China's transgender female prostitutes are among the most vulnerable to HIV infection in the country, according to a recent study published in the Journal of the International AIDS Society.

More than 16 percent of the 220 transgender female prostitutes in the study were HIV positive, much higher than the national average of 0.2 percent among female prostitutes, it said. The national average of HIV prevalence stands at less than 0.06.

Wu Zunyou, director of the National Center for AIDS and Sexually Transmitted Disease Control and Prevention at the China Center for Disease Control, estimated that there are less than 10,000 transgender female prostitutes.

Prostitution is illegal in China, but AIDS control work led by the government has integrated underground practitioners into a nationwide HIV surveillance and intervention radar, he explained.

Transgender refers to those whose gender identity is the opposite of their sex at birth. International studies have found they usually account for 0.3 percent of the population.

The study, conducted by public health researchers from Shanghai Jiao Tong University and the Chinese University of Hong Kong, also found that nearly 27 percent of them had reported not using a condom for anal intercourse with male clients in the past month.

Cai Yong, a researcher at Shanghai Jiao Tong University who led the study, said more surveillance and targeted interventions were needed to curb HIV infections.

Wu agreed, but pointed out that more research is needed to guide future interventions.

The limited research available has shown that transgender female prostitutes are at greater HIV risk, Wu said.

"Current outreach and interventions can hardly reach them," he said.

A research report last year by Asia Catalyst, a nongovernment organization, said transgender prostitutes are 49 times more likely to contract HIV than the general population, and nine times more vulnerable than female sex workers.

Many suffer from social discrimination and economic marginalization, leaving them at high risk of HIV/AIDS, it said.