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Homeless people who gather in outdoor encampments were less likely to test positive for SARS-CoV-2 compared to those housed in shelters, researchers reported.

The rate of SARS-CoV-2 positive tests among 650 people living in shelters in the Denver area was 9%, compared with a 3% positivity rate for 281 people living in tent encampments in the area (P<0.01), reported Sarah Rowan, MD, of the University of Colorado and Bernard F. Gipson Sr. Eastside Family Health Center in Denver, in a presentation at the .

When testing for SARS-CoV-2 antibodies -- a measure of previous infection -- the researchers found that 24% of shelter users were positive compared with 8% of those in encampments (P<0.01). Although Rowan said there was no difference in antibody positivity by race, ethnicity, or sex, the people who tested positive were about 54 years old, while the average age of those who tested negative was 43 years (P<0.01).

"Transmission of SARS-CoV-2 at outdoor encampments may be less common than in shelters," Rowan said. "Continued assessment of mitigation strategies in shelters should be a priority, particularly as shelters densify in cold winter months." The study was performed in July and August, and Rowan suggested the numbers of homeless people might be on the upswing with about 10% of people in Colorado being out of work.

Previous studies and infection-control guidelines indicate that the degree of fresh air flow correlates inversely with risk of contracting COVID-19 -- i.e., the risk is very low outdoors but higher when people are confined in small, poorly ventilated indoor spaces.

Rowan noted that homelessness affects more than half a million people in the U.S.; in Denver the homeless population is estimated to be about 6,000.

She said about one-third of homeless people live in encampments. In Denver, people who show up at temporary shelters -- arenas dedicated to housing the homeless during the pandemic -- are screened upon entrance to the shelters for symptoms and if they have symptoms they are referred for testing. Those with symptoms (fever, cough, or shortness of breath) or who test positive for SARS-CoV-2 are housed in city-rented motel/hotel rooms.

Rowan and colleagues conducted the study at three outdoor encampments and four homeless shelters. During her presentation, Rowan did not address how the study might apply in winter weather, which has already hit the Denver area this week.

In commenting on the study, Robert Glatter, MD, of Lenox Hill Hospital/Donald and Barbara Zucker School of Medicine at Hofstra/Northwell, New York City, told MedPage Today, "Outdoor encampments, weather permitting, offer advantages when trying to mitigate spread of COVID-19. The very fact that they are outdoors reduces risk of transmission, as wind currents and the value of physical distance have a greater impact on reducing transmission."

"The results of the study are impressive and reinforce the added safety of outdoor encampments regarding risk of transmission and infections compared with standard indoor shelters," Glatter said. "We know that outdoors is always preferred over indoor settings, and the study clearly supports this."

In shelters, he said, wearing masks should be mandatory. "Since we know that up to 40% of transmission may occur asymptomatically, wearing a mask is essential inside any shelter, even if physical distancing guidelines are enforced," he said. "We know that airborne transmission by aerosols is a major risk inside such shelters, making ventilation, filtration, along with limits on duration of time spent in a particular indoor space," important to control spread.

He added that mask wearing can help control the spread of SARS-CoV-2 even in areas with adequate ventilation, filtration, spacing, limits on numbers of persons, and duration of exposure. "A mask remains the most important form of source control, catching the majority of droplets and viral particles." he said. "Aerosols may linger in closed spaces for up to 3 hours and scatter up to 20 feet away, making source control [masks] the most critical action to reduce transmission."

Glatter noted that the situation in shelters could become more difficult as winter approaches and encampments become less feasible.

"In addition, the role of relative humidity as a factor in spread of COVID-19 is also important inside any public building such as a shelter," he said, citing humidifiers as a relatively simple intervention. "Increasing relative humidity to 40%-60% can increase the probability that transmission will be reduced, chiefly the result of droplets gaining excessive weight from higher levels of humidity and falling to the ground."

Rowan noted that her study had several imitations, including that it was an observational study requiring voluntary participation, and measured "one point in time during a rapidly changing epidemic." The study is ongoing, she said.

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