Dengue (sometimes known as break-bone fever) is a viral infection that spreads from day-biting mosquitoes to humans. It is more common in tropical and sub-tropical countries, mostly in urban and semi-urban areas (World Health Organization (WHO), 2024).
Dengue is endemic in more than 100 countries worldwide, including in Africa, the Americas, Eastern Mediterranean, south-east Asia and the Western Pacific region. Sporadic cases are also occurring in some European countries. The number of dengue cases worldwide has markedly increased in the past few decades, with large outbreaks reported globally.
Most dengue cases are mild or asymptomatic, but severe, life-threatening disease can occur (UK Health Security Agency (UKHSA), 2024a). Dengue fever is not a risk in the UK, although imported cases in returning travellers are reported annually (UKHSA, 2024b). There is no risk of dengue spreading in the UK from an imported case, as the mosquito species that can carry and spread dengue are not present in the UK (UKHSA, 2024a).
Dengue is caused by a virus of the genus flavivirus, which is part of the family Flaviviridae. There are four different types of dengue virus: DENV-1, DENV-2, DENV-3 and DENV-4. All four have the potential to cause severe dengue, formerly known as dengue haemorrhagic fever (NaTHNaC/TravelHealthPro, 2024a).
Transmission
Dengue virus is spread mainly by bites from Aedes aegypti and Aedes albopictus mosquitoes, which are active during the day (UKHSA, 2024a). Other types of Aedes mosquitoes can also carry the dengue virus, but their contribution is normally secondary to Aedes aegypti mosquitoes. However, in 2023 a surge in local transmission of dengue by Aedes albopictus (tiger mosquito) was seen in Europe (WHO, 2024).
There are rare cases of dengue transmission from blood transfusions, organ transplantation and needle-stick/sharps injuries. Sexual transmission is very rare, but has been reported. Pregnant women can pass the infection to their baby during delivery (NaTHNaC/TravelHealthPro, 2024a).
After a mosquito feeds on an infected person, the dengue virus replicates in the mosquito's gut, before spreading to secondary tissues, including the salivary glands. The time it takes from the mosquito ingesting dengue virus to transmission to a new host is called the extrinsic incubation period. This takes about 8–12 days if the surrounding temperature is between 25–28°C.
Variations in this period are also influenced by daily temperature fluctuations and the dengue virus type. Initial viral concentration can also alter the time it takes for a mosquito to spread the virus. Once infectious, the mosquito can transmit dengue virus for the rest of its life (WHO, 2024).
Infection with one type of dengue virus provides long-term immunity against that specific type, but only short-term protection against the other types (UKHSA, 2024a).
Symptoms
Most people with dengue have mild or no symptoms and will get better in 1–2 weeks. Rarely, dengue can be severe and lead to death.
If symptoms occur, they usually begin 4–10 days after infection and last for 2–7 days. Symptoms may include:
Severe dengue symptoms often come after the fever has gone away:
People with these severe symptoms need to get urgent care immediately (WHO, 2024).
Severe dengue is due to an increase in vascular permeability that can lead to life-threatening hypovolaemic shock. It is more common in children, adolescents and pregnant women. Older people, those taking anticoagulants and anyone with comorbidities, including asthma, bleeding disorders, diabetes, hypertension, obesity and renal disease are also at increased risk of developing severe dengue. The risk for severe dengue is greater during a second dengue infection in people who are then infected with a dengue virus of a different type. It can, however, also occur during the first, third or fourth infection. Severe dengue is rare in travellers (UKHSA, 2024a).
There is no specific treatment for dengue. The focus is on treating pain and symptoms. Most cases of dengue fever can be treated at home with analgesia. Acetaminophen (paracetamol) is often used to control pain. Non-steroidal anti-inflammatory drugs like ibuprofen and aspirin must be avoided as they increase the risk of bleeding. For people with severe dengue, hospital admission is often needed (WHO, 2024). People admitted to hospital with severe dengue need careful management of fever, fluid balance, electrolytes and blood clotting. They may need to be admitted to specialist units such as intensive care or high dependency. With good supportive care, death due to severe dengue is typically less than 1%. Antiviral and steroid therapies have not been shown to help recovery (UKHSA, 2024a).
Epidemiology
Half of the world's population is now at risk of dengue, with an estimated 100–400 million infections occurring annually. Asia represents approximately 70% of the global disease burden. Many temperate regions are now at risk of dengue epidemics, as increasing global temperatures aid the wider geographic distribution of Aedes mosquitoes.
Dengue is spreading to new areas, including in mainland Europe. Although not endemic, since 2010 local dengue transmission has been reported in Croatia, mainland France, Italy and Spain. Cases are reported when viraemic travel-related cases return from endemic areas to areas with suitable environmental conditions and competent vectors (mosquitoes that can carry and spread dengue).
‘The likelihood of a traveller catching dengue is determined by several factors, including destination, length of exposure, intensity of transmission and season of travel’
A major outbreak was reported in 2012 in Madeira, Portugal. In 2023, France reported 43 locally acquired cases and Italy reported 82 locally acquired cases (UKHSA, 2024a).
Dengue risk for UK travellers
All reported UK dengue cases have been acquired as a result of travelling to endemic areas, the majority to South and South East Asia, South and Central America, Africa and the Caribbean (UKHSA, 2024b) (see Box 1).
