NEW YORK (CNN) -- The Centers for Disease Control and Prevention has confirmed cases of swine flu in eight students at a New York preparatory school, Mayor Michael Bloomberg said Sunday.
Some of the students spent spring break in Mexico, according to Mayor Michael Bloomberg.
The students have had only mild symptoms and none have been hospitalized, he said. Some of the students have already recovered.
More than 100 students were absent from school due to flu-like symptoms last week. New York health officials tested samples for eight students Saturday and determined the students were probably suffering from swine flu, and the CDC confirmed the diagnosis on Sunday, Bloomberg said.
The announcement brings the number of confirmed swine flu cases in the United States to 20. Bloomberg and New York Health Commissioner Tom Frieden said there is no sign of a citywide outbreak of the flu, and no sign of a potential outbreak of swine flu at other schools. Watch how the fear of swine flu is contagious »
Some students at the school spent spring break in Mexico, Bloomberg said, but authorities have not determined whether any of the students with a confirmed case of swine flu was in Mexico. Someone who traveled to Mexico may not have had any flu symptoms but passed on the flu to someone else, he noted.
Frieden called on students who are home sick to stay home for 48 hours after their symptoms subside.
If symptoms are normal for a regular kind of flu, there is no need to go to a hospital, said Bloomberg. If symptoms become severe, as with any illness, people should go to the hospital, he said.
St. Francis, which has 2,700 students, announced it will remain closed for two days. Asked whether the students' illnesses have been minor because they're young and healthy or because it is a minor strain of the virus, Frieden responded, "We don't know."
It has been a "relatively mild" flu season, Frieden said.
"We don't know whether this strain has been circulating more widely."