More than two billion people are at risk of developing the Zika virus, scientists have warned. Data on air traveller numbers, analysed by scientists and published in The Lancet Infectious Diseases, found “vast numbers” of people are vulnerable.

People living in India, Indonesia and Nigeria are the most vulnerable to transmission, the research found. It is thought this risk is particularly heightened in summer as people are more likely to engage in travel, especially over long distances.

The number of people travelling to a population can affect likelihood of contracting the virus. Limited access to health resources in countries with high poverty rates and poor infrastructure is another factor, with the Philippines, Vietnam, Pakistan and Bangladesh being most affected by such conditions.

Around the world, more than 70 countries and territories now have continuing Zika transmission. The World Health Organisation has declared it to be a global health emergency. There is currently no known vaccine or drug treatment for it.

Zika was first identified in monkeys in Uganda in 1947. Although outbreaks have occurred since, the current spread is on an unprecedented scale. The recent outbreak was first seen in May 2015 in Brazil. More than 50 cases have been noted in the UK so far.

Zika is spread by the Aedes aegypti mosquito, resulting in quick transmission between people, especially in high-density populations. The World Health Organisation has warned it can also be transferred during sexual contact.

Zika is thought to have resulted in thousands of babies being born with underdeveloped brains. As a result, the Brazilian government warned against women trying to conceive while they are deemed at high risk of developing the virus.

The report’s authors said: “As the Zika virus epidemic in the Americas intensifies and expands, hundreds, and possibly thousands, of infected travellers are now transporting the virus to distant regions of the world. Given the broad global range of aedes mosquitoes and the arrival of summer in the northern hemisphere, these translocation events could catalyse new Zika virus epidemics, in much the same manner that the epidemic in Brazil began.

“The potential for epidemics to occur in parts of Africa and the Asia-Pacific region is particularly worrying given the vast numbers of people who are potentially susceptible to Zika virus and are living in environments where health and human resources to prevent, detect, and respond to epidemics are limited. Our findings could offer valuable information to support time-sensitive public health decision-making at local, national, and international levels.”

The UN health agency convened its expert committee this week to assess the latest status of the epidemic. Dr. David Heymann, the committee's chair, said Friday that considerable gaps remain in understanding Zika and the complications it causes – including resulting in babies with serious neurological problems – and WHO concluded that the outbreak remains a global emergency.

“This extraordinary event is rapidly becoming, unfortunately, an ordinary event,” Dr Heymann said, explaining that health officials around the world should prepare for the imminent arrival of the disease spread mostly by mosquitoes, but also through sex.

In the absence of any effective treatments or vaccines for the disease – and given past failures to wipe out the mosquitoes that mostly spread Zika – Dr Heymann said it will largely be up to individuals to avoid infection. “People have to assume responsibility for this on their own,” he said, adding that people at risk of the disease should wear long sleeves and insect repellent.

