Millions of people in major world cities are at risk of a potential yellow fever outbreak despite living hundreds – and in many cases thousands – of miles from areas where the disease is present. Cities at risk include Miami, Bangkok, Hong Kong, Mumbai and Delhi, which each have millions of residents. The prospect of an outbreak of the haemorrhagic fever in Mumbai and Delhi is particularly worrying because both cities have populations of more than 20m. Other large cities at risk include São Paulo, with a population of 21m, and Cairo, where more than 18m live. The disease is endemic in parts of Africa and south and central America and in the past few years there have been large-scale outbreaks of the disease.
Researchers writing in the Bulletin of the World Health Organization looked at flight information to analyse the destinations of travellers leaving yellow fever endemic areas. They then looked at the number of cities that were suitable for dengue transmission because the same mosquito – Aedes aegypti – transmits both diseases. They also looked at the vaccination policies of the cities at risk and found that – while 89 percent of travellers flying between yellow fever endemic countries were required to provide proof of vaccination – just 35 percent of travellers from endemic cities to those that had the potential for an outbreak were asked to prove that they had been vaccinated. The researchers also found that less than 25 percent of people travelling from non-endemic countries had to provide proof of vaccination when they arrived in countries where they could pick up yellow fever.
Shannon Brent, one of the authors of the paper and an epidemiologist at St Michael’s Hospital, Toronto, said, “We would encourage countries that have policies in place to implement them effectively and this may mean checking [vaccination certificates] more rigorously.” The 2015 yellow fever outbreak in Angola first alerted researchers to the threat of the international spread of the disease. The outbreak spread to the neighbouring Democratic Republic of Congo and then Kenya. And 11 Chinese health workers caring for patients in Angola contracted the disease and brought it back home with them although there was no ongoing transmission. “The theory is that someone is infected with yellow fever and returns home at a time when they could be bitten by a mosquito. That mosquito could then pick up yellow fever and transmit it to the next person. A theoretical risk of a yellow fever outbreak does absolutely exist if all the conditions are right based on the mosquito’s ecology,” said Ms Brent.
Brazil is currently battling a widespread epidemic of yellow fever, with nearly 500 cases between July 2017 and February 2018 and 150 deaths. Some ten travellers have also returned to their home countries with the disease, four of whom died. Two travellers from Chile and a Swiss and a German traveller died after contracting the fever in Brazil. The prospect of the disease running amok in one of the biggest cities in the world is made more worrying by the lack of a yellow fever vaccine which would be deployed to contain any large outbreak. Much of the stock has been used up in the recent African and Brazilian outbreaks and one of the four WHO-approved manufacturers, Sanofi-Pasteur, has had problems manufacturing the drug. Ms Brent urged travellers to ensure that they are protected when they visit an area where yellow fever is known to be active. Researchers wrote, “At a time when global yellow fever vaccine supplies are diminished, an epidemic in a densely populated city could have substantial health and economic consequences. Hence, the global community needs to carefully re-examine existing yellow fever travel vaccination policies and practices to prevent urban epidemics.”
Key facts | Yellow fever
Yellow fever is an acute viral haemorrhagic disease transmitted by infected mosquitoes. The “yellow” in the name refers to jaundice that affects some patients. Symptoms include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue. A small proportion of patients who contract the virus develop severe symptoms and approximately half of those die within seven to ten days. Since the launch of the Yellow Fever Initiative in 2006, significant progress in combating the disease has been made in West Africa, and more than 105 million people have been vaccinated in mass campaigns. Yellow fever is prevented by an extremely effective vaccine, which is safe and affordable. A single dose of yellow fever vaccine which gives life-long protection against the disease. Good supportive treatment in hospitals improves survival rates. There is currently no specific anti-viral drug for yellow fever. World Health Organisation
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Credit: Anne Gulland for The Telegraph, 2 May 2018.