Sexual Transmission of Zika Virus

Zika virus transmission through sexual activity

A person with Zika virus infection can transmit the virus to their partner(s) through vaginal sex, anal sex, and possibly oral sex. Sharing sex toys may also put someone at risk. Zika virus has been detected in semen, vaginal fluids, saliva, urine, and breast milk. There is no evidence that Zika virus can be transmitted through saliva during deep kissing.

There is documented evidence of sexual transmission of Zika virus from male-to-female, male-to-male, and female-to-male sex partners. Female-to-female sexual transmission has not yet been reported but is biologically plausible. Females with Zika infection may pass the virus to sex partners through vaginal fluids or menstrual blood.

Sexual transmission from an asymptomatically infected male has been reported. Limited data suggest the incidence of Zika virus RNA shedding in semen and its persistence after infection are likely similar for symptomatic and asymptomatic men infected with Zika virus. People with symptomatic illness have transmitted Zika virus before they had symptoms, while they had symptoms, and after symptoms resolved.

Using condoms or abstinence is recommended to protect sex partners concerned about Zika virus infection.

Duration of risk of sexual transmission

Among the currently available reports of sexual transmission of Zika virus, the longest period from symptom onset in the index case to potential sexual transmission to a partner was between 32–41 days. However, most reports indicate shorter intervals.

The longest reported period after symptom onset at which potentially infectious virus has been detected in semen by culture or cytopathic effect was 69 days. No other studies reported potentially infectious Zika virus in semen specimens obtained more than 40 days after symptom onset.

Many publications have also reported on detection of Zika virus RNA in semen. However, this might not indicate presence of infectious virus at the time of sampling or correlate with the potential for sexual transmission of infectious virus. In the largest cohort study to date, Zika virus RNA shedding declined during the 3 months after symptom onset. At >90 days after illness onset, less than 7% of participants had detectable Zika virus RNA in semen. The estimated mean time to clearance of Zika virus RNA from semen was 54 days. Similar findings have been observed in smaller cohort studies.

Testing to determine risk of sexual transmission

At this time, testing a person's blood, urine, or genital secretions to determine their potential risk of sexually transmitting Zika virus is not recommended for the following reasons:

• First, a negative blood test or antibody test could be falsely reassuring. This can happen when:
  • The blood test is performed after the virus is no longer in the blood (detected by RNA NAAT [nucleic acid amplification testing]) but could still be present in other bodily fluids (e.g., semen).
  • The antibody test is performed early after infection when the antibody levels are not yet high enough to be detected, or later after infection when the antibody levels have fallen to undetectable levels.
  • The test is falsely negative.

• Second, there is a limited understanding of Zika virus shedding in genital secretions, or of how to interpret the results of tests of semen or vaginal fluids. Zika shedding in these secretions may be intermittent, in which case a person could test negative at one point but still carry the virus and shed it again in the future.