Sexual behavior has long been linked to cervical cancer risk including high number of sex partners, prostitution, and risky sexual behavior of male partners. However, the efficiency of HPV transmission per sexual act has ranged from 5% to 100%. Quantifying this has been extremely difficult for several reasons: 1) transmission studies are difficult to perform since couples often do not remain monogamous throughout studies, therefore the source of infection becomes muddled; 2) studies that include extensive sexual networks have not been performed; and 3) HPV DNA detection does not always reflect an infection, rather detection is often deposition from a partner if recent contact was made.

In vitro studies show that HPV infection of basal cells can take place within 24 hours. However, this requires a wound or inflammation that allows for access to the basement membrane. Hence it would be expected that most sexual acts would not involve trauma. In addition, the time from infection to the release of infectious particles is more variable. This translates into a more successful transmission the longer a couple is together. Modelling studies have estimated a transmission probability of 80% per any new partner (provided that either one is infected while the other one is susceptible).¹

Most studies have focused on immediate concordance (cross-sectional) and have shown high rates between couples. However, the rates depend on how many HPV types were included in the assays and how concordance was defined. The more types in the assay, the more likely “concordance” of at least one type is found, but also these studies were more likely to show high discordance (up to 80%) of other types.^2-4

In a study of monogamous couples tested 6 times over 6 weeks, a concordance rate from genital samples for at least one HPV type of 70% to 86% at any one visit was observed (Figure 1). Unique to the study was that couples were asked to have a baseline visit (V0), then have sex and come in for HPV testing 24 hours later (V1)--then abstain for 48 hours and return for HPV testing (V2). We found that many of the HPV types detected within 24 hours of sex were not detected after abstaining, underscoring the role of “deposition”.²

Transmission rates vary more than concordance and are dependent on intervals between testing (the longer the more likely clearance between visits and therefore lower rates) and length of monogamy (in longer relationships less new infections hence lower rates). Based on these factors, rates have ranged from 21 per 100 person-months to 3.5 per 100 person-months. Female-to-male transmission was higher than male-to-female in 2 of 3 studies.2-4 The greater vulnerability of males to infection is thought to be due to the fact that men often develop superficial infections that quickly resolve, hence there is no time for a memory immune response to develop.

Although concordance rates between the hand and a partner's genitals are relatively high in most studies (around 50%), it remains questionable whether the hand is a true source of transmission (Figure 2). The HITCH study--a prospective cohort of over 250 university female students and their male partner (s) were tested every 4-6 months for 24 months--showed that once values are corrected for the genital type in the partner, the risk of the hand transmission to the genital area is extremely low.5 HPV positivity in the hand was strongly correlated with the person’s own genitals infection, underscoring the role of self-inoculation. The role of the hand in transmission during sexual acts remains unknown. For example, it is feasible that a finger introduced into the vagina and then into the anus may introduce cervical infection into the anus.⁶ High concordance between anal HPV and a partner’s hand and genital HPV is observed.² The role of hygiene habits (such as wiping toilet paper front to back) is also unknown. Hernandez et al showed that detection of anal HPV after cervical HPV detection in a woman was common as well as vice versa.³ In general, transmission from genital to oral appears rare, and male-to-male or female-to-female transmission rates have not been well studied.

In men, the incidence of genital HPV infection is 36% when his sexual partner is positive in the genitals and hand, 23% when HPV-positive in the genitals only, whereas it is 7% when the partner is positive in the hand only and 1% when genital and hand is negative . The same values in women are 13.7%, 17%, 0% and 0.8% respectively. This illustrates the low risk of HPV transmission from hand to genital during sexual intercourse compared to transmission from genital to genital.

ARTICLES INCLUDED IN THE HPW'S SPECIAL ISSUE ON HPV TRANSMISSION:

AG Nyitray. Evidence for HPV Transmission Not Involving Penetrative Sex.

MT Goodman. Oral HPV Transmission and Oropharyngeal Cancer.

AN Burchell. Sexual Behavior and Population Transmission of HPV.

B Steinberg. Recurrent Respiratory Papillomatosis (RRP) Transmission.

DISCLOSURE
ABM is on the Global Advisory Board for Merck and was a paid speaker for Roche.

References

1. Bogaards JA, Xiridou M, Coupe VM et al. Model-based estimation of viral transmissibility and infection-induced resistance from the age-dependent prevalence of infection for 14 high-risk types of human papillomavirus. Am J Epidemiol. 2010;171(7):817-25. Available from: https://academic.oup.com/aje/article/171/7/817/86297

2. Widdice L, Ma Y, Jonte J et al. Concordance and transmission of human papillomavirus within heterosexual couples observed over short intervals. J Infect Dis. 2013;207(8):1286-94. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3603534/

3. Hernandez BY, Wilkens LR, Zhu X et al. Transmission of human papillomavirus in heterosexual couples. Emerg Infect Dis. 2008;14(6):888-94. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2600292/

4. Burchell AN, Coutlee F, Tellier PP et al. Genital transmission of human papillomavirus in recently formed heterosexual couples. J Infect Dis. 2011;204(11):1723-9. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3203235/

5. Malagón T, Louvanto K, Wissing M et al. Hand-to-genital and genital-to-genital transmission of human papillomaviruses between male and female sexual partners (HITCH): a prospective cohort study. The Lancet Infectious Diseases 2019;19:317–26. Available from: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099%2818%2930655-8/fulltext

6. Su Y, Wei F, Huang X et al. Prevalence, concordance and transmission of human papillomavirus infection among heterosexual couples in Liuzhou, China: an observational perspective study. J Infect Dis. 2019. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31074795