As the number of H1N1 flu cases increases sharply, health experts believe the peak flu season is yet to come.

The outbreak is expected to hit students and children hardest.

"Seasonal changes often cause colds, which are more prevalent among children because of their lack of resistance to infection," said Dong Lijuan, a doctor at the Beijing Children's Hospital.

Beijing has reported more than 6,700 H1N1 cases, with about 200 new ones reported daily. About 5,000 flu infections from all strains of the virus are being reported daily in the city.

As the cold weather approaches in the vast northern parts of China, it is likely the number of people infected will increase rapidly, said Pang Xinghuo, an official from the Beijing Centers for Diseases Prevention and Control.

The official said the peak flu season this fall is coming earlier than in previous years, making prevention and control effort even more difficult.

In Beijing Children's Hospital, doctors are treating about 2,000 patients with all strains of flu every day.

Schools and universities are checking students' body temperature every day, and report symptoms of flu, such as fevers and coughs, to the health care department.

"Such efforts are extremely important, because children and teenagers are the hardest hit from H1N1," said Fang Laiying, head of the Beijing municipal health bureau.

"Patients aged between 10 and 20 account for 62 percent of total cases. Most of the mass infections took place in schools. So campuses are the focus of our prevention and control work."

Fang noted that early detection is key.

Health experts say people with weak immune systems are likely to develop a severe illness from the virus, though the virus is relatively mild for many others.

Fang said they are very confident and capable of controlling the spread of H1N1, as long as the vaccination is carried out smoothly in the city.

Beijing started its citywide free vaccination against H1N1 on Oct 21, with middle and primary school students and school staff having priority.

Besides medical staff, people working in transportation, civil servants in important agencies and city residents older than 60 will also be inoculated.

Officials estimated that up to 5 million free flu shots would be available to Beijingers.