MSM transmission of HIV is a critical and growing public health problem in China. The HIV seroprevalence rate among MSM has been reported to be 2.5% (95% CI 0.9% to 3.3%) 
. A national study survey of more than 18,000 MSM in 61 Chinese cities in 2008 reported an HIV prevalence of 4.9%, varying from over 10% in the south-west, to 7% in the east and 4–5% along coast in the south and north-east 
. The empirical prevalence of HIV 4.9% in 2008 is much higher than derived from 26 studies published during 2001–2008 
Since the late 1990s, increasing numbers of newly diagnosed HIV infections in MSM have been observed in many countries with large and visible MSM communities 
. Segura et al reported an incidence of 3.9% (95% CI, 2.0%–6.7%) among MSM in Buenos Aires in a cohort study during 2003 to 2004 
. Hurtado et al observed an increase in incidence from 2002 to 2003 in this population in Valencia, Spain 
Since the nationwide prospective cohort studies are not possible to include in the national surveillance program, regular meta-analyses based on the improved data from cross-sectional and prospective studies may provide important information for developing and monitoring the intervention programs, although the results from the current meta-analysis were not able to exactly represent the HIV incidence among MSM in China considering the limitations in estimation method, sampling strategy, number of studies and sample size, representativeness and coverage of rural areas, etc. With a reasonable geographic coverage consisting of Jiangsu (east China), Beijing (north China), Liaoning (northeast China), Chongqing (southwest China), and Xinjiang (northwest China), reasonable comparability of BED and prospective cohort study in estimation of incidence, and moderate quality of the studies included, the findings from our meta-analysis could a glance of HIV incidence among the particular subgroup in China. In this meta-analysis, we included 12 studies from 11 articles publishing in English or Chinese and covering 5 regions in China, making this the first systematic and comprehensive review of HIV incidence to date. The individual incidence of HIV infection included in this meta-analysis varied widely from 2.6% to 9.4%. The observed disparity reflects differences in study methods and study locations.
The incidence of HIV infection among MSM is much higher than the reports in heterosexual populations. Studies in Yunnan, one of the provinces worst hit by HIV in China, reported the incidence estimates of 5.5%, 1.4%, 0.1%, and 0.2% amongst the discordant couples, female sex workers, pregnant women, and pre-marital couples, respectively 
. HIV incidence among sexually transmitted disease clinic patients was reported to be 0.04% (95% CI, 0.02% to 0.10%) in Guangxi of China 
Chongqing is the largest municipality in southwest China, adjacent to Sichuan and Guizhou, provinces most affected by intravenous transmission of HIV by drug use. Chongqing has a higher HIV prevalence than the national average 
. The increased incidence of HIV in MSM in Chongqing might reflect the combined risk from unprotected sex and unsafe injections. The results of our risk factor meta-analysis showed that baseline syphilis infection, multiple sex partnership, and unprotected receptive anal intercourse in the past six months were significantly associated with HIV seroconversion, which reflects the ‘unsafe sex’ among MSM. Such associations are consistent with findings by Xiao et al 
and van der Bij et al 
, and offer direction for HIV prevention and control programmes. Behaviors of illegal drug use among MSM are not well known although a range of 0.1–44% was reported 
, since some MSM might be afraid of identifying themselves as IDUs. One of the included studies 
reported that 2.8% of baseline MSM population had illegal drug use history in Shenyang. Yang X, et al reported 6.5% (65/1000) of recruited MSM had drug use history in Chongqing 
, which is much higher than other districts, and among 65 MSM who ever had drug use behavior, four ever had injection drug use in the last six months, and two admitted to have shared needles. In addition, southwest China is a region with higher HIV transmission through drug use than many other regions in China. MSM/IDU should be a dual-risk in this sub-population for HIV infection. Further studies are needed to investigate drug use behavior and needle sharing behavior among MSM to clarify the dual risk of HIV infection this subgroup.
