Overview of included studies
Fig. 1 shows the procedure of study inclusion. We identified 4856 articles using the specified search criteria (PubMed: n=732; EMBASE: n=1612; Scopus: n=2277; PsycINFO: n=235). 940 duplicated papers were removed. Based on the inclusion criteria, 3867 papers were excluded. 49 papers remained for full text review, and 13 papers met inclusion criteria.
Table 2 provides details regarding the 13 studies included in the review. Studies included in our review were published between 1996 and 2018. All countries included in our review either remained as low/mid-income or progressed from low to mid-income, and none of these countries progressed to a higher income status, according to the World Bank. Eight took place in Africa (three in Zimbabwe[19, 31, 32], two in South African[20, 33], two in Ethiopia[34, 35] and one in Tanzania), while the remaining five were performed in Asia (three in Thailand[18, 37, 38] and two in China[39, 40]). Two were RCTs[31, 37] and eleven were cohort studies[18-20, 32-36, 38-40]. All studies reported a statistically significant effect on one or more outcomes, which were reported as follows: eight reported HIV risk behaviors[18, 34-40], four reported HIV/AIDS knowledge[18, 37, 38, 40], four reported HCT uptake[19, 20, 32, 33], one reported HIV public stigma, one reported HIV/AIDS attitude and one reported HIV incidence. Risk behaviors included presence of multiple sex partners, commercial sex, recreational drugs or alcohol use before sex, and condom use, etc. According to the UNAIDS, the definition of HIV public stigma was a process of devaluation of people either living with or associated with HIV infection, such as, those who blamed foreigners/migrant workers/prostitutes for spreading HIV/AIDS.
The types and definitions of HIV intervention among factory workers in LMICs are summarized in Table 3. Further details on the risk of bias are reported in Additional file 1: Table S2. In selection bias, seven articles[18, 20, 32, 37-40] were rated as "strong" because those study participants were factory workers, and more than 80% of the selected individuals agreed to participate. Six articles[19, 31, 33-36] rated as "medium" because only 60-79% of the selected individuals agreed to participate. In study design, two studies[31, 37] were randomized controlled trials (RCTs), so they were rated "strong", and eleven studies[18-20, 32-36, 38-40] were cohort studies, so they were rated "medium". In confounders, two of the thirteen studies[35, 40] were rated as "strong", five as "medium"[18, 34, 36, 38, 39] and the remaining six[19, 20, 31-33, 37] as "weak". In blinding methods, twelve[18, 19, 31-40] were rated "medium" or "weak" because the evaluator or participant knew the task of the study group. In data collection methods, the research data in most studies[18, 20, 32-40] were obtained from the survey and proved to be effective, so they were rated as "strong", but there were two studies[19, 31] that did not evaluate the quality of the acquisition method (such as validity and reliability), so they were rated as "weak". Finally, withdrawals and drop-outs were not related to the six studies[20, 33-35, 38, 40], as they did not include subsequent evaluations. Overall, five cohort studies were assessed as strong quality[18, 34, 35, 39, 40], four as moderate quality[20, 32, 36, 38], and two as weak quality[19, 33]. Two RCTs only show that RCT method was used, but did not describe how to control mixing and how to achieve blind method, so their final evaluation results were weak[31, 37].
Efficacy of different intervention methods among factory workers
An educational intervention
Three studies focused on an educational intervention[18, 37, 39]. Two studies indicated that an educational intervention might improve condom use (condomless sex in the last 12 months decreased from 6.9% at baseline to 3.8% at month 12, P < 0.001; use of condom during sexual intercourse in the last 3 months increased from 41% at baseline to 70% at month 3, P < 0.05)[18, 39]. Two studies showed that an educational intervention could improve HIV/AIDS knowledge[18, 37]. For example, workers who learned that antibiotics did not prevent HIV transmission increased from 46.9% to 56.3% (P = 0.03) and that mother-to-child could spread HIV increased from 82.6% to 93.4% (P < 0.05). One study showed that an educational intervention could reduce the proportion of workers with extra-partners (from 16% to 5%, c2=5.32, P = 0.021). One study showed that an educational intervention could reduce the proportion of workers who used recreational drugs (from 2.6% to 0.7%, P < 0.01) or alcohol (from 17.0% to 6.3%, P < 0.01) before sex. In addition, an educational intervention changed HIV attitudes. For example, the proportion of workers who perceived that if they had HIV/AIDS they would not be able to live in society decreased from 46.6% to 30.6% (P < 0.05).
