Since no one has established without doubt that hepatitis C is spread purely by sexual contact, the chance of infection from that activity could in fact be zero.
That is, of course, the most optimistic interpretation of the volumes of inconclusive and conflicting data on HCV risks. Even so, many studies have come to that conclusion.
For example, Italian researchers found in 2005: "The risk of sexual transmission of HCV within heterosexual monogamous couples is extremely low or even null."
Most studies do cite quantifiable risks of HCV transmission via sex, often quite low.
Here are typical numbers: There is a 3% infection rate of sexual partners in relationships that last an average of 10-15 years. This means the yearly risk of transmission could be somewhere between 0.2%-0.3%. A hepatitis C and sex research team reported in 2013 that the percentage among their 500 monogamous couples was 0.07.
Again, there is no firm evidence that any hepatitis C transmission comes solely as a result of sexual activity, but sex certainly is considered a suspect in many otherwise unexplained infections.
"It's a very unusual mode of transmission," says Dr. John Ward, director of the CDC’s Viral Hepatitis unit. "That's not to say it doesn’t happen, but it's so infrequent that we don’t recommend condom use," Ward told Time in February 2012, referring to vaginal intercourse by monogamous heterosexual couples.
Part of the problem for researchers is the predominant role of intravenous drug use in hepatitis C contraction. Self-reporting of illicit needle-drug use is unreliable at best. If no needle use is indicated, sex could be seen as a cause when a partner becomes infected.
Representative of the situation is a risks-of-sex chart from the San Francisco City Clinic, which knows a thing or two about STDs. The chart lists "Known Risks" and "Possible Risks" of getting the major STDs from various sexual acts. Hepatitis C gets its own classification: "Unknown."
"Sexual transmission is possible, but is a rare event, occurring with a probability between 1 in 10,000 and 1 in 100 per year," one hepatitis C research roundup indicates. "Consensus opinion suggests an incidence of 12 per 1000 person-years in partners of HCV infected individuals. These figures translate to a cumulative risk of acquisition of approximately 5% over 20-30 years."
"Based on 8,377 person-years ... the maximum incidence rate of HCV transmission by sex was 0.07% per year," a California-based study of monogamous heterosexual couples found in early 2013.
A frequently cited Austrian hep C study from 1999 study found: "The HCV seroprevalence in spouses of patients with chronic HCV infection and viremia is 5%. Sexual transmission, however, appears possible in only 2.5%, due to the results of HCV genotyping. The real risk of interspousal transmission may even be half that (1.25%). ..."
A report from Turkey indicated that of 600 spouses of hep C sufferers, none tested positive for the virus over a period of three years. (2% were positive going into the study.)
The National Institutes of Health, in 2002, added sexual transmission to its list of exposure risks. The NIH team agreed that heterosexual monogamous couples were at little risk. But, "HCV-infected individuals with multiple sexual partners or in short-term relationships should be advised to use condoms to prevent transmission of HCV and other sexually transmitted diseases."
Many researchers agree that homosexual men who have sex with multiple partners and/or engage in high-risk activities are at risk of getting or passing on hepatitis C.
These activities typically are described as fisting, rimming and unprotected anal sex, especially when it is rough.
The risks of infection for these types of high-risk individuals have been cited at 15% and even higher. Gay and bisexual men who do not have HIV show infection rates about the same as hetrosexuals, researchers say.
A major Canadian study of homosexual men found that 2.9% were HCV infected, mostly from shooting illegal drugs. The study suggested sexual transmission was rare among these men. A similar study in Australia agreed.
Studies strongly suggest that people with HIV are at great risk of acquiring HCV via sex. Researchers in Italy and India independently put the risk factor at three times that of non-HIV individuals.
"A multitude of studies has shown that HCV transmission is not only found, but is becoming more common as an STD among men who have sex with men who are HIV positive," says the CDC's Ward.
What is being called a new strain of HCV apparently has stricken at least 50 HIV-infected men in New York, a researcher said in November 2009. The strain causes rapid damage to the liver, said Dr. Daniel Fierer of the Mount Sinai Medical School, who called its spread an "epidemic."
"It really looks like a sexually transmitted disease," Fierer told the annual meeting of the American Association for the Study of Liver Diseases. The most frequent risk factor, illegal drug needle use, was absent in his study of 21 of the men, the doctor said.