[Written for Big Vang (La Vanguardia) by Adelaida Sarukhan, PhD in immunology and scientific writer at ISGlobal, Jose Muñoz, ISGlobal researcher and Head of Tropical Medicine and International Health of the Hospital Clinic of Barcelona, and Natalia Rodríguez, ISGlobal researcher and MD at the Servicio de Atención al Viajero Internacional of the Hospital Clínic de Barcelona. The Barcelona Institute for Global Health (ISGlobal) is supported by the “la Caixa” Foundation.]
This represents the most important yellow fever activity that South America has seen in many decadesSince this year started, there has been a considerable increase in the number of confirmed yellow fever cases in Brazil, particularly in the states of Sao Paulo and Rio de Janeiro. In both regions, the number of cases exceeds by far the number reported last year for the same period. Authorities have reported a total of 723 cases and 237 deaths between July 2017 and ends of February 2018 (most cases occur during summer in the Southern hemisphere). Although Brazil currently accounts for the majority of cases, other countries like Colombia, Bolivia, Ecuador, French Guyana, Peru and Surinam have also confirmed cases over the last two years. Overall, this represents the most important yellow fever activity that South America has seen in many decades.
Image: Unsplash / Sara Riaño
Why is it re-emerging?
Human to human transmission could start via 'Aedes' mosquitoes [...] perfectly adapted to breed and live in citiesYellow fever is caused by a virus belonging to the same family than Zika and dengue and is endemic (i.e. continually present) in non-human primates of tropical areas of Africa and the Americas. From time to time, the virus “jumps” from monkeys to humans, via tropical forest mosquitoes. For the moment, all cases from the current outbreak are due to “spilling” from this sylvatic cycle. However, growing deforestation is increasingly reducing monkeys’ habitats and bringing the disease closer to urban areas, where human to human transmission could start via Aedes mosquitoes –the same that transmit dengue, zika or chikungunya–, perfectly adapted to breed and live in cities. The last time this type of transmission occurred in Brazil was in 1942.
Why is it worrying?
The risk of urban transmission
One has to keep in mind that yellow fever has a high mortality rate (ranging between 15 and 50%). Urban transmission of the disease (human to human via Aedes mosquitoes) could mean thousands of cases –and deaths– among a non-vaccinated population.
The map of the current outbreak in Brazil shows that the cases are occuring closer and closer to three major cities: in Sao Paulo, 57% of cases were most likely infected barely 15 km north of the city of Sao Paulo, and 45% of cases in Rio have occurred less than 20 km away from the city. In Minas Gerais, 47% of cases live in districts south of Belo Horizonte, where no cases were reported last year.
The risk of spreading to other countries
Thanks to international air travel, an unvaccinated person may get infected with yellow fever in South America or Africa one day and travel thousands of kilometres the next day. If that person reaches a region where the Aedes mosquito is present, he or she can initiate a local (autochthonous) transmission cycle. These regions include half of the USA, Middle East, Southeast Asia, the Pacific Islands, Madeira and the Canary islands.
This happened in 2016, with a yellow fever outbreak in Angola that caused more than 3,800 cases and 360 deaths: a handful of imported cases in China (in fact, the first ever reported cases of yellow fever in the continent) threatened to introduce the virus into a densely populated area without any natural immunity against it.
Image: Pixabay / WikiImages
The vaccine: the best weapon
The good news is that there is a highly effective vaccine to protect against the disease. The problem is the amount of available doses if an epidemic in a large city broke out (the global production of the vaccine is limited), as well as the misinformation generated by unfounded rumours on vaccine safety, leading local people and travellers to take the wrong decision of not getting vaccinated.
The problem is the amount of available doses if an epidemic in a large city broke out: the global production of the vaccine is limitedSeveral cases (and at least three deaths) of European and South American travellers that returned infected from Brazil have been confirmed in the last few months. These include a Dutch tourist that visited the Sao Paulo region, a French tourist that was in Minas Gerais, and a Swiss citizen that died from the disease after returning from Ilha Grande. None of them had been vaccinated. In addition, Chilean authorities confirmed the death of two travellers that visited Ilha Grande (a third companion has fallen sick and is in stable condition), and Argentina also confirmed two imported yellow fever cases (again, none had been vaccinated).
With the goal of protecting local populations and avoiding urban transmission mediated by Aedes mosquitoes, the Brazilian authorities have launched a campaign in which more than 5 million people have been vaccinated (most of them with fractionated doses of the vaccine, which has been shown to be equally protective). The Brazilian Health Minister has even proposed vaccinating the entire country, although it is not clear whether there will be enough vaccine to cover the whole population (207 million people), even using fractionated doses.
For those international travellers planning to visit Brazil, the advice is clear and simple: get vaccinated at least 10 days before travelling if visiting Sao Paulo, Minas Gerais, Rio de Janeiro or jungle areas. The vaccine provides lifelong protection and is offered in the traveller attention services (these services in addition provide practical information and advice tailored to the travel destination). As with all vaccines, when indicated, the benefits largely outweigh the risk of adverse effects.
For those persons who were not vaccinated [...] the best advice is to avoid travelling to areas at riskJust like Brazil has a problem with yellow fever vaccine stocks, Europe is having the same problem, so it is possible that some vaccination centers may lack the vaccine. This is because vaccine production is limited and there is usually no surplus. For those persons who were not vaccinated for this reason or because of contraindications (the vaccine is not indicated for immunocompromised patients or babies under 9 months of age), the best advice is to avoid travelling to areas at risk, if possible.