Human papillomavirus (HPV) infection is one of the most common sexually transmitted infections worldwide, and persistent genital high-risk (HR) HPV infection is the most important etiological factor in the development of cervical cancer. Cervical cancer represents a major health problem in Tanzania, with more than 7500 new cases annually, and around 6000 women die from this disease each year. A high prevalence of HPV infection1 and lack of cervical cancer screening programs are among the main reasons for the high incidence of cervical cancer. Thus, initiatives to decrease morbidity and mortality related to HPV infection are highly needed, and efficient information campaigns about HPV transmission and infection risk factors are important in this effort.
The prevalence of HPV infection differs by geographical area. In sub-Saharan Africa, the combination of factors such as hazardous sexual behavior and a high prevalence of HIV infection may contribute to high HPV prevalence and high HPV transmission rate. Several studies of risk factors for HPV infection have been conducted in different parts of the world, and they have consistently shown that the risk of HPV is increasing with increasing number of sexual partners.2 Risk for HPV infection has also been associated with younger age at first intercourse,3 increasing parity, use of oral contraceptives,4 use of barrier contraceptives,5 and infection with other sexually transmitted diseases.6 In most studies, HPV has been found to be more prevalent in young individuals and decreasing with increasing age.2,7 In some studies, including our recent study of HPV prevalence in women from Tanzania, a second peak in the HPV prevalence has been observed among older women.1,8,9 Relatively few studies of risk factors for HPV have been conducted in sub-Saharan Africa where HIV prevalence is high and other specific lifestyle habits such as multiparity are common. Thus, the complete spectrum of risk factors in play is not yet fully understood, and in addition, it is not known whether specific risk factors apply to different age groups.
We have previously reported on the HPV prevalence and genotype distribution among women from Tanzania.1 The goal of the present article is to examine the risk factors for HR HPV infection and risk factors for HR HPV infection according to HIV status in the same population of Tanzanian women. Furthermore, we also examined the risk factors for HR HPV in different age groups.
MATERIALS AND METHODS
Enrolment and Data Collection
The study has been described previously in detail.1 In brief, we conducted a cross-sectional study, in the period from February 2008 till March 2009, of women living in Tanzania. The study was approved by the Ethical Committee of the Tanzanian National Institute of Medical Research, and all women included in the study were informed verbally about the goal of the study and gave informed consent. Women who were adolescent minor were informed about the study in the presence of their legal guardian and were included when both the legal guardian and them gave their informed consent.
The study population was recruited from urban and rural areas in Tanzania. The urban group consisted of women in the Dar es Salaam Region who spontaneously attended screening at the Ocean Road Cancer Institute screening clinic and a second, community-based group of women who had never responded to the National Cervical Cancer Screening program. In the rural study group, we enrolled women residing in Pwani (northeast and southeast of Dar es Salaam region) and Mwanza (northwestern Tanzania) regions. In all locations, the women were informed through general public announcement that cervical screening was taking place and that they were invited to meet at their nearest hospital or health center for an interview and a gynecological examination. In rural areas, special screening clinics were set up in small district hospitals and health centers.
All women had a gynecological examination, including visual inspection with acetic acid and a conventional Papanicolaou test. Furthermore, cervical cells were collected in sample transport medium (Digene Corporation, Gaithersburg, MD) for HPV testing. The specimens were stored at −20°C, and at the end of the study, they were shipped to the Division of Experimental Virology for analysis (Tuebingen, Germany). Finally, a blood sample was obtained for HIV testing from all women who accepted testing.
We did not use HPV results in the diagnostic workup because the results were only available after the completion of the study. Screening for cervical cancer in the present study was based on visual inspection with acetic acid, which is the current standard of care in Tanzania.
In addition, all women were interviewed by trained female nurses using a structured questionnaire to collect information on socioeconomic and lifestyle factors. Furthermore, weight and height measurements were obtained on all women to determine body mass index (BMI).
HPV DNA Testing
The Hybrid Capture 2 (HC2) test (Qiagen, Hildesheim, Germany) with the HR probe cocktail was used to detect HR HPV DNA in cervical swabs, as described previously.10 A result was considered positive when a woman was found to have on1 or more of the 13 HR HPV types (16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 68) using the Food and Drug Administration–approved threshold of 1.0 relative light unit coefficient.
