In Austin, health officials with the city government and the university have contacted every young person who was on the chartered plane, using flight manifests shared by the Centers for Disease Control and Prevention, the authorities said. City health officials used the case to urge residents of all ages to stay home and to avoid nonessential travel. At least four of the 44 students who tested positive had not shown any symptoms of the coronavirus.

“The virus often hides in the healthy and is given to those who are at grave risk of being hospitalized or dying,” Dr. Mark E. Escott, the interim medical director and health authority for the city of Austin and Travis County, said in a statement. “While younger people have less risk for complications, they are not immune from severe illness and death from Covid-19,” the disease caused by the coronavirus.

U.T. Austin canceled classes on March 13 and resumed instruction online on Monday.

“The incident is a very strong reminder of the importance of taking seriously the warnings of public health authorities on the risks of Covid-19,” said J.B. Bird, director of media relations at the university.

The state’s flagship university has been hit hard by the virus. Its president, Gregory L. Fenves, announced last month that his wife had tested positive, and that his family was self-isolating. Mr. Fenves’s wife, Carmel, began exhibiting flulike symptoms after the couple traveled to New York City for events with alumni and students. Mr. Fenves’s tests later came back negative.