Expert outlines 5 possible reasons the pandemic’s weakening — and why we need to ‘stay humble’

A dose of vaccine being prepared. Vaccinations could be part of the reason why the pandemic is on the decline.Pat Greenhouse/Globe Staff

A Brigham and Women’s Hospital infectious disease specialist says there are five factors that could be behind the weakening of the coronavirus pandemic in Massachusetts and across the nation, but it’s not clear which factor, or combination of them, is responsible.

Dr. Paul Sax, clinical director of the Division of Infectious Diseases at the Brigham, laid out the factors in a column Sunday on the New England Journal of Medicine’s Journal Watch website.

“I was motivated to write it because I saw a lot of discussion going on, a lot of speculation” about the reasons for the decline, he said.

“I thought it was important that we acknowledge that we don’t really know that it’s any one thing, and that it could be multiple things, and it’s important for us to have humility when we’re seeing good news.”

“What I would hate to happen is that we rely on one of the causes of why case numbers are dropping [and let our guard down], and we end up being wrong and the virus surges back,” he said in a telephone interview.

Here are the five factors Sax outlined:

1. The virus could be seasonal. Other coronaviruses have shown seasonal patterns, and “various viral diseases go through communities synchronized with the seasons, especially when school starts or the weather gets colder. Any pediatrician will tell you that,” Sax wrote.

2. Herd immunity could be kicking in. While the Centers for Disease Control and Prevention says there have been nearly 28 million recorded cases of coronavirus in the United States, the agency has also estimated that only 1 in 4.6 COVID infections are reported. “That could bring us up to half the US population with some degree of natural immunity to infection,” Sax wrote. Herd immunity is a situation in which so many people have immunity to a disease that the disease has difficulty spreading.

3. People’s behavior. Sax pointed out that the winter holiday season, a time when public health officials issued dire warnings about indoor gatherings, is behind us. He also noted a hypothesis that the people least likely to follow advice — or unable to follow it due to their work or living situations — may have already had the virus. Meanwhile, those who have “the luxury of staying safe” watched the winter surge and may have stepped up their vigilance “now that a vaccine is in their not-too-distant future,” he wrote.

4. Vaccines. Vaccination is underway around the world, with most places “targeting the people most likely to have symptomatic or severe disease,” Sax wrote. He said that “the data increasingly suggest that the vaccines reduce not just disease, but also the likelihood of transmission.” He noted that vaccinations, which are ramping up, are not yet widespread enough to explain the case declines alone but said they “might be contributing.”

5. The virus itself. Sax raised the possibility that the virus is becoming “less virulent over time.” Perhaps some variants, he said, “in order to gain the ability to transmit, also cause less severe disease.” (He noted that may not be case with the UK variant, which some studies have suggested may cause more severe disease.) “Take the virus’s perspective — yes, think like a virus — and how this would be evolutionarily beneficial. More mild cases, more chance to spread its genetic material to other susceptible hosts. That’s all viruses care about, right?” he wrote.

The most likely explanation, Sax wrote, is a “gemish” — Yiddish for a mixture of things.

Why are Covid19 case numbers dropping? It's probably multi-factorial -- in other words, a gemish* -- and pretending otherwise might get us into trouble. Because there's a lot about this virus we still don't know. Latest post:https://t.co/if8MEG9SpU *word defined in the post

— Paul Sax (@PaulSaxMD) February 21, 2021

“It could be all of the above explanations, in various proportions, and different in various regions — plus things no one has considered,” he wrote.

“It’s a fascinating question, and I hope we get the answer to it,” he said in the interview. “It’s really important that you stay humble with this disease.”

The pandemic, which has killed more than 500,000 people in the United States, has been on the decline for five weeks, CDC Director Rochelle Walensky said Monday, though officials are warning of the arrival of fast-spreading coronavirus variants, saying they could fuel another surge.

Public health officials have urged people to continue to wear well-fitting masks, stay 6 feet apart from people who are not part of their households, wash their hands, avoid travel and crowds, and get vaccinated when it’s their turn.