Two of the world’s biggest drug companies are evaluating whether existing vaccine technology could be used against the Zika virus, as the World Health Organisation warned the mosquito-borne virus was likely to spread to all countries in the Americas except for Canada and Chile.

The WHO director general, Margaret Chan, told the organisation’s executive board that she had asked Carissa Etienne, head of the WHO in the Americas, to brief the board later this week on its response to the outbreak. The mosquito-borne Zika virus has been linked to brain deformities in babies.

“Although a causal link between Zika infection in pregnancy and microcephaly has not – and I must emphasise – has not been established, the circumstantial evidence is suggestive and extremely worrisome,” Chan said. “An increased occurrence of neurological symptoms, noted in some countries coincident with arrival of the virus, adds to the concern.”

Zika is transmitted by the Aedes aegypti mosquito, which is also known to carry the dengue, yellow fever and Chikungunya viruses.

There is no vaccine or treatment for Zika, which typically causes mild fevers and rashes, although about 80% of those infected show no symptoms.

“We’re concluding our feasibility studies as quickly as we can to see if our vaccine technology platforms might be suitable for working on Zika,” Glaxo spokeswoman Anna Padula told Reuters in an email.

France’s Sanofi SA, which won approval late last year for the first dengue vaccine, has also said it is reviewing the possibility of applying its technology for Zika. “However, there are too many unknowns about Zika to reliably judge the ability to research and develop a vaccine effectively,” a spokesman said last month.

The first outbreak of the disease outside of Africa, Asia and the Pacific islands occurred in May 2015, when a case was reported in Brazil.

It was previously considered to have relatively mild consequences for those infected. But in November Brazil’s health ministry said that the virus was linked to a foetal deformation known as microcephaly, in which babies are born with smaller than normal brains.

Brazil has reported almost 4,000 suspected cases of microcephaly, the WHO said last Friday, over 30 times more than had been reported in any year since 2010.

The WHO’s warning came as the Brazilian authorities announced plans to prevent the spread of the disease during this summer’s Olympic Games in Rio de Janeiro, when about 500,000 tourists are expected to descend on the city.

The WHO said the 21 countries predicted to be affected are all those in the region that have Aedes mosquitoes, which carry the virus. Zika has not yet been reported in the continental United States, though a woman who fell ill with the virus in Brazil later gave birth to a baby with brain damage in Hawaii.

The WHO attributed the rapid spread of Zika to the fact that the population of the Americas had not previously been exposed to it and so lacks immunity.

It said in a statement: “The role of Aedes mosquitoes in transmitting Zika is documented and well understood, while evidence about other transmission routes is limited. Zika has been isolated in human semen, and one case of possible person-to-person sexual transmission has been described. However, more evidence is needed to confirm whether sexual contact is a means of Zika transmission.”

It said there was no evidence of Zika being transmitted to babies through breast milk. Pregnant women planning to travel to areas where the virus is circulating are advised to consult a healthcare provider before travelling and on their return.

The Brazilian authorities said inspections of Olympic facilities would begin four months before the Games to get rid of mosquito breeding grounds and daily sweeps would take place during the competition. They added that fumigation would be an option but would be limited because of concerns for the health of the athletes and visitors.

Laura Rodrigues, professor of infectious disease epidemiology at the London School of Hygiene & Tropical Medicine, said evidence of a link with microcephaly was strong. “Evidence includes temporal association between Zika outbreaks and microcephaly outbreaks both in Brazil and French Polynesia,” she said. “The virus is neurotropic. It grows in the brain of the foetus and destroys brain structures so the developing brain is malformed – small – that’s the cause of the microcephaly.”

She said that there had previously been little research on Zika as it was seen as of “no public health importance”, but the suggestion that it could cause congenital infections and microcephaly had changed that view.

Rodrigues suggested that with a major international effort there could possibly be a vaccine within the next couple of years.

Trudie Lang, professor of global health research at Oxford University, said it was important that the response from the research community was faster than with Ebola.

“This is an important emerging disease outbreak situation and we really must apply the lessons that we learnt from Ebola because Zika could be a major public health issue in these countries,” she said. “There are many unknowns and so research is urgently needed to understand what is happening and how to prevent further cases.

“There is no treatment and no vaccine and so this would need addressing through clinical trials as quickly as possible.”