COVID-19 is a highly transmissible disease, but evidence shows that small indoor gatherings and households are where the novel coronavirus is spreading the fastest.
However, there are some settings where COVID-19 is more easily spread. In New York, for example, contact tracing has shown that 70% of new cases come from small gatherings and households.
"Informal gatherings may have played even the biggest role," Brownstein said, "because they are harder to police, they're harder to enforce, and people are probably more lax when it comes to recommendations of mask wearing and social distancing."
When people gather in small groups with friends and family, they are more likely to let their guard down, not wear their masks and stay together indoors for longer periods of time, which makes it easier to transmit the virus.
In a recent study at the University of Mississippi Medical Center, researchers found that for children and adolescents who tested positive for COVID-19, it was small social gatherings -- not school -- that was the most likely place they were exposed to the virus.
The children who tested positive in the study were more likely to have attended social gatherings outside of their homes, had playdates or had visitors at their home where mask wearing and social distancing precautions were not taken.
Early in the pandemic after initial lockdowns were eased and cases began to rise, contact tracing also linked the spread of the virus to restaurants and bars.
In one study published by the Centers for Disease Control and Prevention, people who eventually tested positive for COVID-19 were twice as likely to have eaten at a restaurant in the past two weeks compared to participants who did not test positive for the virus.
"The obvious challenge is that you lose an important control, the mask wearing," said Dr. Joseph Allen, an assistant professor of exposure assessment science at the Harvard T.H. Chan School of Public Health. "Since you have a restaurant with many people who are talking loudly [with] masks off, that leads to higher emission rates of respiratory aerosols, and, depending on how that ventilation system is working in the restaurant, determines how many infectious aerosols people are breathing."
Similar to small home gatherings, people at restaurants are often eating with people who aren't in their immediate household and not wearing masks, and they are in a confined space with poor ventilation.
"Gyms have generally done a good job of adhering to protocols," Brownstein said. "We don't see a lot of super spreader events related to gyms ... because the protocols they had to put in place regarding social distancing and mask wearing and ventilation have generally been pretty good."
Schools are another setting where typically many people are in an enclosed setting. Educational institutions have grappled with decisions on whether to do in-person classroom instruction vs. remote learning to reduce transmission.
But Brownstein said schools have generally been safe. "Of course there are outliers," he added, "but we've seen good evidence that schools have spent time to develop a protocol where social distancing and mask wearing have done relatively a good job."
There is also a lot of guidance with respect to ventilation at schools that makes the environment safer, Allen said.
Ultimately, when it comes to what drives high transmission rates in both homes and restaurants, it's "the commonality in the underlying factors," Allen said. "Time indoors, no masks, low or no ventilation."
For weeks, experts have warned about the surge of new infections that would come with Thanksgiving travel. We are seeing that now. The impending surge from travel and small indoor gatherings during the longer holiday week between Christmas and the New Year could not come at a worse time, the experts warn. However, certain measures can help.
"There is no one silver bullet when it comes to interventions. It's sort of a layered approach," Brownstein said. "Social distancing and mask wearing ... indoor ventilation, limitations on gatherings ... If people are adhering to these general public health guidelines in sort of the broad sense, they've been really successful at driving transmission down."
Adjoa Smalls-Mantey, M.D., D.Phil., is trained in immunology and is a psychiatrist in New York City. She is also a contributor to the ABC News Medical Unit.