Human papillomavirus–positive samples were further analyzed to determine the HPV genotype using the LiPaExtra (Innogenetics, Gent, Belgium). Initially, DNA was isolated in a MagnaPure device (Roche Systems, Indianapolis, IN) from 200 μL of the remaining denatured product from the HC2 test. The INNO-LiPA HPV Genotyping Extra test (Innogenetics) is a line blot assay based on the principle of reverse hybridization. It is designed for the detection of 28 different HPV types covering all currently known HR and probable HR HPV types (16, 18, 26, 31, 33, 35, 39, 45, 51, 52, 53, 56, 58, 59, 66, 68, 73, 82) as well as 7 low-risk HPV types (6, 11, 40, 43, 44, 54, 70) and 3 additional HPV types (69, 71, 74).11 Specific sequences of the L1 region of the HPV genome are amplified using SPF10 primers, and the resulting biotinylated amplicons are then denatured and hybridized with specific oligonucleotide probes. In addition, a set of primers for the amplification of the human HLA-DPB1 gene is added to monitor sample quality and extraction. Using the Auto-LiPA 48 device, a fully automated processing of the strips is performed. Finally, an automatic and objective interpretation of the strips is provided with the LiPA-Scan reading template software assisting for the interpretation of the HPV genotyping results for LiPA HPV.
HIV testing was done according to a standard algorithm and in accordance with Tanzanian Ministry of Health guidelines. Blood specimens were tested with one rapid immunoassay (SD Bioline HIV-1/2 3.0 rapid test; Standard Diagnostics Inc, Republic of Korea). For all samples with positive rapid test results, 2 additional rapid tests from 2 different manufacturers, namely, Determine HIV-1/2 (Abbott Laboratories SA, South Africa) and UNIGold method/Recombigen HIV (Trinity Biotech, Jamestown, NY), were performed consecutively to confirm the initial result.
HIV testing was always preceded and followed by counseling. All HIV-positive women were referred to a care and treatment clinic where they were offered thorough clinical follow-up and treatment when necessary.
We initially analyzed risk factors for overall HR HPV infection, and subsequently, risk factors for HR HPV were examined separately among HIV-positive women and HIV-negative women. Finally, we assessed the risk factors for HR HPV in different age groups.
The association between cervical HR HPV infection and potential risk factors was evaluated by multivariate analysis, estimating the odds ratio (OR) with 95% confidence interval (CI). The evaluation included adjustment for variables selected a priori based on the literature (and the availability in the questionnaire) such as sociodemographic factors (age, place of residence, educational level), reproductive and sexual behavior (number of births, lifetime number of partners, intercourse in the last 12 months, duration of the present relationship, condom use in the past 12 months, circumcision status of current partner), and medical conditions (HIV status, clinically detected genital warts, BMI). Age (age as a continuous variable with a spline at age 50 years) was always included in the statistical model, and the other factors were entered into the model if they had at least a 10% significance level. In the final statistical model, we kept the factors that were statistically significant at a 5% level or changed the estimates of the other variables. In the age-stratified analyses, we divided the women into the following age groups; 29 years or younger, 30 to 39 years, 40 to 49 years, and 50 years or older. The oldest women were initially categorized into 2 groups (50–59 years [n = 424] and ≥60 years [n = 141]), but because the results were similar, we only present the results for the most stable analysis grouping the women 50 years or older in only one group. The analysis of HIV-positive women was adjusted for age. In addition, we made an analysis where the confounders included in the model for HIV negative (age, place of residence, ever giving birth, number of births, lifetime number of partners, condom use in the past 12 months, and BMI) were also included. Because the estimates were virtually unchanged, only the age-adjusted estimates are presented. The age-stratified analyses were adjusted for age and HIV status within each age group. All statistical analyses were carried using SAS, version 9.1.
A total of 3767 women were enrolled in the Prevention of Cervical Cancer in Tanzania study. We excluded 37 women (1.0%) with clinical conditions that prevented a proper examination (eg, total hysterectomy, menstruation, and pregnancy) and 31 women (0.8%) because their samples could not be tested for HPV (insufficient material), leaving 3699 women in the study.
Among the 3699 women constituting the study population, 751 (20.3%) were HR HPV positive, covering 427 women with a single-type HPV infection and 240 women with multiple HPV types, and 84 women were HC2 positive but LiPa negative. A total of 349 women (9.4%) were HIV positive. Among these women, 45.9% were HR HPV positive, whereas among HIV-negative women, the HR HPV prevalence was only 17.3%.
Overall, 24.2% of the women were 29 years or younger, whereas women 50 years or older constituted 15% of the population. Most women (85.8%) were married, and 63.3% had completed primary school level education. Finally, 32% reported having had 1 lifetime sex partner, whereas 9% reported 6 or more partners. Most women (83%) reported that their current partner was circumcised.
