At least 85 teenagers and one adult have tested positive after taking part in a youth summer camp hosted by US megachurch, The Crossing.
In a statement, the department of public health of Illinois (IDPH) said organisers failed to check the vaccine status of members of staff and that people were not required to wear masks indoors. It also confirmed that one adult has been hospitalised.
"The majority of the 85 Covid-19 cases associated with the youth camp are among teens," said IDPH director Dr Ngozi Ezike.
"The perceived risk to children may seem small, but even a mild case of Covid-19 can cause long-term health issues. Additionally, infected youth who may not experience severe illness can still spread the virus to others, including those who are too young to be vaccinated or those who do not build the strong, expected immune response to the vaccine," the statement read.
Illinois is encouraging all those aged above 12 to get vaccinated amid growing concern of the Delta variant spreading across the country. According to recent data, 46 per cent of the state's population is fully vaccinated.
The church has not released an official statement in response to the news but has announced on its website that its next youth camp will be postponed to August due to the recent outbreak.
"Due to a recent outbreak of Covid-19 related to Student Camp June 13-17, we have made the difficult decision to postpone our 4th & 5th-grade Crossing Camp. We were so looking forward to spending time with your campers this weekend, but we believe the best way to value and love our students, difference makers, and staff is to delay camp until a safer time. "
Illinois health authorities also said that at least two people from the camp attended a nearby conference as well, which resulted in eleven additional Covid cases.
Premier has contacted The Crossing church for comment.