Study designs may affect results. According to the result of our subgroup meta-analysis by study design, the cohort studies cited 
had lower HIV incidence than cross-sectional studies 
. Prospective cohort studies accurately estimate HIV incidence if seroconversion is observed, but this method is susceptible to recruitment bias, loss to follow-up, short duration of follow-up, inclusion of essential prevention intervention(s) that modify the results, and other unanticipated factors. Cross-sectional studies have employed laboratory methods, mainly BED assays in China, to measure HIV incidence. However, the accuracy of the BED results has been extensively reviewed, and it may vary by place, time and age 
, or depend on the population sampled and storage of specimens 
. Misclassification of BED, which can lead to overestimated HIV incidence, mainly include HIV-infected individuals under antiretroviral therapy, patients with advanced immunodeficiency, and different HIV subtypes 
. However, in China many of studies on BED-based incidence estimation were conducted on surveillance samples in which the complicating factors might not be common 
. Although earlier studies indicated that the performance of BED assay on HIV-1 subtype C-infected individuals was questionable, Parekh et al 
reported that the HIV-1 BED assay worked well with subtype C. HIV infection with subtype C has been observed among heterosexual contacts in Asian country 
. It may be needed to evaluate the performance of BED assay in estimation of incidence for HIV-1 subtype C in China. Based on the ability of BED to discriminate recent from long-term seroconversion of HIV-1 infection among MSM, further molecular analyses can be possible to investigate the distribution of subtypes and monitor the genetic variation of the HIV epidemic in China and among MSM.
Social stigma in China makes the MSM population very hard to reach. Subjects were recruited for study from internet or MSM venues 
, or through snowball sampling or respondent driven sampling 
. Out of 3 cohort studies one retained less than 60% 
, while another retained near 90% of the initial sample at the 12-month follow up 
. MSM with high risk behaviors may not choose to participate in HIV monitoring programs 
. It has been previously reported that cohort participants who returned regularly for follow-up visits were significantly less likely to report high-risk behaviors compared with those who are lost to follow-up 
. Ruan et al reported that men who had a higher level of education were more likely to retain in the cohort 
. It was reported from previous studies in China that more than one-third of the MSM had ever been married, and more than 70% of well-educated gays or bisexuals had got married or would marry with women 
. Choi et al 
showed that 28% of MSM in Beijing self-reported to ever have sex with both men and women during the last six months. Zhang et al 
reported that 63.6% of MSM in China ever had casual male sex partners, and 50% of MSM ever had sex with women in the last year, and the high risk behaviors among this population implies that MSM might be bridge population for others for HIV transmission.
Several biomedically based interventions for prevention of sexual transmission of HIV have obtained encouraging outcomes while many others are currently under way with biological and clinical investigations. Studies in several countries have shown that daily administration of two oral antiretroviral drugs, emtricitabine and tenofovir disoproxil fumarate could applied provide a significant protection against the acquisition of HIV infection by 44% among MSM 
. This preexposure chemoprophylaxis may provide an opportunity for HIV prevention among this population, but there are still a lot of considerations or concerns in translating this evidence into intervention strategy, particularly in developing countries. An antiretroviral-based vaginal microbicide has proved reduction of HIV acquisition by 39% in women in a recent randomised controlled clinical trial in South Africa 
, but rectal microbicides remain at early stages of clinical investigation 
. In order to curb the increasing epidemic of HIV among MSM in China, more risk-reduction intervention efforts are in urgent need and these intervention efforts should be guided by cultural and social context and be responsive to unique demographic characteristics and risk profiles of different subgroups 
. Health education and behavioral interventions are still primary prevention measures in order to reduce the risk-taking behaviors mainly involved in anal intercourse. More innovative and structural interventions including HIV testing and counseling, treatment of other STIs, integration of STI and counseling and testing services into prevention activities remain a focus of prevention and control efforts to MSM in China.
It is concluded that HIV incidence is high in MSM populations in China, and the high risk behaviors among this population implies that MSM might be bridge population for others for HIV transmission. It is assumed that MSM/IDUs are dual-risky for HIV infection, but further researches are needed. The 2007–2010 Guidelines for Prevention and Control of HIV/AIDS among Men Who Have Sex with Men includes a range of evidence-based HIV preventive measures, but few interventions have proven benefit in study with appropriate design and/or implementation, in China or elsewhere 
. The considerable incidence of HIV infection and unique patterns of high risk behaviors in MSM in China serve as a call for action that must inspire new and innovative social and public health HIV prevention strategies. More studies may be necessary in China in future research, at least including (1) validation of cross-sectional BED assay to estimate HIV-1 incidence among patients on antiretroviral therapy and with advanced disease, and other subgroups as well; (2) validation of cross-sectional BED assay for subtype C of HIV-1; (3) development of modeling methods more appropriate for estimating HIV-1 incidence in Chinese settings; and (4) application of BED assays on molecular epidemiological studies of HIV-1 infection.