Factors Associated With HR HPV Infection
In Table 1, risk factors for HR HPV infection are presented. The risk decreased with increasing age, and women 29 years or younger had 2.3 times (95% CI, 1.5–3.5) higher odds of being HR HPV positive compared with 50- to 59-year-old women. Women 60 years or older had significantly increased odds (OR, 3.0; 95% CI, 1.9–4.9) of being HR HPV positive compared with 50- to 59-year-old women.
Women living in rural areas had higher odds for HR HPV infection compared with women living in urban areas (OR, 1.3; 95% CI, 1.1–1.5). In contrast, educational level was not associated with the risk of HR HPV. Several of the reproductive and sexual factors also played a role. Young age at first birth tended to increase the odds of being HPV positive, and in addition, the odds of HR HPV increased 8% (95% CI, 1.04–1.13) per additional birth. Similarly, increasing lifetime number of sexual partners was statistically significantly associated with detection of HR HPV, with women reporting 6 or more lifetime partners being at a 1.3 time (95% CI, 1.0–1.8) higher risk compared with women reporting one lifetime partner. The odds increased 3% (95% CI, 1.00–1.05) per additional partner. In contrast, the odds of HR HPV infection decreased 3% (95% CI, 0.95–0.99) per additional year spent in the present relationship. Age at first intercourse was not associated with risk of HR HPV infection (data not shown).
Women reporting ever-using condoms during intercourse in the past 12 months had a 1.3 times (95% CI, 1.0–1.6) increased odds of being HR HPV positive compared with those reporting never using condoms. By contrast, circumcision status of the present partner as reported by the woman was not associated with HR HPV infection.
Being HIV positive was the strongest risk factors for HR HPV detection (OR, 4.1; 95% CI, 3.3–5.3), but also having genital warts (detected at the clinical study examination) increased this risk (OR, 2.1; 95% CI, 1.0–4.9). Finally, we found that the odds of being HR HPV positive decreased with increasing BMI, with 12% per additional BMI unit (95% CI, 0.78–0.9).
Factors Associated With HR HPV Infection According to HIV Status
We also examined the potential risk factors for HR HPV infection among HIV-positive and HIV-negative women (Table 1).
Among HIV-positive women, the odds of being HR HPV positive decreased with increasing age, with the exception of women 60 years or older (OR, 4.3; 95% CI, 0.6–29.2). HIV-positive women reporting 6 or more lifetime sexual partners had 1.8 (95% CI, 0.8–3.9) times higher odds of being HR HPV positive compared with HIV-positive women with only 1 lifetime sexual partner (Table 1). We also found a 4% decreasing risk per year spent in the present relationship (95% CI, 0.91–1.02). Furthermore, clinically detected genital warts in HIV-positive women were also associated with increased odds of HR HPV infection (OR, 7.7; 95% CI, 0.9–63.7). Finally, underweight HIV-positive women had increased odds for HR HPV infection (OR, 1.6; 95% CI, 0.7–3.6) compared with normal-weight HIV-positive women and the odds of HR HPV decreased 11% (95% CI, 0.70–1.13) per additional BMI unit. However, none of these estimates reached statistical significance.
The risk factor pattern in HIV-negative women was very similar to that in HIV-positive women, including decreasing odds for being HR HPV positive with increasing BMI (13% decrease per extra BMI unit; 95% CI, 0.79–0.96).
Factors Associated With HR HPV Infection According to Age
Table 2 depicts potential risk factors for HPV infection in different age groups. In all age groups, HIV positivity was a statistically significant risk factor. In contrast, living in rural area was associated with increased odds of HR HPV infection, especially among the younger women (<29 and 30–39 years). Other risk factors found especially among the younger age groups included lifetime number of sexual partners (OR, 1.8 [95% CI, 1.3–2.6], for ≥3 partners vs. 1 partner among women ≤29 years). In addition, among women 29 years or younger who had engaged in sexual intercourse in the past 12 months, the odds of HR HPV was increased, although statistical significance was not achieved (OR, 1.6; 95% CI, 0.7–4.0). In contrast, among the older women (≥50 years), the risk factors included educational level (OR, 1.2 [95% CI, 0.8–2.1], for no formal education vs. primary school: data not shown) and BMI (OR, 2.8 [95% CI, 1.1–6.9], for BMI <18.5 kg/m2 compared with BMI 18.5–25 kg/m2). Finally, the strongest association with age at first birth was seen among the oldest women (OR, 2.1; 95% CI, 0.5–9.9) for first birth at 18 years or younger compared with never giving birth. No association was found in the older women with number of sex partners (Table 2).
