Since early on in the pandemic, public health officials have emphasized that the risk of getting COVID-19 is lower outdoors. In fact, outdoor transmission of COVID-19 has been rare with previous strains of the virus, according to a number of studies.
But as the hypercontagious Delta variant continues to drive up cases in parts of the country, many have begun to wonder if this strain is also more transmissible outdoors, particularly as Labor Day approaches and many Americans are planning outdoor gatherings.
Some government officials have already started to take precautions. On Thursday, Seattle and King County health officer Jeff Duchin issued an order requiring everyone — regardless of vaccination status — attending large outdoor gatherings to wear a mask. A King County executive said the order, which will take effect next Tuesday, is necessary to further prevent the spread of COVID-19.
Dr. Diego Hijano, an infectious disease specialist at St. Jude Children's Research Hospital in Memphis, told Yahoo News that Delta’s outdoor transmissibility has not yet been quantified.
“It's not something that we can determine at this point," he said. "We can’t say, ‘Oh, the Delta [variant], outdoors, is going to be X times more contagious.’”
However, he said recent studies have shown that Delta is much more transmissible than any other version of the virus, leading some experts to believe the risk of outdoor transmission could potentially be greater too.
Delta’s ability to spread has accelerated the pandemic in the U.S. and many other countries. The variant first surged in India in March and April 2021, and soon became the predominant strain of the virus there and then in Britain.
By the end of July, Delta was already responsible for 80 percent of new U.S. COVID-19 cases, according to the Centers for Disease Control and Prevention. The agency says the variant is now the predominant one in the country, accounting for 99.1 percent of all infections nationwide.
Yahoo News Medical Contributor Dr. Kavita Patel said one of the reasons Delta seems to be more transmissible is that the strain “reproduces in people's nasal passages faster — up to 1,200 times higher — than previous coronavirus strains.” That higher viral load makes it easier to spread the virus to others.
According to the CDC, even people who have been vaccinated can become infected with the Delta variant and carry large amounts of virus in their nose and throat, though at lower rates than in those who are unvaccinated. Those infected with Delta can then spread the virus to others even if they are asymptomatic.
As a result, the CDC in late July made a revision to its masking guidelines, recommending indoor mask use — even for those who are vaccinated — in areas of the country with substantial or high transmission.
Public health officials note, however, that the COVID-19 vaccines offer significant protection against Delta by reducing the cases of severe illness, hospitalization and death. Three studies published on Wednesday corroborated this reduction, and also found that breakthrough infections remain very rare.
Nevertheless, some experts warn that the risk of breakthrough cases may increase if people attend large and crowded events, especially if attendees do not practice social distancing or wear masks.
Recent examples of breakthrough cases driven by Delta include a cluster in Provincetown, Mass., in early July. According to a CDC report, 469 cases of COVID-19 associated with multiple summer events and large public gatherings were identified among Massachusetts residents. Those who tested positive for COVID-19 following Fourth of July festivities reported “attending densely packed indoor and outdoor events at venues that included bars, restaurants, guest houses and rental homes.” Approximately three-quarters (74 percent) of cases occurred in fully vaccinated persons, according to the CDC.
In April, 92 people gathered in Texas for a wedding held outside; all guests were required to be fully vaccinated. Despite the precautions, however, six people tested positive for the coronavirus after attending the wedding, Forbes magazine reported, citing a preprint published in medRxiv.
Studies of earlier strains of the virus have found that outdoor transmission is almost 20 times less likely than indoor transmission.
COVID-19 is typically spread via aerosols, or tiny droplets, Hijano said. When someone coughs, sneezes, talks or exhales, these particles are expelled into the air, and in indoor spaces, particularly those with poor ventilation, the viral particles can build up. But when they are released outdoors, better air exchange allows for the viral particles to dilute and disperse, so people are less likely to get infected.
However, since those who are infected with Delta carry higher viral loads, experts believe the exposure time resulting in infection may be shorter, even outdoors. Crowded outdoor spaces may also affect the way in which these particles move around.
“There’s a limit to how much the air currents can dilute and disperse virus, and so if you have a crowded outdoor environment, you may circumvent that beneficial dilution effect,” Charles Haas, a professor of environmental engineering at Drexel University, told the Philadelphia Inquirer.
Hijano said everyone should use common sense when engaging in outdoor activities, as some carry more risk than others. When entering a crowd of people with unknown vaccination status, for example, he said masking up and social distancing are reasonable.
“Outdoors, we always say it’s safer than indoors, but the nature of exposure is very important. If you are outdoors in close proximity with someone that is coughing in your face and you are not wearing a mask, then that outdoor rule does not make any sense,” he said. “At the end of the day, people need to have that sort of common sense and say, ‘If there is somebody coughing, even if it is outdoors, I should probably be distant and have my mask on.”
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