Experimental vaccines to protect against Zika virus are at least 18 months away from large-scale trials, an official with the World Health Organization said Friday.

To date, about 15 companies or groups have expressed an interest in Zika vaccines and most have just started the work, Marie-Paule Kieny, WHO assistant director-general for health systems and innovation, told reporters.

Concerns focus on pregnant women and those of childbearing age because of the suspected association between Zika virus infection and microcephaly or abnormally small heads. 

Brazil Zika Virus

Laurinaldo Alves brings his daughter Luana Vitoria, who suffers from microcephaly, to a treatment centre that provides free health care in Recife, Brazil. The World Health Organization said Friday that a firm link between the Zika virus and microcephaly likely wouldn't be determined for weeks or months. (Felipe Dana/Associated Press)

Brazil has experienced a surge in microcephaly and an epidemic of Zika virus infections. Researchers are closely following pregnant women elsewhere in Latin America, as well as French Polynesia and Cape Verde.

"In a few weeks or months we will see how many of these women will deliver a child with microcephaly and this will make things much clearer," Kieny said. 

Studies are being carried out on medicines and other therapies that could prevent infection, as is done for malaria, Kieny said. "This seems for the moment a more viable and faster option than a curative treatment."

Improved diagnostic tests are also critical. Kieny said new test kits were being developed and could be available in weeks.

Better diagnostics

Scientists who aim to better understand how infectious the virus is in bodily fluids also need better diagnostics. 

The second report of Zika virus found in semen was published by British researchers in the May issue of Emerging Infectious Diseases. The man had symptoms in 2014 after returning from the Cook Islands, and his semen tested positive for the virus 62 days after his illness began. It's not clear if he would have been able to infect a sexual partner so long afterwards.  

On Thursday, Venezuelan President Nicolas Maduro said at least three people in his country have died due to complications related to the mosquito-borne Zika virus. He added that 68 people have been hospitalized with complications confirmed to be related to the virus.

Kieny said WHO hasn't received any primary information from Venezuela yet.

"We haven't seen any report of death of adults directly linked to viral infection, so it could be that this is Guillain Barré in persons known to be infected with Zika virus," Kieny said. 

In Guillain Barré, the nervous system is attacked and paralysis can result. In patients with the immune-associated disease, doctors treat the syndrome itself rather than Zika infection.

Today WHO also suggested pregnant women "consider delaying travel" to Zika-affected areas.

In the affected countries, mosquito control is paramount. For example, Jamaica's new public service announcement shows how to remove stagnant water where mosquitoes breed.

WHO is also assessing new ways to reduce mosquito populations. 

With files from Associated Press and Reuters