The strongest risk factor for HR HPV infection in the present study is HIV status. We find that compared with HIV-negative women, HIV-positive women have a statistically significantly increased risk of being HR HPV, which is in line with other studies12,13 and a meta-analysis.14 The increased risk of HR HPV infection among HIV-positive individuals has been shown to be caused by decreased CD4+ cell count and lack of an immune response capable of clearing HPV infection.15,16 Interestingly, recent studies have shown that being HPV positive increases the risk of becoming HIV positive by mechanisms that are still not completely understood.17–19 Consequently, it could have a great health impact not only on HPV-related diseases if HPV infection could be prevented in countries where HIV infection is prevalent.
Apart from increasing lifetime number of sex partners, which increased the risk of testing HR HPV positive, the duration of the present relationship also plays a role in our study. For every year longer in the present relationship, the risk of HR HPV infection decreased with 3% (95% CI, 0.95–0.99). This could be explained if a longer duration of the present relationship reflects a decreased likelihood of new partners and otherwise less risky sexual behaviors and therefore decreased risk of exposure to HPV infection. In the present study, women reporting no current relationship also have a statistically significantly increased risk of HPV infection, probably indicating that these women in our study may have a higher likelihood of shorter casual sexual relationships, increasing their risk of being exposed to new HPV infections.
We also find that both increasing number of births and young age at first birth increase the risk of being HR HPV, suggesting that both repeated cervical trauma during consecutive births, but probably also cervical changes caused by hormonal adjustment such as the ones experienced during and after pregnancies, may be important factors influencing susceptibility to HPV infections.20 The association between number of births and HPV prevalence is in line with results from other study populations where multiple births were common.21
In the present study, the risk of being HR HPV positive is related to the BMI of the woman. Both overall, but especially among HIV-positive women, the risk decreased with 11% to 13% per additional BMI unit, that is, the risk was increased in underweight women compared with overweight women. A recent study showed that being underweight was a good clinical predictor of a low CD4+ cell count and low CD4/CD8 ratio in HIV-infected patients, decreasing the likelihood of resistance to infection.2,22 Nevertheless, that we observe an effect also in HIV-negative women, although less pronounced, may suggest that having a lower BMI alone represents a risk factor for HR HPV infection. Our findings are in line with a review pointing to the fact that deficiency of nutrients occurring in malnutrition has a negative effect on host immune system independently of HIV status.23 Our results are also in agreement with a study conducted among sex workers in Spain showing that women with high BMI had lower risk of being HR HPV infected.24
We failed to observe a lower HPV prevalence in women who reported the current partner to be circumcised. However, it is a limitation that our results are exclusively based on circumcision status reported by the female partner. Nevertheless, our results may be in line with a study from Uganda showing that circumcision of HIV-positive men did not reduce transmission of HR HPV to their female partners25 and a recent review concluding that although male circumcision more than halved the acquisition of HIV in clinical trials, an additional trial found no direct reduction in HIV risk for female partners of circumcised men.26 However, results are conflicting, and some studies have showed a reduced risk for HPV infection in women where the partner was circumcised because it has also been demonstrated for HIV infection.27,28 This has led to the introduction of World Health Organization recommendations encouraging circumcision in regions of the world high HIV prevalence and low male circumcision prevalence.29
Like in most previous studies, we found age to be an important risk factor for HPV infection. The risk of HR HPV is high among younger women and decreases gradually with age until age 60 years, whereafter we observed a significant second peak. This picture was consistently seen for overall HR HPV infection and also among HIV-positive and HIV-negative women. The increased risk of HPV in older women has been suggested to be caused by a lower rate of HPV clearance and increased duration of HPV infection induced by a decreasing immunity with age,8 nonmonogamous sexual activity of women or their current partners leading to new HPV infections,30 or a reactivation of latent HPV infection induced by changes in hormonal or immunological factors, or alternatively, it may be the result of a cohort effect.8
In the present study, we attempted to further understand the increased prevalence of HR HPV in older women by assessing the potential risk factors for HR HPV detection in different age groups. We find that women 50 years or older have a significantly higher risk for HPV infection when they are HIV positive. In addition, the risk was significantly increased in underweight women, even when estimates were adjusted for HIV status. This may indicate a role of decreased immunity leading to increased risk of reactivation and/or increased susceptibility to new HPV infections. Young age at first birth was associated with higher risk for HPV infection, especially among women 50 years and older. In accordance with our results, a recent study conducted in Nigeria also showed that young age at sexual debut and at first pregnancy were more strongly associated with HPV infection among older women (≥56 years age).31 Finally, our study showed that lifetime number of sexual partners and sexual activity in the past 12 months increased risk of HR HPV in the younger women (<29 and 30–39 years), whereas these factors had no effect in older women.
