When Sun Weisi returned home from the United States on Tuesday, she thought the temperature check at the airport and the health declaration form she submitted there were all that were needed as a precaution against H1N1 flu.

But the phone rang the next morning, and the voice at the other end suggested she begin a seven-day medical observation at home.

The call was from the community health service center of Xiaoguan Street in Beijing's Chaoyang district where Sun and her parents live.

"They call me four times a day," said Sun, a sophomore student at the University of Iowa.

The health workers check on temperature and whether she has any flu symptoms, and report to the local office of the Center for Diseases Control twice a day.

Should any flu-like symptoms be found, the CDC will dispatch an ambulance to collect the patient.

Such in-house prevention and control measures directed at people returning from overseas, particularly students coming home for the summer vacation, were adopted in Beijing on Tuesday.

The Beijing Health Bureau has also asked parents of returning students to take prevention measures and have their child stay home for seven days for medical observation.

The quick implementation of grassroots monitoring stems from the community-level network established after the 2003 SARS outbreak.

There are two channels for health workers to gather information on any suspected cases: One is from the CDC which sorts out information from airport health declaration forms and relays it to community centers, said Yuan Li, deputy director of the center, which has more than 20 full-time and part-time employees.

The other is from the residents or management offices in apartment buildings.

"These people are our ears and eyes," she said. "They know who enter the building and who leave."

By Wednesday, more than 9.6 million travelers had been screened and 1,318 cases with fever and respiratory symptoms were found.

The Health Ministry has expanded its national monitoring network by adding another 119 laboratories and 167 hospitals to deal with H1N1 flu.

Yuan, a veteran community health worker who was involved in efforts to fight SARS, said she thinks the bottom-up prevention and control measures are essential.

"Maybe the wolf will not come in the end. But we are doing our best to protect our sheep."