Travel and clinical advice on yellow fever including symptoms, diagnosis and epidemiology.
Yellow fever is a viral disease that is transmitted by several species of mosquito. It is caused by the yellow fever virus, which belongs to the Flaviviridae family. It is endemic in tropical and sub-tropical regions of Africa and South and Central America (including Trinidad), with 90% of cases occurring in Africa. A modelling study published in 2014 estimated there were 130,000 severe cases and 78,000 deaths in Africa in 2013.
Recent notable yellow fever outbreaks
There is significant yellow fever transmission in Brazil for the second season in a row - the season there runs from 1 July to 30 June the following year. As of 17 April 2018, there were 1,157 confirmed human cases and 342 reported deaths in Brazil since 1 July 2017 compared to 733 confirmed cases and 241 deaths in the previous season. Cases rapidly increased in the states of Rio de Janeiro, São Paulo, and Minas Gerais in January 2018. The Brazilian Ministry of Health launched mass vaccination campaigns for residents in these regions, but in March they extended these to the whole country to protect all residents should yellow fever continue to spread. See the Brazil Ministry of Health website for the latest information.
WHO extended the areas in Brazil that are considered a risk for yellow fever transmission. See the National Travel Health Network and Centre (NaTHNaC) website Travelhealthpro for up-to-date vaccine recommendations for travellers to Brazil.
A large outbreak that started in September 2017 is ongoing in Nigeria although as of March 2018 it had started to slow down. Further information, including current case numbers, is available from the Nigeria Centre for Disease Control. In January 2018 the Government of Nigeria launched a mass vaccination campaign, the largest to date in the country’s history.
In 2016, a large outbreak in Angola that first started in December 2015 resulted in international spread to the Democratic Republic of Congo, which then experienced its own outbreak. Kenya and China reported imported cases in people who had been working in Angola. Further details about this outbreak are available on the WHO website.
Yellow fever is rarely reported in travellers as there is a safe and effective vaccine. Between 1970 and 2015, 11 yellow fever cases were reported among unvaccinated international travellers.
Following the large outbreaks in Angola and Brazil, cases and deaths in travellers have increased, all in unvaccinated individuals.
- 11 imported cases (4 fatal) in Argentina (3), Chile (3), the UK (1), the Netherlands (1), France (1), Switzerland (1) and Romania (1), all associated with travel to Brazil.
- 1 imported case in Netherlands associated with travel to Suriname
- 1 imported case in Denmark associated with travel to Bolivia
Humans and monkeys are the principle reservoirs for the virus. The most common types of mosquito that transmit the yellow fever virus are Aedes spp (including Aedes aegypti), Haemagogus spp and Sabethes spp. These mosquitoes are not present in the UK and are unlikely to establish in the near future as the UK temperature is not consistently high enough for them to breed.
The last known case to have occurred in the UK before 2018 was in 1930 in a laboratory worker who had been working with yellow fever virus.
There are 3 main transmission cycles. Sporadic cases result from sylvatic (jungle) transmission and are seen in South America and Africa. The intermediate cycle of transmission occurs in the moist savannah zones of Africa only, when semi-domestic mosquitoes infect both animals and humans and may cause small epidemics in rural villages. Urban transmission can occur where the virus is introduced into urban areas and the domestic Aedes aegypti mosquito is widespread. This can lead to large outbreaks if the virus is introduced into unvaccinated populations where most people have little or no immunity.
There are 2 stages to yellow fever; 3 to 6 days after infection symptoms may include fever, headache, nausea or vomiting, muscle pain (often with backache), and loss of appetite. Most people will make a full recovery after 3 to 4 days, however a small number (approximately 15%) will go on to develop jaundice, abdominal pain, renal failure and haemorrhage (bleeding).
Up to half of those who develop the severe symptoms may die. Infection results in life long immunity in those who recover.
Health professionals should consider yellow fever in the differential diagnoses for illness in unvaccinated individuals returning from yellow fever risk areas. Hospital doctors can contact the Imported Fever Service (IFS) after discussion with their local microbiology, virology or infectious disease consultant. See Viral haemorrhagic fever: sample testing advice for information about testing samples from patients with a possible viral haemorrhagic fever (VHF).
Send the appropriate samples (with full clinical, travel and vaccination history including relevant dates) to PHE’s Rare and imported pathogens laboratory (RIPL). RIPL is a specialist centre for advice and diagnosis of a wide range of unusual viral and bacterial infections including yellow fever.
Prevention and advice for travellers
Yellow fever is rare in travellers. However, the unusual increase in travel-associated cases in 2017 and 2018 is consistent with an increased circulation of yellow fever virus in South America, particularly Brazil. This re-emphasises the importance of vaccination and seeking advice before travel.
The yellow fever vaccine is very effective and safe, although there have been reports of rare adverse events associated with its use. One dose is considered to provide lifelong immunity in most travellers. Possession of an International Certificate of Vaccination or Prophylaxis (ICVP) for yellow fever vaccine is an entry requirement for some countries under the International Health Regulations (2005); yellow fever vaccine is also recommended for those travelling to areas with a risk of disease transmission even if there is no certificate requirement.
Travellers to yellow fever risk areas are advised to check the latest information regarding vaccine recommendations or certificate requirements on the NaTHNaC country information pages before they travel as recommendations can change.
All travellers should take insect bite avoidance measures during daytime and night time hours to reduce the risk of infection with yellow fever and other mosquito borne diseases.
Read the mosquito bite avoidance for travellers leaflet.
The latest updates on the yellow fever situation worldwide are available on the NaTHNaC website.
Find details of your nearest yellow fever vaccination centre (YFVC)
Information for travel health professionals
The NaTHNaC Yellow Fever Zone provides clinical and administrative resources for health professionals and other staff responsible for administering yellow fever vaccine and for those managing a YFVC.