The strengths of this study include the large sample size and a wide age range. In addition, the relatively high proportion of HIV-positive women gives us the opportunity to examine the risk factors for HR HPV according to HIV status. However, the study also has some limitations that should be considered. The cross-sectional design of the study limits the possibility to address causal relationship. In addition, although we aimed at including women from different areas and with different cervical cancer screening history, we cannot exclude selection bias, and the results may therefore not be entirely generalizable to all women in Tanzania. Finally, it is a limitation that we do not have information on CD4 counts as a measure of immune status.
In conclusion, we showed that age, time in the present relationship, number of child births, number of sex partners, and also decreasing BMI were all important factors associated with the risk for HR HPV infection among women in Tanzania. Risk factors for HR HPV among HIV-positive and HIV-negative women were very similar, but the strength of association tended to be greater among HIV-positive women. In line with the results by Clarke et al.,31 we were not able to identify a clear explanation for the high prevalence of HR HPV among older women. However, our results could potentially indicate that risk factors related to new HPV infections such as number of sex partners and having had sex in the last 12 months, number of births, and HIV positivity were seen primarily in younger women. In contrast, risk factors related to a potentially decreasing immunity such as decreasing BMI and being HIV positive were prevailing among older age groups. No association between HPV positivity and number of sex partners or having had sexual intercourse within the last 12 months was observed among older women. This may, in theory, point to reactivation of a latent HPV infection as one explanation for the second peak in the age-HPV prevalence curve. However, more studies are needed in this area.
1. Dartell M, Rasch V, Kahesa C, et al. Human papillomavirus prevalence and type distribution in 3603 HIV-positive and HIV-negative women in the general population of Tanzania: The PROTECT study. Sex Transm Dis 2012; 39: 201–208.
2. Nielsen A, Kjaer SK, Munk C, et al. Type-specific HPV infection and multiple HPV types: Prevalence and risk factor profile in nearly 12,000 younger and older Danish women. Sex Transm Dis 2008; 35: 276–282.
3. Kahn JA, Rosenthal SL, Succop PA, et al. Mediators of the association between age of first sexual intercourse and subsequent human papillomavirus infection. Pediatrics 2002; 109: E5.
4. Vaccarella S, Herrero R, Dai M, et al. Reproductive factors, oral contraceptive use, and human papillomavirus infection: Pooled analysis of the IARC HPV prevalence surveys. Cancer Epidemiol Biomarkers Prev 2006; 15: 2148–2153.
5. Herrero R, Castle PE, Schiffman M, et al. Epidemiologic profile of type-specific human papillomavirus infection and cervical neoplasia in Guanacaste, Costa Rica. J Infect Dis 2005; 191: 1796–1807.
6. Trottier H, Franco EL. The epidemiology of genital human papillomavirus infection. Vaccine 2006; 24 (suppl 1): S1–S15.
7. Franceschi S, Herrero R, Clifford GM, et al. Variations in the age-specific curves of human papillomavirus prevalence in women worldwide. Int J Cancer 2006; 119: 2677–2684.
8. González P, Hildesheim A, Rodríguez AC, et al. Behavioral/lifestyle and immunologic factors associated with HPV infection among women older than 45 years. Cancer Epidemiol Biomarkers Prev 2010; 19: 3044–3054. Epub 2010 Oct 15.
9. Smith JS, Melendy A, Rana RK, et al. Age-specific prevalence of infection with human papillomavirus in females: A global review. J Adolesc Health 2008; 43 (4 suppl): S5–S25, S25.e1–e41. Review.
10. Kjaer SK, Breugelmans G, Munk C, et al. Population-based prevalence, type- and age-specific distribution of HPV in women before introduction of an HPV-vaccination program in Denmark. Int J Cancer 2008; 123: 1864–1870.
11. Iftner T, Villa LL. Chapter 12: Human papillomavirus technologies. J Natl Cancer Inst Monogr 2003; 80–8. Review.
12. Blossom DB, Beigi RH, Farrell JJ, et al. Human papillomavirus genotypes associated with cervical cytologic abnormalities and HIV infection in Ugandan women. J Med Virol 2007; 79: 758–765.
