BOSTON (CBS) – Guns N’ Roses kicked off a summer concert series at Fenway Park Tuesday night that packed the park with thousands of fans. As the highly contagious Delta variant continues to cause COVID-19 cases to tick up in Massachusetts, experts say outdoor transmission remains rare.

“The studies on outdoor transmission. There are a couple good ones. One showed that the risk of outdoor transmission is one-thousandth the risk of indoor transmission,” said Tufts Medical Center epidemiologist Dr. Shira Doron.

Doron says the Delta variant is proving to be about twice as contagious as the Alpha variant, with more virus in the mouth and nose of infected individuals. But the movement of air outside disperses droplets, making it more difficult to inhale enough virus to become infected.

“We also have very, very good data that shows that it is much harder for a fully vaccinated person to get infected. When they are infected, they’re not infected for as long as someone who is unvaccinated,” said Dr. Erin Bromage, a biology professor at UMass Dartmouth.

Guns N’ Roses at Fenway Par, August 3, 2021. (Photo credit: Valeska Thomas – Live Nation)

Bromage says he would take more precautions in crowded indoor parts of concert arenas, like the line for the bathroom.

“I would mask up going into those spaces,” Bromage told WBZ-TV.

Asked about the uptick in cases, Gov. Charlie Baker said he continues to support the Department of Public Health’s guidance recommending that fully vaccinated people wear masks indoors around immunocompromised individuals and the unvaccinated.

“I think our guidance is really focused on vulnerable populations and people who are most at risk,” Baker said.

Last week the CDC pointed to evidence that fully vaccinated people can become infected with the virus and transmit it, but those cases remain rare. In Massachusetts, only about 0.2% of all vaccinated people have become infected with COVID-19.

Louisa Moller