Geneva: European countries should start preparing now to protect their populations against the Zika virus, which has never been transmitted in Europe but could spread with the onset of spring and summer, the World Health Organization (WHO) said on Wednesday.
"Now is the time for countries to prepare themselves to reduce the risk to their populations," WHO's Europe chief Zsuzsanna Jakab said in a statement. "Every European country in which Aedes mosquitos are present can be at risk for the spread of Zika virus disease.
"A number of travellers infected with Zika have entered Europe, but the disease has not been transmitted further, as the mosquito is still inactive. With the onset of spring and summer, the risk that Zika virus will spread increases."
Meanwhile, the WHO voiced concern on Wednesday over a report the Zika virus had been transmitted in the United States and called for further investigation.
The first known case of Zika virus transmission in the United States was reported in Dallas, Texas on Tuesday by local health officials, who said it likely was contracted through physical contact and not a mosquito bite.
"We certainly understand the concern. This needs to be further investigated to understand the conditions and how often or likely physical transmission is, and whether or not other body fluids are implicated," WHO spokesman Gregory Hartl told Reuters.
"This is the only the second mooted case of transmission," he said, referring to media reports about a case of an American man who returned from Senegal in 2008 and is suspected of having infected his wife.
The virus, linked to babies born with abnormally small heads and birth defects in Brazil, is spreading rapidly in the Americas and the WHO declared an international public health emergency on Monday about the condition known as microcephaly.
The United Nations agency, which is leading international coordination on the outbreak, said on Tuesday the virus could spread to Africa and Asia, which have the world's highest birth rates, as well as to southern Europe.
The WHO has not recommended travel or trade bans with affected countries, but says that it is drawing up advice to pregnant women.
The WHO global response team will discuss the transmission report among other issues at its daily meeting, Hartl said.
"There are many things we don't know about Zika," he said. "Lots of surveillance is needed... We have our team set up and are sure there will be lots of progress quickly."
For now, the key in infected areas is to try to control mosquitoes and for people to wear adequate clothing, use insect repellent and sleep under bednets, Hartl said.
Pfizer Inc, Johnson and Johnson and Merck & Co Inc said they were evaluating their technologies or existing vaccines for their potential to combat Zika.
Japan's Takeda Pharmaceutical Co Ltd said it had created a team to investigate how it might help make a vaccine, a day after Sanofi SA said it would launch a Zika vaccine programme.
Meanwhile, making a shot to generate an immune response against Zika virus shouldn't be too hard in theory. However, producing a safe, effective and deliverable product to protect women and girls who are at risk is not easy in practice.
For a start, scientists around the world know even less about Zika than they did about the Ebola virus that caused an unprecedented epidemic in West Africa last year.
Ebola, due to its deadly power, was the subject of bioterrorism research, giving at least a base for speeding up vaccine work. This time, the knowledge gap is more daunting.
There are just 30 mentions of Zika in patents, against 1,043 for Ebola and 2,551 for dengue fever, according to Thomson Reuters Derwent World Patents Index. And there have been only 108 high-profile academic papers on Zika since 2001, against more than 4,000 on Ebola, as found in the Web of Science.
Still, the US National Institutes of Health, the Public Health Agency of Canada and the Butantan Institute in Brazil have started work on potential candidates for a Zika vaccine, and several biotech firms are in the race.
They include NewLink Genetics, which helped develop the first successful Ebola vaccine with Merck & Co.
Canadian researcher Gary Kobinger told Reuters he believes an experimental Zika shot might be able to be used on a limited emergency basis as soon as late 2016, although full regulatory approval will take years.
Ben Neuman, an expert on viruses at Britain's University of Reading, says there are many hurdles ahead. "To be useful, a Zika vaccine would need to be effective and safe, but it's difficult to do both," he told Reuters. "It's a balancing act."
That's because a good vaccine works by provoking the immune system into a strong response - but not enough to make a person sick - and there is no simple way to assess the right immune response for Zika, according to one drug company expert.
Zika infection is so mild in the vast majority of cases that its victims are unaware they are even infected, so this group of potential patients is unlikely to need or want immunisation.
The crucial target group is women who may be pregnant, since the disease's greatest suspected threat is the possible link to severe birth defects.
All of this makes developing and testing a vaccine highly complex, especially since pregnant women are often excluded from clinical trials until the safety of new drugs or vaccines is well-established in other population groups.
"It raises special safety considerations in vaccine development because you want to make sure any vaccine is safe for both mother and child," Takeda's vaccine head Rajeev Venkayya told Reuters.
It also makes for an uncertain and potentially limited market for any Zika vaccine.