Last weekend, hundreds of thousands of people made their way through Chicago's Grant Park for the Lollapalooza music festival, despite a surge in the highly transmissible Delta variant of COVID-19.

The four-day event drew about 100,000 people each day, and this year's lineup – featuring the likes of Foo Fighters, Miley Cyrus and Tyler, the Creator – was one of the first major festivals in the US since the start of the pandemic.

Photos of the event's enormous crowds have shocked many, including epidemiologist Marylouise McLaws from the University of New South Wales, who is among a group of experts who have provided advice to Lollapalooza's parent company, Live Nation, about how to hold future events safely in Australia.

Live Nation has a controlling stake in Australian festivals such as Splendour in the Grass and Falls Festival, and the Australian iteration of Download Festival.

To attend this year's Lollapalooza, ticketholders in Chicago had to either be fully vaccinated (and prove this with a vaccination card) or receive a negative test result in the previous 72 hours.

Live Nation worked with local health officials and adhered to their regulations, the company said. This meant there was no mask mandate, until day three when masks were made compulsory in the festival's indoor spaces.

Chicago is currently seeing an uptick in COVID-19 cases and hospitalisations.

A little more than 50 per cent of the population is fully vaccinated, but the city of 3 million people is recording about 250 new cases per day – similar to what Sydney has been recording in the past week of its Delta outbreak.

The Delta variant now makes up more than 80 per cent of new COVID-19 cases in the US, according to the country's Centers for Disease Control and Prevention.

Upon seeing images of the huge crowds at Lollapalooza, University of Chicago infection prevention and control executive medical director Emily Landon took to Twitter to express her concerns, which have been dismissed by Chicago Mayor Lori Lightfoot as coming from "critics on the sidelines".

Doctor Allison Arwady from the Chicago Department of Public Health said while no event was completely safe from COVID-19, she was happy with the precautions in place at Lollapalooza.

"It's not a zero-risk situation... We expect to see some cases associated with it, just because of the sheer number of folks," she told WGN.

Since the festival, some Lollapalooza attendees have shared their COVID-19 test results on social media.

"I'm fully vaccinated and woke up with body aches, sore throat, stuffy nose, and a headache," one attendee wrote on Reddit, adding that they later tested positive.

"Based on when I tested [positive] I probably had it while I was at [Lollapalooza] but contracted it before," wrote another.

Video on social media also shows a crowd breaking down a fence to gain entry to the festival, without being checked for vaccination or a negative test.

Lollapalooza organisers claimed 90 per cent of attendees showed vaccination cards proving they were fully inoculated, but vaccinated people can still transmit the virus and vaccination cards can be forged.

Chicago Tribune photo intern Vashon Jordan Jr, who covered the festival, said that fake vaccination cards were being used by some people, despite earlier warnings of "hefty fines and prison time" from the FBI.

UNSW's Professor McLaws said festival organisers should remember that COVID-19 vaccines weren't 100% effective.

"People can still get breakthrough infections and symptomatic disease, and while the viral load may be lower, given there's no social distancing, this will increase the risk of spread," she said.

In an op-ed published by Billboard, Lollapalooza founder Perry Farrell said he was proud of this year's event, but couldn't guarantee there wouldn't be COVID-19 infections.

"I'm not going to say that nobody is going to catch anything, because it's beyond me, but I can tell you that we really set a course in the right direction — and for that, I'm happy," he said.

How have other festivals fared?

Several COVID-19 cases have reportedly been linked to hip-hop festival Rolling Loud (also operated by Live Nation), which took place in Miami, Florida last month.

The event did not require attendees to wear masks, be vaccinated or show a negative test, in accordance with local laws, despite the state experiencing a spike in new cases and hospitalisations.

Rapper Dess Dior announced she had tested positive for COVID-19 after performing at Rolling Loud.

As did some attendees who posted their positive results to social media.

Last month in the Netherlands, unvaccinated people were allowed to attend the Verknipt music festival in Utrecht, as long as they had a negative test within 40 hours of entering.

There were 20,000 people in attendance with no masks or social distancing, and about 1,000 festival-goers are known to have become infected with COVID-19 over the two-day event.

An outlier last month was Serbia's EXIT Festival, which welcomed about 45,000 people per day for four days, and recorded no COVID-19 infections according to a study published a week after the event.

All attendees were required to be fully vaccinated or return a negative test on entry.

Story By | Tom Williams