If any New York state student or staffer tests positive for the new coronavirus, their school will close for at least 24 hours, Gov. Andrew Cuomo announced Monday morning as the number of people who tested positive for the illness grew over the weekend. 

“If a child gets it — this virus has a very limited effect on children, but the child goes home and kisses his grandmother [and] grandmother has an underlying illness, now we have a problem,” Cuomo said. 

Twenty people in New York City had tested positive for the coronavirus as of Monday afternoon, according to Mayor Bill de Blasio.

The guidance, released Monday evening, will require a child’s school to be closed for at least 24 hours to clean the campus and “to assess facts and circumstances of that school,” such as figuring out how the child got the virus, Cuomo said. The guidance comes less than a week after the state education department said schools should decide whether to close after consulting with their local health department. 

Emily DeSantis, a spokesperson for the state education department, said her department is “closely monitoring” the coronavirus, and that includes tracking guidance out of the Centers for Disease Control and Prevention, or the CDC. The guidance will also apply to teachers and school staff, who are more likely than young children to become ill with the coronavirus. 

“We will continue to update this guidance as the situation warrants,” DeSantis said in a statement. 

The guidance also requires school districts to notify state health officials about infected students and teachers so they can “determine what additional steps are needed for the school community.” Additionally, students and staff who are taking care of or share a home with an infected person must not attend school and should follow quarantine instructions from state health officials, who will decide when it’s safe for them to go back, the guidance says.

The guidance makes recommendations on a variety of fronts, including how school districts should be communicating with families, how to clean school buildings, when to separate students from the rest of the school, ensuring students have access to meals during a school closure, and how to reduce stigma and discrimination related to coronavirus.

All private and public schools should notify the state education department if they decide to close buildings because of the coronavirus, department officials said.

No New York City public schools have closed as of Monday afternoon. Three teachers who had recently traveled abroad were tested for the illness, with negative results. The city’s health commissioner issued an order last week requiring certain public employees, including teachers, to be tested for the coronavirus if they’ve possibly been exposed to it. 

De Blasio said he agreed with the state’s guidance. If an ill child or staffer prompted a school closure in New York City, the city health department will investigate and decide if a shutdown longer than 24 hours is needed, according to an education department spokesperson. Officials would first clean the building, which would largely be reassurance for families, as city officials have repeatedly said the virus lives for only a few minutes on surfaces and is primarily caught through person-to-person contact, such as getting someone’s saliva in your mouth or eye. 

The World Health Organization is uncertain about how long the virus lives on surfaces, but previous coronaviruses lasted between hours and days on surfaces. 

Also during any closure, “disease detectives” would investigate who else had close contact with the person with the illness, such as other classmates and family. 

Still, de Blasio emphasized that closures would be a “last resort” and would be brief since such a move can present enormous challenges for low-income families, who often rely on their schools for childcare and meals. 

The city is attempting to find ways to deliver meals to children who need it in case their school closes, de Blasio said. The education department is prepared to serve “grab-and-go breakfast and lunch for any student who wants it” during a 24-hour closure, said Miranda Barbot, spokeswoman for the department.

Officials are also preparing for remote learning, but de Blasio offered no details on how that would work, adding “that’s not going to be our preference.”

“We want to make sure that a school is closed only for the period of time it needs to be because there are so many parents who depend on that school,” de Blasio said. 

The mayor continued to urge people to wash their hands and stay home from work and school if they are displaying any symptoms of the illness, which include fever, cough, or shortness of breath. On CNN, he said closure isn’t necessary while  awaiting test results for someone at the school, and that there’s only reason for concern if a child “really only had close contact with a handful of adults or kids in the school, like really close contact.”

 Last week the city said it was creating a contingency plan for closing schools, should the need arise, yet details of such a plan had not yet been released.

The mayor also announced on Sunday a few other directives for schools in response to the global health crisis. All international school trips sponsored by the education department would be canceled for the rest of the school year. A total of 85 nurses would be sent to school buildings this week to ensure they had a health professional on hand, and department officials would be spot checking schools for soap and paper towels.

Public schools have been closed outside of New York City, including in the suburb of Scarsdale in Westchester County. On Monday, Cuomo said they could close public schools “for weeks” in New Rochelle, after receiving guidance from the CDC as illnesses grow in Westchester. Schools there are open today.