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CHICAGO — Lollapalooza opened its doors Thursday to an estimated 100,000 people, but the state’s surge in COVID-19 cases is still causing concerns.

Health officials have acknowledged there will likely be cases tied to the four-day festival. They are urging people to stay home if they’re sick.

While there’s cause for concern, Lollapalooza officials said Thursday that over 90% of attendees presented proof of a vaccination. Around 600 people were not allowed to enter the festival due to lack of paperwork.

Mayor Lightfoot and festival organizers believe the right precautions are in place despite a surging number of COVID-19 cases due to the Delta variant.

Attendees must show proof of a vaccine or a negative COVID-19 test, taken within 72 hours of entry, and masks are encouraged.

The FBI is warning that creating, buying or selling fake vaccination cards is illegal and those who do can face fines and prison time.

A sign outside the festival reads, “We have taken enhanced health and safety measures for you, our artists and employees. You must follow all posted instructions while attending Lollapalooza. An inherent risk of exposure to COVID-19 exists in any public space where people are present. COVID-19 is an extremely contagious disease that can lead to severe illness and death. According to the Centers for Disease Control and Prevention, senior citizens and guests with underlying medical conditions are especially vulnerable. By attending Lollapalooza, you voluntarily assume all risks related to exposure to COVID-19. Please help keep each other healthy!”

On Thursday, Cook County and several other area counties reached “substantial transmission,” defined by the CDC.

Headliners include Foo Fighters, Post Malone, Tyler the Creator and Miley Cyrus. Other performers announced are DaBaby, Marshmello, Illenium, Journey, Megan Thee Stallion, Modest Mouse — among many others.

The festival gates open at 11 a.m. and closures are in place around Grant Park. Both Metra and CTA will have additional service. Masks are required on trains and buses, and no alcohol allowed.

If you have questions about COVID-19 vaccines or to find a vaccination location near you, please visit