Spreading COVID outside was possible, but not probable, experts advised in 2020, urging cooped-up citizens to turn to Mother Nature as an antidote to the isolation of lockdowns. Events, dining, and even entire classrooms were moved outside, when feasible.

But Omicron was a game changer, in more ways than one.

The original Wuhan strain of COVID-19 had a reproductive rate—also known as an R0 or R-naught value—of around 3.3, meaning that each infected person infected another 3.3 people, on average. That put COVID-19 among the least transmissible human diseases.

Slightly less transmissible were the 1918 pandemic strain of flu, which had an estimated R0 of 2, as does Ebola. On the higher end of the spectrum, mumps has an R0 of 12; measles tops the list at 18.

In order to outcompete, successful COVID variants have become more transmissible with time. Delta had a slightly higher reproductive rate of around 5.1. Then came Omicron, with an reproductive rate almost twice as large: 9.5.

So called “stealth Omicron,” nicknamed for its ability to evade detection on PCR tests, was about 1.4 times more transmissible than BA.1, so its reproductive rate was around 13.3, Adrian Esterman, a professor of biostatistics and epidemiology at the University of South Australia, recently wrote on academic news website The Conversation.

New studies suggest that BA.4 and BA.5, currently sweeping the U.S. and countries around the globe, have a growth advantage over BA.2 similar to the growth advantage BA.2 had over BA.1. Thus, the latest dominant COVID subvariants have a reproductive rate of around 18.6, tying or surpassing measles, the world’s most infectious viral disease, according to Esterman.

Greater transmissibility means greater transmissibility in any setting, indoors or outdoors—even if outside is still safer, Maimuna Majumder, an assistant professor at Harvard Medical School and a computational epidemiologist at Boston Children’s Hospital, recently told NPR.

Upping the ante is the fact that recent subvariants like BA.4 and BA.5 are the most immune-evasive yet, with the ability to dodge antibodies from both vaccines and prior infection.

All this to say your protection outdoors isn’t what it was in 2020—and it may be time to begin thinking more critically about outdoor gatherings.

For those hosting events, Majumder recommended decreasing the amount of attendees at gatherings, a move that can “drastically” reduce transmission. She also suggested making sure guests are vaccinated, have recently tested negative, and are symptom-free.

If an outdoor event is crowded, especially with singing or yelling—perhaps a concert or protest—masking is a good idea, she advised.

While outdoor events are safer than indoor events, they’re “not 100% safe,” Majumder told the news outlet. “The more crowded an outdoors space is, the more it begins to mimic an indoor space in terms of our exposure to shared air.”

She cautions that outdoor tents that don’t have flaps that let air in are “not that different from being indoors,” as far as COVID transmission risk goes.

As for indoor activities outside of home—mask up, even if your trips are brief, she recommended: It’s more possible than ever to catch COVID in passing.