The Chicago data confirms, though, that huge mass gatherings can be held safely even in the age of Delta. Hold them outdoors and limit admission to the vaccinated (or confirmed COVID-free) and you can party without much worry.

Chicago’s health commissioner, Allison Arwady, claims she told her staff before the festival began that they could expect around 200 cases given the size of the crowds that were anticipated and the precautions being taken. Number of confirmed cases so far: 203.

We are now 14 days past the first day of Lolla and we are continuing to investigate cases of COVID. There have been no unexpected findings at this point and NO evidence at this point of “super-spreader” event or substantial impact to Chicago’s COVID-19 epidemiology.

— Dr. Allison Arwady (@DrArwady) August 12, 2021

As of now, there is a total of 203 cases identified with attendance at Lolla and symptom onset (or if asymptomatic, test date) on or after attendance. 58 were Chicago residents, 138 were non-Chicago Illinois residents and 7 out of state residents.

— Dr. Allison Arwady (@DrArwady) August 12, 2021

As predicted, the vaccinated were less likely to be infected than the unvaccinated although the combination of outdoor ventilation and the negative test requirement seems to have held down transmission for everyone. That 203 includes people who tested positive while the festival was ongoing, which may mean they were infected when they got there, as well as people who tested positive at any point in the ensuing two-week period, which may include transmissions unrelated to Lollapalooza.

In other words, the true number of infections that happened at the event itself may be considerably smaller than 203.

Some scientists had wanted the event shut down before it began. “I think it has the makings [of a superspreader event],” said one epidemiologist in July. “When we’re in a place where rates are rising, we need to put prevention strategies in place. I don’t see how a large festival like this could meet that criteria of slowing the spread.” Arwady conceded today that if Chicago were seeing the sort of rampant spread that New Orleans is now enduring, they would have had to cancel. But since COVID is spreading more slowly in the north at the moment, she was confident that it could be done safely if precautions were taken. And they were:

“We checked with [attendees] every single day, multiple times a day. We had our people at the screening checkpoints,” Lightfoot told Chicago radio station WVON-AM earlier this month . “Every single day, they turned hundreds of people away, either who didn’t have the right paperwork or had an expired test that wasn’t [taken] within 72 hours. That tells me there is a rigor around the protocols that they were using to screen people.”

The event may have even had benefits. “We saw younger attendees were more likely to say that attending Lollapalooza was an incentive for them to be vaccinated,” Arwady told reporters, citing a survey of concert-goers Chicago conducted afterward.

One obvious point is that people who are likely to attend Lollapalooza skew young and the young have more robust immune systems. Combine that with the high vaccination rate among the crowd and transmission was more difficult for the virus than usual. An older crowd might not have done quite as well. Also, it’s not clear to me how the city went about confirming these 203 cases. Did they just call around to a random sample of attendees afterward and ask them if they’d been tested? The tricky part with that is that young adults, especially ones who’ve been vaccinated, may have mild enough symptoms from being infected that they assume it’s a cold rather than COVID — or, even if they suspect COVID, they don’t bother getting tested because they assume they’ll be over it quickly. A number of cases could have been missed that way.

But still, good news. Here’s hoping for good news at Sturgis too.