At least 46 people who attended the party, or are close contacts of those who did, tested positive for COVID-19, the Centers for Disease Control and Prevention (CDC) revealed on Monday.
Two of the secondary cases were among student athletes, which led the school to cancel in-person classes for two weeks due to teachers not being able to work because they had to isolate.
The report is a cautionary tale that serves to warn businesses to keep enforcing mask and social distancing rules as states begin relaxing restrictions.
A bar in rural Illinois held an opening party indoors, where attendees often didn't wear masks or practice social distancing (file image)
Within two weeks, 26 people who attended the event and three staff members tested positive for COVID-19, along with 17 close contacts - which included two student athletes and a hospitalized long-term care facility resident - for a total of 46 cases linked to the party
For the report, published in the CDC's weekly MMWR report, the team looked at an opening event held indoors for a rural Illinois bar in early February 2021.
It's unknown how many people attended the event but the space accommodates about 100 people and six employees were working that day.
Attendees said there was inconsistent mask use and people often did not maintain a distance of six feet or more apart.
On February 17, the Illinois Department of Health received a notification of a possible outbreak linked to the event.
Of those who were at the event, 26 attendees tested positive for coronavirus as did three staff members.
At least four people who were later confirmed to have COVID-19 said they had symptoms the day they went to the bar. and one asymptomatic attendee was diagnosed with the disease the day before the event.
According to the report, the attendees were in close contact with at least 71 people not at the bar.
Of that group, 37 people were tested for COVID-19 and 17 were confirmed to be positive within 14 days of contact.
Two of the close contacts were student athletes who attended indoor sports practice and in-person classes, and were later diagnosed with coronavirus.
On the week beginning February 18, the school - which has 650 children - closed for two weeks because 13 staff members were in isolation, in quarantine, or absent because their own child was quarantined due to the two athletes.
In addition, three of the contacts were long-term care facility residents or staff members who were infected because on of the bar attendees worked as a certified nursing assistant at the facility.
He or she was asymptomatic and tested positive during routine COVID-19 testing at the center four days after the event.
One of the residents was hospitalized after testing positive, but was discharged the same day.
The CDC said that before the event, the county was recorded a seven-day rolling average of 41 to 42 cases per 100,000 people.
Just 14 days after the event, the incidence more than doubled to a seven-day rolling average of 86 to 87 cases per 100,000 people.
'Bars can play a role in community spread of COVID-19 because of limited mask use while eating or drinking and lack of consistent physical distancing,' the authors of the report wrote.
'These findings show that SARS-CoV-transmission originating in a business such as a bar not only affects the patrons and employees of the bar but can also affect an entire community.'