Battling the zika virus - in pictures Show all 19 1 /19 Battling the zika virus - in pictures Battling the zika virus - in pictures A worker of the Salvadorean Ministry of Health fumigates a house in Soyapango, 6 kilometers from San Salvador, El Salvador. Salvadorean authorities have began a three days campaign of fumigation to reduce the presence of the mosquito that transmit the Zika virus. EPA/Oscar Rivera Battling the zika virus - in pictures A Health Ministry employee fumigates a home against the Aedes aegypti mosquito to prevent the spread of the Zika virus in Soyapango, six km east of San Salvador. Health authorities have issued a national alert against the Aedes Aegypti mosquito, because of the link between the Zika virus and microcephaly and Guillain-BarrÈ Syndrome in foetuses. AFP PHOTO/Marvin RECINOSMarvin RECINOS/AFP/Getty Images Battling the zika virus - in pictures A pediatric infectologist examines a two-months-old baby, who has microcephaly, on 26 January 2016 in Recife, Brazil. Getty Images Battling the zika virus - in pictures A woman walks through the fumes as Health Ministry employee fumigate against the Aedes aegypti mosquito to prevent the spread of the Zika virus in Soyapango. Marvin RECINOS/AFP/Getty Images Battling the zika virus - in pictures A health ministry employee sprays to eliminate breeding sites of the Aedes Aegypti mosquito, which transmits diseases such as the dengue, chicunguna and Zica viruses, in a Tegucigalpa cemetery on January 21, 2016. The medical school at the National Autonomous University of Honduras (UNAH) recommended that women in the country avoid getting pregnant for the time being due to the presence of the Zika virus. If a pregnant woman is infected by the virus, the baby could be born with microcephaly. AFP PHOTO/Orlando SIERRA Battling the zika virus - in pictures A man walks away from his home with his son as health workers fumigates the Altos del Cerro neighbourhood as part of preventive measures against the Zika virus and other mosquito-borne diseases in Soyapango, El Salvador REUTERS/Jose Cabezas Battling the zika virus - in pictures A three-months-old, who has microcephaly, in Recife, Brazil. Getty Images Battling the zika virus - in pictures A pregnant woman waits to be attended at the Maternal and Children's Hospital in Tegucigalpa. The medical school at the National Autonomous University of Honduras (UNAH) recommended that women in the country avoid getting pregnant for the time being due to the presence of the Zika virus. If a pregnant woman is infected by the virus, the baby could be born with microcephaly. ORLANDO SIERRA/AFP/Getty Images Battling the zika virus - in pictures Army soldiers apply insect repellent as they prepare for a clean up operation against the Aedes aegypti mosquito, which is a vector for transmitting the Zika virus in Sao Paulo, Brazil. AP Photo/Andre Penner Battling the zika virus - in pictures Workers disinfect the Sambadrome in Rio de Janeiro to fight the spread of the Zika virus Battling the zika virus - in pictures Dr. Vanessa Van Der Linden, the neuro-pediatrician who first recognized the microcephaly crisis in Brazil, measures the head of a 2-month-old baby with microcephaly in Recife Battling the zika virus - in pictures Mother Mylene Helena Ferreira cares for her son David Henrique Ferreira, 5 months, who has microcephaly, on January 25, 2016 in Recife, Brazil. In the last four months, authorities have recorded close to 4,000 cases in Brazil in which the mosquito-borne Zika virus may have led to microcephaly in infants Getty Images Battling the zika virus - in pictures U.S. women who are pregnant from traveling to many South American countries Battling the zika virus - in pictures In the last four months, authorities have recorded close to 4,000 cases in Brazil in which the mosquito-borne Zika virus may have led to microcephaly in infants. Getty Images Battling the zika virus - in pictures Dr. Vanessa Van Der Linden, the neuro-pediatrician who first recognized the microcephaly crisis in Brazil, examines a two-month-old baby with microcephaly on January 27, 2016 in Recife, Brazil Battling the zika virus - in pictures Washington Post Battling the zika virus - in pictures Battling the zika virus - in pictures Battling the zika virus - in pictures

WHO said it was also unknown just how big the risk is for pregnant women. Although Zika has been proven to cause a range of neurological problems in babies, various studies have put the risk anywhere from 1 to 30 percent. “We don't have a definitive answer,” said Dr. Peter Salama, WHO's director of emergencies. “The risk is relatively low, but significant.”

Despite Zika's spread to more than 70 countries and territories, Brazil has the vast majority of cases of microcephaly, or infants born with abnormally small heads. Dr Heymann said that studies are ongoing in the country and that the explanation could involve numerous factors. Peter Salama, head of disease emergencies and outbreaks at the WHO, said the virus was likely to keep spreading.

Florida officials on Thursday said they have trapped the first mosquitoes that tested positive for the Zika virus in the Miami area, further confirming reports of local US transmission of the illness. There have been 49 cases of Zika in people believed to have contracted the virus in a small area of Miami