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A disturbing new trend is making the coronavirus even harder to control: A rising proportion of infections are occurring at informal gatherings of family and friends.

At the beginning of the pandemic, there were many outbreaks in congregate facilities such as nursing homes and prisons. State and local authorities put into place infection-control measures that have substantially reduced transmission in these high-risk settings.

As the fall school term approached, universities thought that they could effectively implement similar protocols on campus. They instituted safeguards including reduced class size, improved classroom ventilation and daily symptom checks. The problem? Students got infected when they engaged in risky behaviors off-campus. At the University of New Hampshire, 11 cases were traced to a fraternity party hosting about 100 guests without masks. Medical degrees apparently provide no guarantee of safe gathering: A single party resulted in covid-19 infections in 18 anesthesiology residents and fellows training at the University of Florida.

At K-12 institutions, the hard work administrators and teachers are doing to enforce mask-wearing and physical distancing will be undone if kids gather after school without similar protections. The risk extends to parents, and increases as families come together with friends and relatives with whom they are likely to let down their guard.

I spoke this week with several public health officials from different parts of the country who noted this same trend. “In the beginning, we focused our efforts on homeless shelters, jails and workplaces that the city has the ability to regulate and enforce,” said Allison Arwady, the commissioner of the Chicago Department of Public Health. “But over the last few months, only 5 to 6 percent of our cases are being linked to these congregate settings.” She identified the current primary source of spread as informal social gatherings: barbecues, retirement parties and birth celebrations.

Nilesh Kalyanaraman, health officer for Anne Arundel County in Maryland, agrees. He said to me that when his team looks at the data, “We see that family gatherings are coming up as one of the biggest sources of spread.”

Why? “There’s a lot of magical thinking when it comes to covid,” Arwady said. “It’s natural to feel safe among those you know and love.”

Our society’s efforts to restrain covid-19 in formal settings are working — as is public awareness of the need to wear masks and limit interactions outside the home. Retail stores, gyms and hair salons that follow strict protocols are safely remaining open. Companies have improved workplace safety, and people are exercising caution around strangers.

But we have a blind spot. We must now turn our attention to informal settings with those we know.

This requires a substantial shift in our mentality. Half of all covid-19 spread is by people who have no symptoms, and someone you know could just as easily be an asymptomatic carrier as a stranger. If you wouldn’t sit in a room, maskless, with random people, you shouldn’t have an indoor dinner party with close friends, either. We should be just as careful in relatives’ homes as we are in grocery stores and doctors’ offices.

This is not to say that we should never see our loved ones. Rather, we need to recalibrate our perception of risk. I talked to a patient who couldn’t fathom taking a 15-minute bus ride, but regularly hosts groups of friends in her house, without masks, for hours at a time. I told her that seeing them outdoors — or if absolutely needed, indoors, with masks on, at a distance, and with windows open — would help protect not only herself and all her guests, but their friends and families, too.

Policymakers need to take this blind spot into account. According to the American Academy of Pediatrics, infections among children now constitute 10 percent of all coronavirus cases, up from 2 percent in April. It’s tempting to tie this uptick to school reopening, but we should look deeper at the data before shutting down schools. If higher-risk activities are occurring outside of school, parents should be instructed to pause extracurriculars and to stop play dates.

Many cases are linked to indoor bars and restaurants, but shutting down these establishments might not be effective if people instead gather — without precautions — in one another’s homes. Restrictions on informal gatherings are much harder to enforce than those on institutions, but the effort could spare the economic damage of closing businesses.

Our public messaging needs to change. States that are removing restrictions on public settings must emphasize that we continue to face a very contagious virus. Masks, physical distancing and extra caution at private gatherings should to be framed as the cost of getting our economy back. Organizers of large events such as rallies and sports should discuss safety protocols not only for the event, but also the other social activities that occur around it. Businesses and schools should educate people on how to keep safe not only in those settings but at home as well.

All of this requires hard work and constant vigilance. But it can be done. We’ve learned that when we pay attention and implement safety protocols in formal settings, we can reduce transmission. We can do the same in our homes and with our loved ones. As we enter this new phase of living with the coronavirus, the power — and the responsibility — is more than ever in our hands.

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