Delta variant’s outdoor transmission is still unclear. Here’s how Philly experts are thinking about COVID-19 risk.

As the delta variant pushes COVID-19 case counts upward again, people are being more careful about wearing masks and distancing indoors. But what about the outdoors, which has been a refuge from the pandemic isolation because of reduced transmission risk?

Last weekend, photos of maskless revelers at Lollapalooza, an outdoor music festival in Chicago, prompted concern about whether packed crowds might offer a feeding ground for the delta variant. Philadelphia is planning to host a similar music festival, Made in America, next month on the Benjamin Franklin Parkway. In past years, the concert has drawn as many as 50,000 music fans a day.

Here’s what to keep in mind before you buy tickets to the next summer festival, game, or other outdoor event.

It seems likely, said Aaron Richterman, an infectious-disease clinical fellow at the Hospital of the University of Pennsylvania. Transmission hasn’t been conclusively quantified yet, but we do know something about how much virus is in an infected person — their viral load — which can play a role in how much transmission occurs.

The delta variant seems to lead to a larger viral load when it infects someone (which happens at a higher rate in people who are unvaccinated, but can also occur in vaccinated people). This might mean that it is more transmissible — and may also boost risk of infection even outdoors.

But outdoor transmission has been very rare with earlier strains. “The context to keep in mind is that the baseline outdoor risk with all of these is extremely low,” Richterman said. Studies of earlier strains found that outdoor transmission was almost 20 times less likely than indoor transmission.

Outdoor transmission is less common because the virus is spread through aerosols, tiny droplets on which viruses can hitch a ride when someone coughs, sneezes, talks, or exhales. Indoors, these particles can build up in stagnant air. Outside, however, air moves around more freely, carrying viral particles away from their source and diluting them so there are smaller quantities in any one spot, said Charles Haas, a professor of environmental engineering at Drexel University.

Virologists are still trying to figure out why the delta variant seems to be more transmissible, and the reason might affect how we approach risk and precautions indoors and outdoors.

The virus might be more transmissible because people who are infected produce more of the virus, or because the virus is somehow better able to infect cells, Haas said. It might also be a question of timing, Richterman said: The delta variant might make viral loads spike when people are still asymptomatic, allowing it to spread as the unsuspecting host goes about their daily routine. One study suggests that the delta variant might be present for a longer period of time in a host.

With earlier strains, Richterman was reassured by how few reports of cases involved outdoor transmission, so he is monitoring similar reports involving the delta variant to determine risk. Just last week, the CDC released a report on one of the most prominent delta-driven clusters described in the United States so far, where people in Provincetown, Mass., were reported to be interacting both indoors and outdoors.

“We have to have humility here and understand that, of course, we have to be prepared for that evidence to come up if it does,” Richterman said.

Crowds like those pictured last weekend from Lollapalooza — thousands of people packed in close quarters with few wearing masks — complicate the dynamics of how a virus might 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,” Haas said.

Certain activities, like loud shouting and singing, may also increase risk of transmission, Haas said.

Last year, crowded outdoor events like protests did not lead to the massive outbreaks of previous COVID-19 strains that some expected. But there are other reasons to be wary of outdoor events. Concerts and sporting events tend to bring together people who might otherwise never cross paths, and then send them back to their homes. “That’s what I would be worried about,” Richterman said. “People coming from all over the place, gathering with people, mixing with people who are outside of their ordinary social networks.”

He also pointed out that even at outdoor events, there are plenty of reasons to go inside: pit stops at bars, returning to hotels in the evenings. The relative safety of the outdoors might make people lower their guard and forgo precautions when festivities relocate indoors, where the risk of transmission is higher.

Vaccination is Haas’s number one recommendation for staying safe at large outdoor events — or anywhere else. Being vaccinated decreases the risk of being infected, requiring hospitalization for severe illness, or dying.

“Wear a mask, and stay away if you can from shouting, singing, and cheering,” Haas said. “But if you’re vaccinated, the risk and the consequences are expected to be quite low in any event.”

The COVID-19 vaccines take around two weeks after the second shot to provide protection against the virus, so if you wait to get vaccinated until right before your event, you won’t be able to reap the full benefits. Plan in advance to get vaccinated sooner rather than later.

When entering a crowd of people with unknown vaccination status, even outdoors, masking up is reasonable, Richterman said. It can protect you from potential loads of delta variant that someone else might be spewing, while also keeping others from contracting any virus you might unknowingly be infected with, Haas added.