Outdoor music festival Lollapalooza brought thousands of people to Chicago's Grant Park.

Photographs from Lollapalooza, which took place from Wednesday to Sunday, show large groups of un-masked concertgoers cheering in densely packed crowds. Organizers have not released official attendance numbers, but over 100,000 people were expected to attend each day of the four-day festival.

Lollapalooza's safety measures ahead of the event were straightforward: The festival required attendees to present their vaccination card indicating that they were fully or partially vaccinated. Unvaccinated individuals had to show proof of a negative Covid test within 72 hours of attending and wear a mask.

But even with the safety protocol in place, is it safe to attend a crowded outdoor concert at this stage of the pandemic with the more transmissible delta variant dominating?

It's too early to say whether there will be an outbreak due to the Lollapalooza concert.

But we know from data out of a Provincetown, Massachusetts outbreak that people who are fully vaccinated are capable of getting infected with and transmitting Covid, Dr. Jorge Parada, medical director of Loyola Medicine's Infection Control Program, tells CNBC Make It.

"The more crowded the venue, the more likely they'll happen," he says.

"At this point, for everybody, it's really about balancing risk," says Anna Bershteyn, assistant professor at New York University Grossman School of Medicine, who specializes in infectious disease modeling.

Here's what experts want you to know about the risks, as well as some safety measures to keep in mind if you're planning to attend a large outdoor concert or gathering:

Requiring vaccination or a negative Covid test is not enough to ensure safety

In July, the Verknipt outdoor festival in the Netherlands followed similar rules to Lollapalooza (attendees had to show a QR code demonstrating that they were fully vaccinated, proof of a negative Covid test or recent infection), but ultimately, more than 1,000 people tested positive following the event.

Mercedes Carnethon, vice chair of preventive medicine at Northwestern University Feinberg School of Medicine, refers to these types of measures as "hygiene theater," because they give the appearance of safety but do not ensure that the risk is reduced. In reality, people could forge a negative Covid test or create a counterfeit vaccination record (although Lollapalooza's guidelines remind attendees that doing so is illegal).

"When your safety measures rely on the honesty of individuals, it's a little tough to count on," Carnethon says. "I think the vast majority of people will try to do the right thing, but some will not."

Plus, there is new evidence that even vaccinated people can become infected with and transmit the delta variant, though it is more rare. Although breakthrough cases in vaccinated individuals tend to be milder, they can still spread Covid to people who are unvaccinated or who are vulnerable to severe illness from Covid.

If you're someone who lives with a person who is ineligible for vaccination or who is immunocompromised, consider skipping the event, Bershteyn says. "Even though [a vaccinated person] might not get sick, you might be part of a transmission chain that could make somebody else quite sick," she says.

Travel increases your risk

Music festivals often attract people from out of the state or even the country.

Chicago was "doing fairly well" with vaccination rates, Carnethon says. In Cook County, where Chicago is located, 60% of the population is fully vaccinated for Covid, according to the CDC. "But inviting people from around the country into the city for [the Lollapalooza] festival, that is largely targeting the age range who is least likely to be vaccinated, certainly did pose some risks," she says.

Travel increases the risk of Covid transmission and infection. Once you arrive at a music festival from out of town, you might be staying in a busy hotel or taking public transportation like trains and shuttle buses to get to and from shows, Carnethon. "All of those certainly are places where the virus is at risk of spreading," she says.

If you must go, here's how to be safer...

The most important thing you can do to ensure you're safe at an outdoor concert or gathering is to get vaccinated, Parada says.

Then, even though the CDC doesn't require fully vaccinated people to wear a mask in outdoor settings, given the packed environment of a concert, and the fact that many people will be screaming or cheering, it's a good idea to mask up in a crowd, Carnethon says.

Wear a mask for any encounter where you're in a small and constrained space, Carnethon says. For example, using the portable toilets, waiting in line for drinks or standing in a space where people are pressed up very close next to you, she says.

"What we're learning about this particular variant is that the viral loads are higher initially, so the time of exposure is shorter," she says.

Though it may not create the most exciting concert-going experience, it's safest to stands on the outskirts of a crowd where you are able to maintain social distance from other people, Carnethon says.

... and what to do afterwards

After attending a festival, it's wise to monitor for symptoms of Covid 10-14 days following, Carnethon says. Typically, surges occur two weeks following an event. With any event, it's difficult to attribute an uptick in cases to the actual gathering, or if it's indicative of a broader trend.

"We're all a little bit unsure what Lollapalooza will do with regards to being a potential major Covid spreader event," Parada says.

If there's someone in your household who you know is vulnerable, like your children or an elderly person, you should quarantine from those people if you've gone to a mega-event like a music festival, Bershteyn says.

"You should absolutely quarantine from those people if you're aware that there were cases at that event."

Lollapalooza and Verknipt did not immediately respond to CNBC Make It's request for comment.

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