The private primary school in Shanghai where 45 students were sickened and sent to the hospital for treatment after lunch on Friday said many of the students have recovered.

"None of the students are in critical condition," according to the director of student affairs at Shanghai Tongyuan Experimental Primary School, who only gave her surname, Zhang.

"Some are still suffering minor symptoms, such as stomach cramps and nausea. The district CDC suggested the students not return to school until 72 hours after the symptoms have fully disappeared."

Officials at the center for disease control and prevention in the Jing'an district, where the school is located, took test samples from 13 patients randomly on Saturday. Ten tested positive for norovirus, a common cause of acute gastrointestinal illness.

Zhang said the local CDC had taken samples of the food served for lunch on Friday for testing. Lab results are expected in two weeks.

Wang Xiaohai, whose 8-year-old daughter was among the students who suffered stomachache, fever and vomiting, said the girl had been diagnosed with acute gastroenteritis.

"The sufferers are mainly first- and second-graders and their classrooms are close to each other," he said.

Norovirus is transmitted mainly through contaminated food, water and tableware. It can also be passed in stool and vomit, the local CDC said. Infected people generally don't need treatment and usually recover in two days.

The Jing'an education commission ordered the school to strengthen its disinfection procedures in key areas, including classrooms, canteen and restrooms, and to bolster daily health checks for students to detect and cope with possible infections in a timely manner.