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Work-related mobility and experiences of gender-based violence among female sex workers in Iringa, Tanzania: a cross-sectional analysis of baseline data from Project Shikamana

Objectives To examine how work-related mobility among female sex workers (FSWs) is associated with gender-based violence (GBV) in Iringa, Tanzania. Design Cross-sectional analyses were conducted on baseline data gathered between October 2015 and April 2016 from FSWs participating in Project Shikamana, a community empowerment-based combination HIV prevention intervention. Setting Participants were recruited for the baseline study using venue-based time-location sampling in two communities in Iringa, Tanzania. Participants FSWs were eligible for participation if they were 18 years or older and had exchanged sex for money within the past month. Four-hundred ninety-six FSWs participated in the baseline survey. Primary and secondary outcome measures Any recent experience of GBV was examined by recent work-related mobility among FSWs. Any recent experience of GBV was also disaggregated by severity for analyses. All bivariate and multivariate binary and multinomial logistic regressions adjusted for intraclass correlations among women recruited from the same venues. Results Forty per cent of participants experienced recent physical or sexual violence, and 30% recently experienced severe physical or sexual violence. Thirty-three per cent of participants recently exchanged sex for money outside of their district or region, and 12% were both intraregionally and inter-regionally mobile for sex work. Intraregionally and inter-regionally mobile FSWs had 1.9 times greater odds of reporting recent GBV (adjusted OR: 1.89; 95% CI: 1.06 to 3.38; p=0.031) compared with non-mobile FSWs and a 2.5 times higher relative risk for recent experience of severe GBV relative to no recent GBV (relative risk ratio: 2.51; 95% CI: 1.33 to 4.74; p=0.005). Conclusions Mobility for sex work may increase FSWs’ exposure to GBV, particularly more severe GBV. The vulnerability of mobile FSWs to violence, particularly severe forms, demands inclusive services that are accessible to mobile FSWs.

Study Design: Time elements in the study are not aligned in all measures. The outcome of interest (GBV) and explanatory variable of interest (work-related mobility) both extend to a six-month recall period; however, inclusion criteria allow for FSWs enrollment among those who have exchanged sex for money in (at least) the last month only. These unequal timeframes may produce lower-thanexpected figures in work-related mobility measures, as the experience of a FSW who enrolls in the study with less than six months of sex for money exchange may only speak to her time engaged in this work. Likewise, FSW's odds of recent GBV, as associated with work-related mobility, may be lower for those individuals. Though GBV may still be associated with FSW mobility in contributing to behaviors of exchanging sex for money, as a result of stigma and discrimination, the study does not specifically measure this unique variant in the cohort with fewer than six months experience exchanging sex for money. It is advised that the authors mention this as a limitation.
Article Summary: The text in this section does not seem to fit the heading "Strengths and Limitations," yet these bullets serve as a basic article summary; this sub-heading may be unnecessary.
Alternatively, perhaps the Article Summary section is incomplete, as the numerical list has only one item. Please either 1) remove the "Strengths and Limitations" sub-heading, or 2) adjust bulleted text to properly address strengths (limitations are briefly covered), and (if applicable) insert the remaining items in this list for text copy.
Data Collection: The manuscript does not mention who conducted the surveys for the baseline study. While the study under review analyzes the baseline as secondary data, it would be helpful for the reader to understand these additional details for possible bias in participant responses.
Clarity of Text: • The description of venue type as categorical for "most often" selected venue by FSWs is first mentioned in the Results section. To clarify that respondents could only choose one venue as a preferred work environment, please include this brief explanation (or the phrase "most often") in the Methods section (sub-section 2, page 8, paragraph beginning with line 167).
• Line 309 (p14): sub-Saharan Africa rather than Sub-Saharan Africa • Line 310 (p14): Please clarify word choice or what is meant by "concrete effects" in this sentence. For example, stigma and discrimination may affect treatment outcomes (different results than intended), or treatments should consider the effects of stigma and discrimination? References: • 5 -Please complete the open space after the dash in (80-), or remove.
• 20 -This source is incomplete without a date and concluding period.