A community intervention
One study conducted a community intervention. The study indicated that workers having high risk behaviors were more likely to take HCT, and proactive provision of HCT could increase the detection rate of HIV (relative risk [RR]: 1.87, 95% confidence interval [CI]: 1.01 to 3.61) and sexually transmitted diseases (STDs) (RR: 3.47, 95%CI: 2.51 to 4.89). Moreover, HIV seroconversion was higher among subjects who obtained their test results at the first follow-up visit compared to those who did not (19.5% vs. 16.7%, respectively, P = 0.01).
Combination of a lottery intervention and a community intervention
Two studies focused on a lottery intervention combined with a community intervention, both of which analyzed the changes in HCT uptake before and after the intervention[20, 33]. Moreover, these studies had demonstrated that a lottery intervention could improve HCT uptake (from 30% to 85% (P < 0.001) and from 27.3% to 53.6% (P < 0.001)). In addition, a lottery intervention could also reduce HIV public stigma. For example, the proportion of subjects who thought that foreigners/migrant workers/prostitutes were to blame for spreading HIV/AIDS decreased from 22.2% to 9.6% (P < 0.05).
Combination of an educational intervention and a community intervention
Four studies conducted an educational intervention combined with a community intervention[34-36, 40]. Three studies demonstrated that an educational intervention combined with a community intervention reduced the proportion of workers with casual sex (from 12.0% to 6.1%, P = 0.03; from 17.5% to 3.5%, P < 0.001; from 8.8% to 4.6%, P < 0.01). Two studies showed a decrease in the proportion of workers having sex with sex workers[34, 35], but only one report had statistically significant result (from 11.2% to 0.75%, P < 0.001). One study reported a decrease in the proportion of workers who started with more than one sex partner. For example, the proportion of workers with two sexual partners decreased from 17% to 10% (P < 0.05) and with three or more sex partners decreased from 4.7% to 2.0% (P < 0.05). In addition, the combination of these two interventions increased condom use (from 7.6% to 27.3%, P = 0.002) , reduced premarital sex (10.9% in intervention group, 31.3% in control group, P < 0.001), and improved HIV knowledge (P < 0.05)  and an increased awareness of the locations providing free health educational counselling (from 3.5% to 6.7%, P < 0.001) .
Combination of a peer education and a community intervention
Two studies focused on a peer education combined with a community intervention[19, 31]. One study indicated that a peer education reduced incident HIV infection rate (1.51 vs. 2.52 per 100 persons-years, P < 0.05). Another study concluded that a peer education rendered more workers to take their partners to HCT (odds ratio [OR] = 1.37, 95% CI: 1.04-1.79), but statistical significance was not found for individuals to take up HCT (OR = 1.05, 95% CI: 0.92-1.20). In addition, workers with STDs (OR = 2.78, 95%CI: 2.25-3.43), commercial sex (OR = 1.27, 95%CI:1.09-1.49) and multiple sex partners (OR = 1.31, 95%CI: 1.14-1.50) in the last 6 months were more likely to take up HCT.
Combination of a policy intervention and an educational intervention
One study conducted a policy intervention combined with an educational intervention. This study indicated that combination of these intervention increased HIV/AIDS knowledge (t = 2.84, P = 0.005), perceived condom accessibility (OR = 2.80, 95% CI: 1.13-6.86, P < 0.05), and condom use with regular partners (OR = 1.25, 95% CI: 1.01-1.54, P < 0.05) at the last sex.