Authorities are “closely monitoring” data after a number of people who attended the Boardmasters Festival in Cornwall tested positive for the virus.
The five-day event in Newquay finished on Sunday, which means any impact on infections will not be officially known until later this week.
But several people have claimed they tested positive for Covid since attending.
All ticket holders had to show proof of double vaccination, natural immunity or a recent negative lateral flow test to enter the site.
People attending Boardmasters were also urged to attend a walk-in vaccine clinic, which was set up in the festival car parks on Monday.
Coventry councillor Nathan Griffiths claimed he caught the virus at the event.
He tweeted: “Like many, I caught Covid at Boardmasters despite the mandated day 1 + 3 lat-flows. No complaints as I knew the risks, however, means either lat-flows aren’t reliable for such events, or Covid-positive people faked negative results. A lot to learn here for gov/event organisers.”
Another attendee said: “Went to Boardmasters and got Covid, everyone I know has it too. Not fun, hope everyone else with it recovers soon.”
Newquay and the surrounding area had one of the country's highest Covid infection rates before the festival began.
Last week, more than 1,000 people per 100,000 had tested positive for the virus.
Experts warned the current spike had been caused by the G7 and Cornwall attracting young holidaymakers who are less likely to have been fully vaccinated.
Professor Gary McLean, a molecular immunologist at London Metropolitan University, told MailOnline: “Since the G7 summit in early June, case numbers have been increasing at a rate that is amongst the highest in England.
“This was likely seeded by the summit and influx of people at that time and has potentially been amplified by continued 'staycation' tourism to the region during the school holidays.
“This is of great concern for the region and for those returning home from holidays in Cornwall - it will need to be monitored very carefully over the next few weeks.”
Last week, Cornwall Council’s deputy director of public health Ruth Goldstein told the BBC the areas experiencing the highest infection levels were “holiday hot-spots” which attracts young people.
She said: “It's what we expect because we know that the younger people have only had one, and in some case no, vaccine.”
Cornwall councillor Jane Kirkham added infection levels had been high in the area “off-and-on” since restrictions were lifted and the G7 was held in the area.
A spokesman for Boardmasters said: “Since the government allowed live events to return, we have worked closely with experts from Cornwall Council’s Public Health service, wider Cornwall Council departments, the NHS and emergency services to put in place a variety of measures to manage the risk of infection at what is effectively a pop-up town with 50,000 temporary residents and visitors.
“Like any town, we cannot eliminate risk entirely and, while it is still too early to see from the data if there has been any impact on Covid-19 cases, there are likely to be anecdotal reports of some people who were at the festival among those who test positive in the days following.
“Cornwall Council’s public health team will be closely monitoring the data in the coming weeks, as will we.”
A Cornwall Council spokesman added: “It is too early to say whether the festival has had any impact on Covid cases numbers in Cornwall, but we will be monitoring the data closely as we have done throughout the pandemic. Our advice to residents and anyone visiting Cornwall remains the same – if you have Covid symptoms then isolate immediately and book a PCR test.”