GENERAL COMMENTS
The paper is well written and covers an important and under researched topic on work-related mobility and GBV among female sex workers (FSWs) in Tanzania. The paper analyses association of inter-and intra-mobility and the GBV risks of FSWs in Tanzania from baseline survey data. Based on the available literature on sex work mobility and GBV risks, the paper fills an important gap in knowledge by reporting on participants characteristics, the mobile and non-mobile experiences, and work related mobility and experiences of GBV among FSW in Tanzania. The introduction provided by the authors is appropriate. The review of literature is adequate given the limited and narrow scope of studies surveying hazards of sex work mobility in most low income settings including in the sub-Saharan African region. Thus, the paper somewhat covers unfamiliar grounds in terms of its findings. With that said, my main concern is the explanation given for sample size (line 127) that states that a description of the sample size is given elsewhere in another paper. It would be important for the authors to briefly describe how sample size was derived and then refer readers to a more detailed description of sample size in the paper cited. Also, I would like the authors to indicate where ethical approval to conduct the study was obtained and give a detailed account of how ethical concerns were handled in the study. I did not find reference of this made anywhere in the paper. Please document if there were mechanisms put in place to refer respondents for GBV services. With these changes the paper will make important contributions to the literature. Comments: General comment: Please ensure that the revised manuscript does not exceed the words allowable. Introduction; In addition to providing a generalized account of female sex workers mobility, the authors would do well by focusing on similar aspects that affect sex workers in Tanzania such as those that likely increase their health risks such as sex work environment, existing laws and policies touching on this population group. This would likely give the reader the much needed background of the environment in which sex work occurs and associated risks. Methods; My question is whether there was an ethical approval for the study. If so can the authors state where permission to conduct the study was received? Also what was the response/refusal rates during the survey implementation?
Results: General comment: The results are presented well. Conclusion: The author could talk about how specific interventions could be tailored (to target female sex workers) given the repressive laws and existing environment of stigma and discrimination, high risk behaviours and pervasive abuse in Tanzania as well as other similar contexts.
Addressing FSW health needs requires targeted locally informed and culturally relevant messages to raise personal awareness of risk and change socio-cultural norms around sex-for-money exchanges. Generally, there are few countries/programs with interventions dealing with the impacts of intra and inter mobility patterns among female sex workers.

REVIEWER
Tsitsi B Masvawure Dept of Sociology and Anthropology College of the Holy Cross Worcester, MA, USA REVIEW RETURNED 24-May-2018

GENERAL COMMENTS
Thank you for the opportunity to review this manuscript. Overall, the manuscript is very well written with methods, results and significance well articulated. The conclusions are well supported by the data presented. Below are a few minor comments for consideration: 1. The authors state in the methods (p7, line 140) that they dichotomized severity of GBV into two: less severe forms and severe forms. I think that measuring severity of GBV has important programmatic implications. However, the authors do not present any data that shows if severity is significantly associated with workrelated mobility. They mention severity in the Discussion but not in the Results or in any of the Tables presented. Related to this, it is also interesting that FSWs who reported having fewer clients per week had more experiences of violence (see table 2). Again, how much of this is related to the severity of violence experienced? It would be great if the authors also commented on this curious finding.
2. Page 11, lines 244-246: the authors state that "a higher percentage...had sex in private vehicles". This statement is misleading considering that only 15 FSWs in total reported having sex in private vehicles compared to 481 who did not. I understand the comparison that the authors are making but given the very small numbers involved the statement sounds over-blown. 3. Page 10, line 225: I think there is an error here: it should be Table  2, not Table 1.

