The woman contacted the St. Louis County coronavirus hotline on Thursday about her illness and was directed to go to Mercy Hospital in Creve Coeur for testing, KTVI reports.
The woman was discharged after it was determined she did not need to be hospitalized and health officials told her that she and her family would have to self-quarantine at home.
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The woman complied with the self-quarantine, but officials said some of her family members did not, visiting a school dance Saturday night.
On Saturday, the woman was confirmed to have COVID-19, becoming Missouri's first case. A statement from the school said that the two family members left the dance after being told that the woman had tested positive with COVID-19 and that they need to maintain self-quarantine.
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County Executive Sam Page said that the woman in her 20s was a student at an out-of-state college and had been in Italy studying abroad. Page also said that if family members of the infected woman do not comply with the requirement to stay home, they can be compelled to do so by force of law.
The woman had returned from Italy to O'Hare Airport in Chicago and traveled to St. Louis via an Amtrak train, Amtrak said in a statement.
Two schools in St. Louis, Villa Duchesne and Oak Hill, are closed Monday. The woman who tested positive has a younger sister who attends Villa Duchesne.
Both of the schools are independent Catholic schools. Villa is an all-girls school for grades 7 through 12 and Oak Hill is a coed preschool through grade 6.
In a statement released by the schools on Sunday, the school leaders say they are cooperating with St. Louis County Department of Public Health to mitigate risk and maintain a safe environment for students staff and the public.