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What’s behind the dip in coronavirus cases? We ask specialists

Medical assistant Suleika Nunez peered out to see if there's a line at the NEW Health COVID-19 testing site in Charlestown.
Medical assistant Suleika Nunez peered out to see if there's a line at the NEW Health COVID-19 testing site in Charlestown.Jessica Rinaldi/Globe Staff

After a dramatic post-holiday surge in COVID-19 infections, Massachusetts seems to have found something of a reprieve: Throughout most of January, daily case counts fell steadily, marking the longest period of transmission decline since late spring.

But the drop in cases, however welcome, does not necessarily mean the worst is behind us, disease experts said. And it is unclear precisely why cases are dropping, though more cautious behavior and state-imposed restrictions on restaurants and other gathering spaces could be playing a role.

The scientists also warned that much remains in flux — emerging coronavirus variants, a halting vaccine rollout, people’s continued exhaustion with and resistance to pandemic rules, and redoubled efforts at reopening workplaces and schools. Any of these, they said, could undermine the progress the state and country have seen in recent weeks.

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“It’s fabulous that it is [declining],” said Dr. Ashish Jha, dean of the Brown University School of Public Health. “What’s interesting is, it’s happening across the country so it’s not so easy to point to one state’s policies [as the cause].”

Massachusetts has seen a 59 percent decrease in its seven-day average case count, to 2,543.4 on Sunday, down from an all-time high of 6,242.3 on Jan. 8. The United States as a whole also saw a drop-off over that period, with average daily cases falling nearly 40 percent.

Individuals may have changed their behavior in the wake of the most recent surge, doubling down on mask-wearing and reducing their travel and number of contacts, Jha said.

“There’s been a bit of this throughout the entire pandemic, that when things get really bad, you do see a pulling back of activity,” he said. “Given all the concerns and all the news about how badly the hospitals are stretched, you see people pull back and be more careful.”

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Epidemiologists said it is probably too early to see large-scale effects of vaccination tamping down the virus, given how few people have been immunized thus far.

But Jha suggested that the large number of people who have some level of resistance to COVID-19 infection, whether through vaccination or previous infection, could have played a role in blunting the December surge. In Massachusetts, that population makes up an estimated 23.8 percent of the state’s residents, according to models by Youyang Gu, an independent data scientist whose work Jha follows.

Jha warned the estimate still falls far short of so-called herd immunity, the point at which the virus can no longer spread widely. While the exact threshold for COVID-19 remains unknown, researchers estimate it is at least 70 percent. For now, “we still can have big outbreaks,” Jha said.

Vaccinations could be helping slow COVID-19 transmission among one population, said Samuel Scarpino, a Northeastern University epidemiologist. Long-term-care facilities, where residents and workers have begun to be vaccinated, have been tied to a reduced number of COVID-19 clusters in recent weeks, based on state data analyzed by Scarpino and postdoctoral researcher Brennan Klein, both of the Network Science Institute at Northeastern.

For the past four weeks, the state reported 74 new clusters — defined as two or more confirmed cases with a common exposure — tied to long-term-care, compared to 122 clusters reported for the four-week period from Nov. 29 to Dec. 26.

“I’m hesitant to say there’s any evidence that things are slowing because of the vaccines [in the general population], but it’s certainly something that we want to watch really closely,” Scarpino said.

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He added that the data revealed another site of slowed transmission: restaurants.

Governor Charlie Baker on Dec. 8 imposed additional restrictions on indoor dining, reducing capacity to 25 percent, limiting groups to six people or fewer, and requiring masks whenever patrons are not actively eating or drinking. Scarpino said he believed those steps helped reduce spread at restaurants, which have seen 61 cluster-related cases over the past four weeks, down from a high of 249 cluster-related cases reported for Nov. 22 to Dec. 19.

Continued restrictions and vaccination can help keep the worst of COVID-19 behind us, Scarpino said, but he and others warned against taking continued decline in transmission for granted.

“I really would advocate for us being a little bit more cautious to make sure that things aren’t going to start ticking back up again as people returned to work,” Scarpino said.

Experts agreed that despite having cause for hope, Massachusetts and the country still face a number of variables that could cause the pandemic to take another turn for the worse. People who were more cautious could let down their guards as cases decline. Some businesses that closed or reduced operations over the holidays have now reopened. The pace of vaccine distribution remains slow and uneven, and variants of the coronavirus threaten to further complicate the path to slowed transmission and herd immunity.

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“I am concerned that we are still spending too much time in the weeds of the moment and not enough planning for potential serious challenges,” Dr. Sarah Fortune, professor of immunology and infectious diseases at Harvard T.H. Chan School of Public Health, said in an email to the Globe.

Several variants have been discovered in recent months. Massachusetts reported its first cases of one more-contagious variant, originally identified in the United Kingdom, last month. Another variant first detected in South Africa and now reported in South Carolina and Maryland is also of concern, with several available vaccines offering less protection against that particular strain. Scientists warn that other variants are likely to emerge, some of which could be more transmissible or deadly.

“There is a lot of viral genetic diversity out there for natural selection to act upon,” Fortune said.

Maintaining the safe behaviors Massachusetts residents have displayed over the past several weeks will be key going forward, said Dr. David Hamer, an infectious disease expert at Boston University and a physician at Boston Medical Center.

“There’s been more of a focus on social distancing and mask use, and so the potential for transmission in public places is reduced,” Hamer said. “I think that if people continue to adhere well, to do testing, isolate, do contact tracing — all the things that are happening to try and reduce spread — [declines in cases] will continue.”


Dasia Moore can be reached at dasia.moore@globe.com. Follow her on Twitter @daijmoore.