GENERAL COMMENTS Comments
The authors' research is relevant especially within the African context and the manuscript is clear and interesting to read. Below are my comments: -The different themes within the introduction should be arranged in a somewhat systematic manner; i.e. information on Iringa specific contexts should be moved further down in the introduction and the section could start with a more general perspective and then flow down to the more specific/ localized setting of the study.
-The authors need to provide more description of the time location sampling in the methodology section and/or include a reference if this has been adequately described in a previous paper -Although dichotomization of the variables for GBV simplifies the interpretation and presentation within the paper, I fear this is gained at the cost of loss of information and statistical power. The authors may consider splitting this variable into more severity categories to enrich the information provided. The categorization herein for example, have categorized the severity of gang rape/rape with being hit with a fist as same.
-Line 318-321 discusses social cohesion data which is not found anywhere within the manuscript. I don't see the relevance for this inclusion within the discussion. I recommend that the authors either include the data in results section if it is available or removing it entirely from the discussion -Authors should include within the results and discussion, the av. weekly number of clients. It would be interesting to know how this relates with the outcomes of GBV and mobility.
-Remove the first sentence in the conclusion as it has no significance to the specific outcomes of the study Wonderful to see a manuscript on this important topic! It has great potential to influence GBV prevention efforts with FSWs. I recommend it for publication, with minor revisions. Please see comments below: Study Design: Time elements in the study are not aligned in all measures. The outcome of interest (GBV) and explanatory variable of interest (work-related mobility) both extend to a six-month recall period; however, inclusion criteria allow for FSWs enrollment among those who have exchanged sex for money in (at least) the last month only. These unequal timeframes may produce lower-thanexpected figures in work-related mobility measures, as the experience of a FSW who enrolls in the study with less than six months of sex for money exchange may only speak to her time engaged in this work. Likewise, FSW's odds of recent GBV, as associated with work-related mobility, may be lower for those individuals. Though GBV may still be associated with FSW mobility in contributing to behaviors of exchanging sex for money, as a result of stigma and discrimination, the study does not specifically measure this unique variant in the cohort with fewer than six months experience exchanging sex for money. It is advised that the authors mention this as a limitation.
Response #2: Thank you very much for this important point. We agree with the reviewer's point, and although the recall period for the outcome of interest and the explanatory variable of interest are the same, it is an important point that both of these variables have time periods that differ from the inclusion criteria, which has those who have exchanged sex within at least the last month. It is important to note that participants who had exchanged sex for money for shorter periods of time have less time to have been able to be mobile for work, which may mean that their work-related mobility measures are lower than would be expected, as you describe, than those who had exchanged sex for money for at least six months. We have included a discussion of this point in the discussion on p. 17, lines 350-357.
Article Summary: The text in this section does not seem to fit the heading "Strengths and Limitations," yet these bullets serve as a basic article summary; this sub-heading may be unnecessary. Alternatively, perhaps the Article Summary section is incomplete, as the numerical list has only one item. Please either 1) remove the "Strengths and Limitations" sub-heading, or 2) adjust bulleted text to properly address strengths (limitations are briefly covered), and (if applicable) insert the remaining items in this list for text copy.

Response #3: Thank you very much for your comment. As part of the journal's style, a strengths and limitations section, termed the "Article Summary" is required. The section is supposed to highlight the methodological strengths and limitations of this article, rather than the strengths and limitations more generally. We have revised this section to meet the journal's formatting requirements (see p. 3, lines 54-68). Strengths include strengths in the study design (e.g. participatory), the use of venue-based time-location sampling, and the disaggregation of gender-based violence based on severity. Limitations include generalizability of the findings and potential recall bias (see p. 3, lines 54-68).
Data Collection: The manuscript does not mention who conducted the surveys for the baseline study. While the study under review analyzes the baseline as secondary data, it would be helpful for the reader to understand these additional details for possible bias in participant responses.