13. Wang C, Wright TC, Denny L, et al. Rapid rise in detection of human papillomavirus (HPV) infection soon after incident HIV infection among South African women. J Infect Dis 2011; 203: 479–486. Epub 2011 Jan 7.
14. Clifford GM, Gonçalves MA, Franceschi SHPV and HIV Study Group. Human papillomavirus types among women infected with HIV: A meta-analysis. AIDS 2006; 20: 2337–2344.
15. Silva RJ, Casseb J, Andreoli MA, et al. Persistence and clearance of HPV from the penis of men infected and non-infected with HIV. J Med Virol 2011; 83: 127–131.
16. Kang M, Cu-Uvin S. Association of HIV viral load and CD4 cell count with human papillomavirus detection and clearance in HIV-infected women initiating highly active antiretroviral therapy. HIV Med 2012; 13: 372–378. 10.1111/j.1468-1293.2011.00979.x.
17. Auvert B, Lissouba P, Cutler E, et al. Association of oncogenic and nononcogenic human papillomavirus with HIV incidence. J Acquir Immune Defic Syndr 2010; 53: 111–116.
18. Smith JS, Moses S, Hudgens MG, et al. Increased risk of HIV acquisition among Kenyan men with human papillomavirus infection. J Infect Dis 2010; 201: 1677–1685.
19. Smith-McCune KK, Shiboski S, Chirenje MZ, et al. Type-specific cervico-vaginal human papillomavirus infection increases risk of HIV acquisition independent of other sexually transmitted infections. PLoS One 2010; 5: e10094.
20. Hernández-Girón C, Smith JS, Lorincz A, et al. High-risk human papillomavirus detection and related risk factors among pregnant and nonpregnant women in Mexico. Sex Transm Dis 2005; 32: 613–618.
21. Hildesheim A, Gravitt P, Schiffman MH, et al. Determinants of genital human papillomavirus infection in low-income women in Wachintong, D.C. Sex Transm Dis 1993; 20: 279–285.
22. Crum-Cianflone NF, Roediger M, Eberly LE, et al. Infectious Disease Clinical Research Program HIV Working Group. Impact of weight on immune cell counts among HIV-infected persons. Clin Vaccine Immunol 2011; 18: 940–946. Epub 2011 Apr 27.
23. Gerriets VA, Rathmell JC. Metabolic pathways in T cell fate and function. Trends Immunol 2012; 33: 168–173. Epub 2012 Feb 17. Review.
24. del Amo J, González C, Belda J, et al. Prevalence and risk factors of high-risk human papillomavirus in female sex workers in Spain: Differences by geographical origin. J Womens Health (Larchmt) 2009; 18: 2057–2064.
25. Tobian AA, Kong X, Wawer MJ, et al. Circumcision of HIV-infected men and transmission of human papillomavirus to female partners: Analyses of data from a randomised trial in Rakai, Uganda. Lancet Infect Dis 2011; 11: 604–612. Epub 2011 Apr 12.
26. Templeton DJ. Male circumcision to reduce sexual transmission of HIV. Curr Opin HIV AIDS 2010; 5: 344–349.
27. Gray RH, Serwadda D, Kong X, et al. Male circumcision decreases acquisition and increases clearance of high-risk human papillomavirus in HIV-negative men: A randomized trial in Rakai, Uganda. J Infect Dis 2010; 201: 1455–1462.
28. Serwadda D, Wawer MJ, Makumbi F, et al. Circumcision of HIV-infected men: Effects on high-risk human papillomavirus infections in a randomized trial in Rakai, Uganda. J Infect Dis 2010; 201: 1463–1469.
29. WHO and UNAIDS. New data on male circumcision and HIV prevention: Policy and programme implications. Geneva, 2007. Available at: http://data.unaids.org/pub/Report/2007/mc_recommendations_en.pdf.
30. Syrjänen K, Kulmala SM, Shabalova I, et al. Epidemiological, clinical and viral determinants of the increased prevalence of high-risk human papillomavirus (HPV) infections in elderly women. Eur J Gynaecol Oncol 2008; 29: 114–122.
31. Clarke MA, Gage JC, Ajenifuja KO, et al. A population-based cross-sectional study of age-specific risk factors for high risk human papillomavirus prevalence in rural Nigeria. Infect Agent Cancer 2011; 6: 12.