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Model suggests sexual transmission accounts for 3% of new Zika cases
A mathematical model featured in Scientific Reports estimated that sexual transmission accounts for 3% of new Zika cases. In addition, the model estimated that one initial case of Zika in an average population will lead to two more people becoming infected.
“Zika is a complicated virus,” Shigui Ruan, PhD, developer of the model and professor of mathematics at the University of Miami, said in a press release. “It is not as simple as passing a cold back and forth.”
Studies have shown that Zika can be transmitted sexually, the researchers wrote. In fact, Zika has been detected in semen up to 62 days after the onset of febrile illness. Despite an increase in knowledge since the outbreak in the Americas began in 2015, the role of sexual transmission in spreading Zika virus is not well-understood.
Ruan and colleagues developed a deterministic model to investigate Zika’s ability to be transmitted sexually. They accounted for mosquito-borne transmission and transmission by sexual intercourse. In their model, the investigators ignored human births and deaths and made the following assumptions:
- Mosquitoes cannot be infected with Zika by biting people who are asymptomatically infected;
- The male to female human ratio is 1:1;
- The end of the Zika viremic period happens simultaneously with the disappearance of symptoms; and
- Frequency of sexual intercourse over all age groups is once per week and twice per week in sexually active age groups.
The researchers calibrated their model to Zika epidemic data from Brazil, Colombia and El Salvador. They determined that Zika’s basic reproductive number is 2.055 (95% CI, 0.523-6.3), which is the number of infections created by one initial infection in a population. This number was most sensitive to mosquito biting rate and mortality rate. In addition, the researchers found that the percentage contribution from sexual transmission was 3.044% (95% CI, 0.123-45.73), which also was determined to increase the risk for infection and epidemic size.
Shigui Ruan, PhD, professor of mathematics at the University of Miami College of Arts and Sciences
Source: University of Miami
Ruan and colleagues said this suggests that sexual transmission alone is unlikely to initiate or sustain an outbreak. In the absence of reducing sexual activity, however, decreasing Zika’s basic reproductive number below one will not be enough to stop an outbreak.
“You could conceivably have somebody who was infected and did not know they were infected, carrying the disease around for a while, have some sexual encounter, and then infect somebody else,” Chris Cosner, PhD, a professor of mathematics at the University of Miami, said in the release. “In theory, that could result in a source for an outbreak that seems to come from nowhere. So for this disease, because of the complexity of the transmission routes and the fact that some people can stay in the infected phase for a long time, it is more complicated than your average disease.”
Disclosure: The researchers report no relevant financial disclosures.