Response #4: Thank you for your attention to detail and for identifying this important aspect of the methods that is in need of further clarification. We have incorporated details about who conducted the surveys for the baseline on p. 7, lines 140-143.
Clarity of Text: • The description of venue type as categorical for "most often" selected venue by FSWs is first mentioned in the Results section. To clarify that respondents could only choose one venue as a preferred work environment, please include this brief explanation (or the phrase "most often") in the Methods section (sub-section 2, page 8, paragraph beginning with line 167).
Response #5: Thank you for identifying the lack of clear language for this variable. We have clarified the definition of the variable. Women were asked what type of establishment they work in and provided a single response (see p. 9, line 187-p. 10, line 190). The sentence in the results that used "most often," has been revised to clarify that a higher percentage reported working in modern bars and guesthouses rather than traditional bars (see p. 12, lines 241-244).
• Line 310 (p14): Please clarify word choice or what is meant by "concrete effects" in this sentence. For example, stigma and discrimination may affect treatment outcomes (different results than intended), or treatments should consider the effects of stigma and discrimination?
Response #7: Thank you for this point. We have clarified the language to specify that stigma and discrimination can be internalized and can negatively influence how people treat and interact with FSWs. See p.16, lines 330-331. The paper is well written and covers an important and under researched topic on work-related mobility and GBV among female sex workers (FSWs) in Tanzania. The paper analyses association of inter-and intra-mobility and the GBV risks of FSWs in Tanzania from baseline survey data. Based on the available literature on sex work mobility and GBV risks, the paper fills an important gap in knowledge by reporting on participants characteristics, the mobile and non-mobile experiences, and work related mobility and experiences of GBV among FSW in Tanzania. The introduction provided by the authors is appropriate. The review of literature is adequate given the limited and narrow scope of studies surveying hazards of sex work mobility in most low income settings including in the sub-Saharan African region. Thus, the paper somewhat covers unfamiliar grounds in terms of its findings.

References
Response #9: Thank you very much for these comments! This is an under researched topic and we are happy to contribute to the literature.
With that said, my main concern is the explanation given for sample size (line 127) that states that a description of the sample size is given elsewhere in another paper. It would be important for the authors to briefly describe how sample size was derived and then refer readers to a more detailed description of sample size in the paper cited. Also, I would like the authors to indicate where ethical approval to conduct the study was obtained and give a detailed account of how ethical concerns were handled in the study. I did not find reference of this made anywhere in the paper. Please document if there were mechanisms put in place to refer respondents for GBV services. With these changes the paper will make important contributions to the literature.
Response #10: Thank you very much for highlighting these places for further elaboration. We have expanded the discussion of the sample size calculation to explain how the final sample was derived (see p. 7, lines 137-140). Thank you also for emphasizing the importance of ethical approval. We had removed the names of the IRB institutions for the purposes of peer review, but we have added back in those names (see p. 8, lines 147-149). We have emphasized the importance of privacy and confidentiality during data collection (see p. 7, lines 142-143) and also how consent was received (see p. 8, lines 146-147). In addition, we have also elaborated on the mechanisms put in place by the study to refer respondents as necessary for services (see p. 8, lines 149-151).

Comments:
General comment: Please ensure that the revised manuscript does not exceed the words allowable.
Response #11: Thank you for your comment about the word limit. We have ensured that the revised version of this manuscript does not exceed the length allowable. It is currently 3987 words (under the 4000 word limit).
Introduction; In addition to providing a generalized account of female sex workers mobility, the authors would do well by focusing on similar aspects that affect sex workers in Tanzania such as those that likely increase their health risks such as sex work environment, existing laws and policies touching on this population group. This would likely give the reader the much needed background of the environment in which sex work occurs and associated risks.
Response #12: Thank you for your comment about the introduction. We agree about the importance of context and environment for how sex work occurs and the associated risks in Tanzania. As a result, we have elaborated on the context of sex work in Tanzania to describe the environment as well as existing laws and policies that are relevant to FSWs (see p. 6, line 113p. 7, line 124).
Methods; My question is whether there was an ethical approval for the study. If so can the authors state where permission to conduct the study was received? Also what was the response/refusal rates during the survey implementation?
Response #13: Ethical approval was received from three institutions for this study. We have clarified the institutions and elaborated on ethical issues related to this study (see p. 7, lines 140-p. 8, line 151). This survey was administered as a baseline as part of a phase II communityrandomized trial of the Shikamana intervention. The enrollment rate was 65%, with only 5% explicitly refusing participation among those eligible (24 of 762 FSWs). We have clarified this on p. 8, lines 145-146.
Results: General comment: The results are presented well.

