The World Health Organisation has convened an emergency committee to discuss the “explosive” spread of the Zika virus, which has been linked to thousands of birth defects in Latin America.“Last year the disease was detected in the Americas, where it is spreading explosively,” Margaret Chan, the WHO director general, said at a special briefing in Geneva. It was “deeply concerning” that the virus had now been detected in 23 countries in the Americas, she added.One WHO scientist estimated there could be 3-4m Zika infections in the Americas over the next year.The spread of the virus has prompted governments across the world to advise pregnant women against going to the areas where it has been detected. There is no vaccine or cure for Zika, which has been linked to microcephaly, a serious condition that can cause lifelong developmental problems.Chan said: “The level of alarm is extremely high. Arrival of the virus in some cases has been associated with a steep increase in the birth of babies with abnormally small heads.”She added: “A causal relationship between Zika virus and birth malformations and neurological syndromes has not yet been established – this is an important point – but it is strongly suspected.“The possible links have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions. The increased incidence of microcephaly is particularly alarming as it places a heartbreaking burden on families and communities.” Chan outlined four reasons for alarm: “First, the possible association of infection with birth malformations and neurological syndromes. Second, the potential for further international spread given the wide geographical distribution of the mosquito vector. Third, the lack of population immunity in newly affected areas. Fourth, the absence of vaccines.” This year’s El Niño weather patterns meant mosquito populations were expected to spread, Chan added. “For all these reasons, I have decided to convene an emergency committee under the international health regulations,” she said. The committee will meet on Monday and will advise on the international responses and specific measures in affected countries and elsewhere.Brazilian authorities estimate the country could have up to 1m Zika infections by now, and since September, the country has registered nearly 4,000 cases of babies with microcephaly.The Zika outbreak and spike in microcephaly cases have been concentrated in the poor and underdeveloped north-east. But the south-east, where São Paulo and Rio de Janeiro are located, is the nation’s second hardest-hit region. Rio de Janeiro is of particular concern, since it will host the Olympic games this summer. The president of the International Olympic Committee, Thomas Bach, said the IOC was in “close contact” with Brazilian authorities and the WHO, and that all national Olympic bodies would be advised on how to deal with the virus before the Games started.The Brazilian president, Dilma Rousseff, has pledged to wage war against the Aedes aegypti mosquito that spreads the virus, focusing on getting rid of the insect’s breeding grounds.The US Centers for Disease Control and Prevention said there had been 31 cases of Zika infection among US citizens who travelled to areas affected by the virus, but so far there had been no cases of transmission of the virus through mosquitoes in the US itself. The White House said its experts were most concerned about its potential impact on women who are pregnant or could become pregnant.US officials said the country had two potential candidates for a vaccine, and might begin clinical trials in people by the end of this year. But experts in disease control have warned they do not expect to have a vaccine available in 2016.Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said on Thursday that previous research into dengue fever, the West Nile virus and the chikungunya virus would give scientists an “existing vaccine platform” which could be used as “a jumping-off point” for finding a cure to the Zika virus.