In two of the cases, the virus has been confirmed in women whose only known risk factor was sexual contact with an infected male partner who had recently traveled to an area with active Zika transmission through mosquito bites, the Centers for Disease Control and Prevention said on Tuesday. The new reports, which include pregnant women, suggest that sexual transmission of the virus may be more of a concern than was initially thought.
With only a few sex-related cases documented, scientists had previously believed sexual transmission of the virus to be very rare, the New York Times reported. Little is known about sexual transmission of the virus, however, the CDC reported that the virus is present in semen longer than in blood.
The CDC also said that it is unknown whether a woman can transmit the virus to her sex partners.
"We think mosquito-borne spread is the most common route of transmission, but we want to make people aware that sexual transmission is also a risk," a CDC official told Reuters.
Despite the two confirmed infections, eight of the cases are still being investigated and four confirmatory tests are pending, the Associated Press reported.
The CDC said several of the women involved in the reports are pregnant, but officials did not give further details.
The flavivirus has plagued areas in Central, South America and the Caribbean in recent years. There is no cure or treatment for the virus, which is primarily transmitted through mosquitoes.
There have been 82 "travel-associated" reports of the virus in the U.S. and, unlike other flaviviruses, Zika can be transmitted to fetuses through a pregnant woman's amniotic fluid.
As researchers work to find a cure or develop a vaccine, the CDC has offered up recommendations for travelers, including using insect repellent while in Zika outbreak areas to prevent mosquito bites.