Response #14: Thank you very much for this comment!
Conclusion: The author could talk about how specific interventions could be tailored (to target female sex workers) given the repressive laws and existing environment of stigma and discrimination, high risk behaviours and pervasive abuse in Tanzania as well as other similar contexts.
Addressing FSW health needs requires targeted locally informed and culturally relevant messages to raise personal awareness of risk and change socio-cultural norms around sex-for-money exchanges. Generally, there are few countries/programs with interventions dealing with the impacts of intra and inter mobility patterns among female sex workers.
Response #15: Thank you very much for this recommendation. We agree with the reviewer's comments about the importance of tailoring interventions. We appreciate that few countries/programs have interventions that deal with the impacts of intra and inter mobility patterns on FSWs and therefore have elaborated on our discussion of specific interventions and development of messaging based on the findings of this article. Please see p. 18, lines 379-384 as well as p. 18, lines 388-395 in the "Public Health Implications" section for further elaboration.
Reviewer: 3 Reviewer Name: Tsitsi B Masvawure Institution and Country: Dept of Sociology and Anthropology, College of the Holy Cross, Worcester, MA, USA Please state any competing interests: None declared Please leave your comments for the authors below Thank you for the opportunity to review this manuscript. Overall, the manuscript is very well written with methods, results and significance well articulated. The conclusions are well supported by the data presented. Below are a few minor comments for consideration: Response #16: Thank you very much for these kind words. We have addressed the comments described below.
1. The authors state in the methods (p7, line 140) that they dichotomized severity of GBV into two: less severe forms and severe forms. I think that measuring severity of GBV has important programmatic implications. However, the authors do not present any data that shows if severity is significantly associated with work-related mobility. They mention severity in the Discussion but not in the Results or in any of the Tables presented. Related to this, it is also interesting that FSWs who reported having fewer clients per week had more experiences of violence (see table 2). Again, how much of this is related to the severity of violence experienced? It would be great if the authors also commented on this curious finding.
Response #17: Thank you for much for acknowledging the importance of dichotomizing severity of GBV. We have described this variable in the methods section on p. 8, lines 158-161. We would also like to clarify that data in Table 4 clarifies the associations between workrelated mobility and severity of GBV. These findings are also discussed in the results section on p. 14, lines 289-295. We have copied this language below: "In adjusted multinomial models disaggregating experiences of GBV by severity, those reporting both intra-and inter-regional work-related mobility had a significantly higher relative risk (RRR: 2.51) of more severe GBV relative to no GBV (95% CI: 1.33-4.74; p<0.01) as compared to those not mobile for sex work (Table 4). FSWs reporting recent general mobility, a higher average number of clients per week, higher sex work-related stigma, and hazardous/harmful drinking practices similarly had significantly higher relative risks of more severe GBV relative to no GBV, as compared to their counterparts (Table 4)." The association between the number of clients and GBV was an important component of the analyses conducted, and we want to clarify the results for the reviewer as well. The average number of clients is positively associated with gender-based violence (45.7% of those with recent GBV had more than 2 clients on average per week vs. only 30.8% among those with no recent GBV (p=0.001). A higher average number of clients per week was significant associated with increased odds of recent GBV (dichotomous) in unadjusted models, but was not significantly associated in adjusted models. The association with GBV (disaggregated by severity) showed that in both unadjusted and adjusted models, those reporting a higher average number of clients per week had a significantly higher relative risk of more severe GBV relative to no GBV (See Table 4). We have, as per the recommendations of multiple reviewers, expanded the discussion of these findings in the results and discussion (see p. 13 line 280-p. 14, line 295; p. 16, lines 336-342).