized the importance of getting vaccinated. “Vaccination is basically the No. 1 thing that will help everyone ... to interact and celebrate in the upcoming weeks with friends and family.”

Massachusetts is a national leader in getting people shots, and experts say the state will fare better this winter than last winter — and better than other less-vaccinated states. The state says more than 81 percent of the total population has received at least one dose.

“I expect that the number of hospitalizations, severe illnesses, and deaths will be lower this winter season as compared to last winter season,” said Andujar Vazquez.

William Hanage, an epidemiology professor at Harvard’s Chan School, said, “We are probably in one of the regions of the country which has the least cause for anxiety.”

But he said people should be sure to get their vaccinations and booster shots. “The pandemic is not over. People should behave responsibly,” he said.

He also warned, “It’s really important to note that the Delta variant will find you. You’re not going to be able to dodge Delta. What you want to do is make sure you have the best preparation.”