That's according to AEG Presents, the organizer of Firefly.
The festival is scheduled from Sept. 23 to 26 at The Woodlands.
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AEG will require all concertgoers and staff to show proof of full vaccination in order to be at all its upcoming concerts and venues beginning no later than Oct. 1.
"Leading up to Oct. 1, AEG Presents will be implementing a policy of showing proof of vaccination or a negative COVID test taken within 72 hours of show date where permitted," the company said in a release Thursday.
The company says the rise in COVID cases, fueled by the delta variant, made the policy necessary.
AEG owns or is a partner in venues such as Webster Hall and The Roxy, as well as Coachella and the New Orleans Jazz Fest.
Chairman and chief operating officer Jay Marciano said he expects some pushback but is confident they are doing what's best for everyone.
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The four-day music festival in Delaware is returning after being canceled last year because of the pandemic.
The 2021 lineup features more than 120 bands across seven stages. Billie Eilish, The Killers, Tame Impala and Lizzo are the headliners.
Other performers include Megan Thee Stallion, Cage the Elephant, Wiz Khalifa, Roddy Ricch, Diplo, Portugal. The Man, Glass Animals, Phoebe Bridgers, Nelly, Blackbear, Taking Back Sunday and Machine Gun Kelly.
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The Associated Press contributed to this report.