Coronavirus cases are rising in Mass. How high will they go?

Maria Morataya smiles after receiving her second Covid-19 vaccination at Gillette Stadium on June 14, 2021.Christiana Botic for The Boston Globe

The number of coronavirus cases detected in Massachusetts is sharply increasing. Why now? And how high will they go?

Experts say they expected increases as the weather got colder and people began gathering inside, where the virus can circulate more easily.

“In winter, we get indoors, all of us. We feel safer indoors so we let down our guard and take down our masks,” said Ali Mokdad, a professor at the University of Washington Institute for Health Metrics and Evaluation.

But there are other reasons for the increases as well. Those include Massachusetts having a lower number of previously infected people who have some degree of protection from natural immunity, and the waning protection offered by vaccines, Mokdad said. Massachusetts may have done a good job getting residents vaccinated, he said, but now they need to get their boosters.

“The booster uptake has not been quite as high as we hoped,” said Andrew Lover, an assistant professor of epidemiology in the School of Public Health and Health Sciences at UMass Amherst.

In Massachusetts, COVID-19 case totals have gone up more than 150 percent and hospitalizations have nearly doubled in the past several weeks. Deaths have not risen as sharply, the Globe reported Thursday. The increases have come even as people have been worrying about the potential impact of the newly detected Omicron variant. Currently, the Delta variant is estimated to account for nearly 100 percent of US cases.

How high will cases in Massachusetts go? The IHME’s closely watched model currently suggests that case numbers will rise by about another third by the end of January. But Mokdad said modelers haven’t yet fully factored in the waning protection provided by vaccines — and when they do, the predicted numbers will likely be even higher.

Lover said it was possible that Massachusetts cases could go up by another 50 percent from current levels.

“This is the reason that everyone is very concerned. Everyone had assumed we’d be in a very different place than last year and things look pretty comparable,” he said.

He said cases could follow the same trajectory as last year, when they ramped up after Thanksgiving and the winter holidays. “It’s hard to imagine a similar pattern not happening,” he said.

Matthew Fox, a professor of epidemiology and global health at the Boston University School of Public Health, said he wouldn’t venture an opinion on how high cases would go, calling it a “complete unknown.”

But he said believed the increases wouldn’t be as bad as the state has previously seen.

“We’re clearly going to go through a surge, but I don’t think it’s going to be as bad as any of our previous surges,” he said.

He said the warm early fall may have delayed the increases, but the Thanksgiving holiday took what was already an increasing caseload and accelerated it, and we’re “likely to see the same thing happen over the winter holidays.”

Mokdad urged people to make sure they get their booster shots. “We vaccinated on purpose early on people who are at high risk. We gave them priority,” he said. With immunity from vaccinations waning, “Unless we rush and give them a booster, we’re going to be in a bad position.”

He also suggested that people be authorized to get the booster three months after their original vaccination series with the two-dose Pfizer and Moderna vaccines, rather than six months after, which is the current recommendation from the federal government. (Those who received the one-dose Johnson & Johnson vaccine can get their boosters two months after their single shot.)

The current vaccines and boosters offer protection against severe disease and death from the Delta variant, he said. But, he said, Omicron poses an unknown threat, potentially increasing cases as well as severe disease.

“With Omicron, we don’t know how much the effectiveness of the vaccine will go lower. We know it will, but not how much,” he said.

“What we are hearing right now from our colleagues in South Africa is vaccine effectiveness went down and cases are going up and hospitalizations are up. It’s not only that more people are getting infected, but Omicron is sending more people to the hospital,” he said.

There are also indications that people who have been previously infected with the coronavirus are getting reinfected when they encounter Omicron, he said.

“Go get your booster,” he said. “Upgrade your masks. Be careful when outside your house. Let’s be responsible before the holidays. Let’s be careful and protect each other.”

Lover said, “At this point, we really just don’t have enough information to know how big the potential threat is” from Omicron. He said Omicron looks “very, very different” and there is concern that the vaccines developed for the original coronavirus strain, which emerged in China, will not provide the same protection.

He said people should “definitely get vaccinated and definitely get boosted as soon as you can and ensure that everyone in your family who’s eligible gets vaccinated.”

He said people should also consider minimizing or altering holiday travel plans “at least until we have better information on Omicron. It’s definitely here and we don’t really understand anything yet.”

Fox said Omicron is “a big wild card. We do know that the vaccines are pretty effective against Delta. …. That’s still an unknown for Omicron.”

One indicator that experts say could predict the trajectory of cases continued to show worrisome increases on Friday. The MWRA wastewater numbers, which have been zigzagging and recently turned sharply upward, continued to climb toward levels reached during last winter’s surge.

Dr. Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital, noted on Twitter Friday the increases in Massachusetts coronavirus metrics and urged people to get vaccinated — and boosted — and take precautions.

Hey Boston/MA, take a look at these data (h/t @SDrydenPeterson) -- covid tracking like last year, sharp ⬆️ in wastewater, cases, inpatients.

- vax all eligible
- 3rd dose if 6 mos since #2 (mRNA if J&J)
- mask indoors
- improve ventilation
- frequent use of rapid home tests pic.twitter.com/ebAIX1HdPQ

— Paul Sax (@PaulSaxMD) December 3, 2021

Ryan Huddle of the Globe staff contributed to this report.