During spring break, Monica Gandhi tested her confidence in coronavirus science. She and her two unvaccinated children stood in a packed crowd near a well-known bridge in Austin, Texas, to enjoy watching the emergence at dusk of thousands of Brazilian free-tailed bats.

She and her kids were the only ones in the group wearing masks; the state had lifted its mask mandate in early March. But Gandhi, an infectious-diseases physician and professor of medicine at the University of California, San Francisco, had read studies showing it is rare for people to transmit the coronavirus outdoors. And she knew that the regional infection rate was low. So, she decided to take off her mask, as well as those on her 11- and 13-year-old children.

“I was really nervous, but I did it,” she said. “You’ve just got to do it.” Neither Gandhi, who was vaccinated, nor her children got sick in the coming weeks.

Gandhi is one of many health experts encouraging Americans to embrace the outdoors as they make plans for their second summer of the pandemic. The outdoor transmission rate for the coronavirus is very low, and much lower than the indoor rate, several lines of evidence suggest.

A study in Wuhan, China, last year identified only one case of outdoor transmission (the case is thought to have resulted from a conversation between two people, both of whom became infected) among 7,324 infections. In figures released by health authorities by health authorities in Ireland, only 1 in 1,000 coronavirus infections was traced to outdoor transmission.

But it’s a bit of an art form to apply those findings to specific outdoor activities this summer, especially at full-capacity venues and in the context of each individual’s risk for COVID-19. For instance, how should vaccinated parents of an unvaccinated child consider the myriad scenarios and risks associated with, say, a major-league baseball game in San Diego, an outdoor concert at Tanglewood in Massachusetts or the backyard birthday party of a neighborhood friend?

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Finding hard numbers

Some experts, including Gandhi, advise people considering outdoor options to lean heavily on local coronavirus infection rates, COVID hospitalization statistics or vaccination rates. But other public-health and infectious-diseases experts continue to advocate for even more nuanced and individualized calculations that are sensitive to the size and density of crowds, airflow and health factors, such as your vaccination status and the overall strength of your immune system.

Gandhi hopes that people making summer plans look at local public-health numbers that can be found at online coronavirus data dashboards, as she did for her Austin trip. Federal officials have not clearly stated a numerical threshold of daily average cases or COVID hospitalizations to use in making risk assessments, but Gandhi and two colleagues sketched a framework in an essay published recently.

They advised pegging an end to masking and distancing restrictions to COVID hospitalization rates of fewer than 5 daily cases (meaning infections) per 100,000, averaged over several days.

“I think you could do something like 3 cases per 100,000 or 2 hospitalizations per 100,000,” Gandhi said. “You need to come up with a metric and say it’s safe to unmask outdoors even in crowded settings.”

Peter Chin-Hong, a medical professor and infectious-diseases expert at the University of California, San Francisco, pointed to an average daily case rate of two per 100,000 people as a threshold for considering outdoor travel destinations.

“Travelers should look at COVID-19 burden in the proposed destination like they do the weather report,” he said, adding that this data is most relevant for people who are unvaccinated or have suppressed immune systems. The local vaccination rate might be the most telling figure on regional coronavirus dashboards, Chin-Hong said, because it impacts infections and other COVID metrics. Coronavirus infections started to drop this year in California and Israel when the percentage of vaccinated adults reached about 50%, he said, suggesting that figure as a threshold.

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Mask safety and signals

Vaccination is a bottom line for all mingling considerations, including those outdoors, said Gregg Gonsalves, an public health researcher at the Yale School of Public Health. “If you’re vaccinated, it’s really hard to think of a reason to dissuade you from outdoor activities this summer — even crowded ones, say a big wedding — if everyone is vaccinated,” he said.

Nonetheless, he plans to wear a mask at places where people are in close contact for hours — say a roof party in Brooklyn or a day at the beach this summer — if the group includes people who have yet to be vaccinated or if he doesn’t know everyone’s vaccine status.

Such behavior would be more stringent than this past month’s updated guidance from the Centers for Disease Control and Prevention stating that vaccinated people no longer need to wear masks in nearly all settings. But Gonsalves said he would do so to show support for mask-wearing among unvaccinated people.

For travel, he recommended checking local vaccination rates and looking out for surges in infection rates to gauge the risk of encounters with unvaccinated people infected with the virus. “If you’re in a part of the country where you have 30% or 40% vaccination, instead of where you have 60% or 70% vaccination, you’re more likely to come in contact with somebody who is potentially positive and shedding virus,” he said, adding that traveling families with unvaccinated kids and/or adults should continue to wear masks outdoors this summer.

People who are vaccinated and have no underlying health conditions probably need not worry about visiting theme parks or attending a concert or baseball game in an undomed stadium, said Linsey Marr, an engineering professor at Virginia Tech who has expertise in how viruses move in air. (Stadiums with domes or roofs can trap air, thus increasing the risk of viral spread.)

However, in hot spots with COVID vaccination rates below 30% or 40% or where dashboards show more than an average of 10 daily infections per 100,000 people in a seven- or 14-day period, Marr said she would wear a mask in a crowded outdoor setting, despite being vaccinated.

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And unvaccinated children and adults should mask up while standing in lines, where people tend to clot for long spells, Marr said. She would want her two children, one of whom is partially vaccinated, to wear masks at a theme park any time they came within 6 feet of others; she would don hers too, in solidarity.

With vaccines anticipated to be authorized for use in American children under 12 no sooner than late this summer, decisions about outdoor activities with groups of children are more complex. Hold children’s parties in a park or big yard, if possible, and keep them small, Gonsalves said. A party of 30 kids with 50 or so parents, some of whom are unvaccinated, would make him wary. Marr said that bounce houses at outdoor children’s parties should be limited to half their capacity.

Blurred lines

In November, Nooshin Razani, an associate professor of epidemiology and biostatistics at the University of California, San Francisco, was an author on a systematic review of published outdoor virus-transmission studies that has become influential among policymakers. Her research turned up findings of respiratory diseases spreading at big gatherings that took place largely outdoors but also included indoor dining and indoor overnight stays.

“When we talk about indoors versus outdoors, it’s not very binary actually,” Razani said.

Absolute statements about outdoor coronavirus risks this summer trouble Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. Many people incorrectly assume that outdoor activities are entirely free of risk when it comes to the coronavirus, he said. And he opposed advising people to make outdoor plans based on local coronavirus infection, COVID hospitalization or vaccination numbers, because individual and community circumstances vary too much.

“If you’re going to a family reunion, you can look at the rates of infection in your community,” he said. “But at a lot of big outdoor events, if you have a person coming from an outside area, you have no idea where they are coming from in terms of risk.” He thinks some coronavirus transmission that happens outdoors is unreported or never traced to its outdoor origin, unlike a cluster of cases traced to the 10-day motorcycle rally in August in Sturgis, South Dakota, or those thought to have spread in July at a concert in Minnesota.

People at an increased risk for COVID — be that from being unvaccinated, an underlying condition, or a suppressed or weakened immune system — should wear an N95 respirator mask if they are stationary in a big crowd outdoors for hours, he said. Those who are not young, healthy, vaccinated adults should still avoid large, packed crowds, especially for extended periods of time, as well as any areas where air is trapped.

At this point in the pandemic, people have come to crave clear lines around coronavirus safety issues. “And unfortunately, this is not Oz,” Osterholm said, referring to the land where wizards and witches can know all. “There is not a simple answer.”