Zika virus has already been linked to brain damage in babies and paralysis in adults. Now scientists are facing another ominous possibility: that on rare occasions, the virus might be transmitted through sex.

The evidence is very slim; only a couple of cases have been described in medical literature. But a few experts feel the prospect is disturbing enough that federal health officials should inform all travelers, not just pregnant women, of the potential danger.

Officials at the Centers for Disease Control and Prevention, however, say the evidence is insufficient to warrant such a warning. While the two instances suggest a “theoretical risk” of sexual transmission, they note the primary vector is clearly mosquitoes.

Dr. Márcio Nehab, a pediatrician and infectious disease specialist at Fiocruz, a research institute in Rio de Janeiro, said that much more research was needed to be done to definitively prove that Zika can be transmitted during sex.