Drugs companies are developing a new vaccine to combat the various flu strains that have swept China this winter, a respiratory expert told Guangzhou Daily on Wednesday.

"The most dominant strain this flu season has been B/Yamagata, which the current seasonal vaccine does not cover," said Zhong Nanshan in Guangzhou, Guangdong province. Zhong is also an academic with the Chinese Academy of Engineering.

"There's also been a slight mutation to the virus," he added.

B/Yamagata is a virus with a long incubation period that has not caused a widespread epidemic for years. Because of its long silence, the general population has basically lost immunity to this strain, Zhong said.

Quadrivalent influenza vaccines provide coverage against two influenza A virus strains and two influenza B virus strains－Yamagata and Victoria－while the traditional trivalent vaccines guard against two influenza A virus strains and only one influenza B virus strain－Victoria.

"The influenza vaccine used this year ... does not include Yamagata. To some extent, it has caused the soaring number of flu cases nationwide," Zhong said.

According to the national influenza sentinel hospitals, in the past three weeks there was a 40 percent increase in flu cases compared with the same period last year, and those were mainly B strains.

Data from the Chinese Center for Disease Control and Prevention show that in the past three weeks the outpatient and emergency clinics across the country have reported more flu cases than in the same period in the previous three years.

He Xiong, deputy director of the Beijing Center for Disease Control and Prevention, said flu outbreaks are up across China compared with previous years, but the epidemic remains at normal levels and is much less severe than the Severe Acute Respiratory Syndrome outbreak.

"The new quadrivalent influenza vaccine can reduce the chances of massive flu infection and ease flu symptoms. Vaccination is recommended for those at higher risk of complications, such as children, the elderly and those with chronic diseases," He added.

The flu outbreak that started in December is expected to be further reduced and die down by the end of this month, Zhong said.