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The voluntary quarantine system that states are using to combat the spread of the new coronavirus in communities can work only if people follow it. So how can health officials be sure that people who agree to self-quarantine are at home?

The weaknesses in the system became apparent over the weekend in Missouri when a man broke quarantine and took one of his daughters to a dance.

The unidentified dad was already cutting the rug at a hotel Saturday when he got confirmation that his other daughter, the one who had stayed home, had tested positive for the virus, St. Louis County Executive Sam Page said during a news conference Sunday.

Father and daughter headed straight home, but Page said they are still trying to determine how many people they had been in contact with, and he warned that the dad would be slapped with a court order "to remain in the house by order of the sheriff" if he ventures out again.

"When we ask someone to quarantine and they tell us they will, we have to trust that," said Page, who is also a doctor. "'Quarantine' means stay in your home."

St. Louis County Public Health Department workers had been in touch with the family by phone every day since Thursday, when the daughter who has the disease went to get tested and the county told the family to "self-quarantine," Page said.

In this case, the daughter who contracted the virus "conducted herself responsibly and maturely," Page said. "The patient's father did not act consistently with the health department's instructions."

"The way the family has reacted to this situation is really a tale of two reactions but a study of how people should and should not react to the coronavirus," Page added.

Still, Page added, "we are not currently monitoring the family. ... We depend on the common sense and goodwill of the people we communicate with."

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The dance was held at the Ritz-Carlton, St. Louis in Clayton for students who attend Villa Duchesne and Oak Hill School, county officials have said. In the aftermath, the parochial school announced that it would close this week and undergo a "hospital-grade cleaning."

In an era when most people have cellphones and fewer people have landlines, how can officials be sure people are where they are supposed to be?

"They can't," Dr. Robert Murphy, who heads the Institute for Global Health at Northwestern University, said in an interview. "Even if they had landlines, one could leave in-between check-in calls."

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It's the fear of getting caught and being quarantined in a secure facility with guards as well as doctors that keeps people in line, Murphy said.

"Basically, a person who agrees to self-quarantine has to agree to some specific terms," Murphy said. "If they break the rules and get caught, they could be forcibly quarantined in a secure location and might have to pay a fine. That is both totally legal and reasonable.

"I think it would make a person who is supposed to stay home think twice about going to a father-daughter dance," Murphy said.

What is quarantine?

Quarantine means physically separating people who have been exposed but do not have the disease. The patients are viewed as being at risk. If the patients are confirmed to have the infection, that's when they go into isolation or hospitalization. The incubation time for the coronavirus is 14 days, meaning if people have been exposed but have no symptoms and test negative, they are released from quarantine.

What happened in the St. Louis suburbs is not an isolated incident.

In New Hampshire, a man who came down with coronavirus symptoms after returning home from Italy was told to stay home by his doctor until the test results were in, NBC News reported.

Instead, the man — a health care worker at Dartmouth Hitchcock Medical Center — attended a mixer Friday for doctors and students at Dartmouth College.

Three days later, he was confirmed as New Hampshire's first person to have been infected.

In New York City, health workers also rely on "common sense and goodwill" to keep the virus from spreading.

More than 2,700 people are under "volunteer home isolation" in New York City, where there were 20 confirmed cases of COVID-19 as of Monday and Mayor Bill de Blasio said he was considering imposing a citywide quarantine similar to what Italy has done in the northern corner of the country.

"It's a possibility, but I think people are getting a little ahead of ourselves, and we should be careful," the mayor said.

Asked how authorities make sure quarantined people stay home, Michael Lanza, assistant spokesman of the New York City Department of Health and Mental Hygiene, said in an email that city workers regularly call or text.

"New Yorkers recognize the importance of following Health Department guidance and protecting their fellow residents when it comes to staying home. In the event that something like the situation you've described happens, we could pursue a commissioner's order to keep New Yorkers safe."

Lanza did not immediately explain what a commissioner's order entails.

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