Health officials are investigating more than a dozen new reports of the Zika virus possibly being transmitted sexually, suggesting the method may be more probable than originally believed.

The virus is most commonly spread when an infected mosquito bites a person, but the Centers for Disease Control and Prevention (CDC) announced Tuesday that United States public health departments had received 14 new reports about the virus potentially being spread through sex.

Several of those cases involved pregnant women, the CDC said.

Earlier this month, the World Health Organization (WHO) declared a Public Health Emergency of International Concern in response to the Zika virus. Scientists are still investigating whether the virus can lead to microcephaly, a rare condition in which children are born with unusually small heads, or Guillain-Barre Syndrome, a nervous system illness that can cause paralysis.

The WHO says the evidence about a potential link between Zika and the disorders “remains circumstantial,” though a growing amount of data that suggests the virus plays a “causal role.”

There is currently no vaccine against Zika, though the WHO says some are being developed.

The CDC maintains the best way to prevent Zika infections is to prevent mosquito bites, and has issued travel notices for several countries where the virus is currently being transmitted.

However, the new reports about possible sexual transmission prompted the organization to reiterate its advisories for people who have traveled to those countries. The CDC says pregnant women and their partners should use condoms throughout their pregnancy, or abstain entirely.

The CDC said it has confirmed two cases of Zika in women whose only possible exposure to the virus was “sexual contact” with infected men who had recently traveled to countries where mosquitos are known to be transmitting Zika. Another four potential cases are currently awaiting confirmation from lab tests, and the other cases are currently being investigated.

“These new reports suggest sexual transmission may be a more likely means of transmission for Zika virus than previously considered,” the CDC said in a news release.

In all 14 cases, the CDC said the travelers were male and began displaying symptoms about two weeks before their “non-traveling female partner” reported an illness. The CDC says Zika stays in semen longer than blood, though scientists do not know exactly how long the virus lingers.

The CDC says it’s not yet known if women can transmit Zika to their partners through sex.

To date, the CDC has not reported any instances of people acquiring Zika from mosquitos in the United States. However, there have been 82 cases of people getting sick after returning home.

The organization says only about 1 in 5 people infected with Zika display symptoms, and they usually experience a mild illness that lasts about a week.