Current UK dengue vaccine advice and recommendations
Pre-travel dengue risk assessment
The likelihood of a traveller catching dengue is determined by several factors, including destination, length of exposure, intensity of transmission and season of travel. Risk is thought to be higher during periods of intense mosquito feeding activity: 2–3 hours after dawn and during the early evening. All travellers to dengue-endemic countries are at risk, although determining individual risk is difficult. True dengue incidence in travellers is probably underestimated as, in many countries, dengue reporting is not obligatory. Also, due to absent or non-specific symptoms, dengue is probably under-diagnosed (NaTHNaC/Travel Health Pro, 2024a).
Travellers who spend long periods of time in endemic areas, including aid workers, expatriates or people visiting friends and family, are at increased risk. However, even short-term visitors can be exposed to dengue (NaTHNaC/TravelHealthPro, 2024a).
Currently available UK dengue vaccine
There is only one dengue vaccine licensed for use in the UK. It is a tetravalent live attenuated vaccine called Qdenga® (https://www.medicines.org.uk/emc/product/14663/smpc). Qdenga is produced in Vero cells by recombinant DNA technology and contains serotype-specific surface protein genes of the four dengue serotypes, engineered into a dengue type 2 backbone. Another dengue vaccine, Dengvaxia®, is available in some countries, but not in the UK. No interchangeability data are available and travellers should not complete their Qdenga vaccine course overseas with Dengvaxia, and vice versa.
Following dengue infection, an infected individual has short-lived cross protection against all dengue types. For this reason, consideration should be given to delaying administration of a Qdenga vaccination for a period of one year after a laboratory confirmed dengue infection (UKHSA, 2024a).
Dengue vaccine dosage and schedule
Qdenga should be administered as a 0.5 mL dose at a two-dose (0 and 3 months) schedule, by subcutaneous injection, preferably in the upper arm in the region of deltoid. The need for a booster dose has not been established (UKHSA, 2024a).
Rationale UK for dengue vaccination recommendations
The objective of the UK dengue vaccine programme is primarily to provide those at risk of dengue who have already experienced dengue infection in the past, with protection from a secondary (and potentially more severe) infection.
The Qdenga vaccine is not recommended for seronegative individuals (people with no evidence of previous dengue infection) as the UKHSA advised that the trial data are currently insufficient to make a recommendation for these individuals (UKHSA, 2024a).
Offering dengue vaccine
The UK Joint Committee on Vaccination and Immunisation (JCVI) has advised that the dengue vaccine can be considered for individuals aged 4 years of age and older who have had dengue infection in the past and are:
Blood tests for previous dengue infection may not be 100% reliable and assessment must be made of previous tests for dengue, likely exposure and clinical history (NaTHNaC/TravelHealthPro, 2024a). The UKHSA Immunisation Against Infectious Disease Dengue chapter has further details on this and the use of Qdenga: https://travelhealthpro.org.uk/media_lib/mlib-uploads/full/the-green-book-chapter-15a-dengue-october-2024.pdf.
Prescribing dengue vaccine
Any decision to vaccinate should depend on obtaining a reliable history of dengue infection. Clinicians need to obtain as many details as possible, including previous travel, illness and vaccination history, and consider any laboratory testing information to make this assessment. In the absence of a reliable history of confirmed dengue infection, epidemiological factors, such as growing up in an endemic area, may also be considered to support a decision to test and/or offer vaccine.
‘Travellers who spend long periods of time in endemic areas, including aid workers, expatriates or people visiting friends and family, are at increased risk’
Resources
Where there is any uncertainty about a previous history, the potential risk of vaccination should be clearly explained. Previous dengue infection can only be reliably confirmed if the traveller was tested at the time of illness (usually by a PCR or antigen test). Travellers may have the results from PCR or antigen tests available or be able to provide a reliable history of confirmed infection (UKHSA, 2024a).
The UK JCVI has taken a precautionary approach for UK travellers because of a theoretical risk of severe dengue if a seronegative individual is vaccinated and subsequently exposed to dengue virus DENV3 or DENV4 (UKHSA, 2024a).
The JCVI is an expert scientific advisory committee that advises the UK government on vaccination and immunisation issues. More information about the JCVI, including their code of practice and details of membership is available here: https://www.gov.uk/government/groups/joint-committee-on-vaccination-and-immunisation.
Contraindications
Qdenga vaccine should not be given to (UKHSA, 2024a):
Pregnancy
There is limited data and research regarding the health outcomes of dengue infection in pregnancy and effects of maternal infection on the developing baby (US Centers for Disease Control and Prevention, 2024). Pregnant women who catch dengue at are at risk of significant complications, including an increased risk of developing severe dengue.
Mother-to-child transmission of dengue during delivery is associated with severe dengue in newborns. As the dengue vaccine is contraindicated in pregnancy (UKHSA, 2024a), pregnant women should be informed about the risks associated with dengue infection in pregnancy for both mother and baby. Considering avoiding unnecessary travel to dengue-affected countries is an option to discuss with pregnant travellers. This is a particular concern in the third trimester, due to the risk associated with mother-to-child transmission (NaTHNaC/TravelHealthPro, 2024b).
Pregnant women should also be advised of any other risks relating to their planned destination, including other mosquito and insect-spread illnesses, such as oropouche and Zika.
Women of childbearing potential should avoid pregnancy for at least one month following dengue vaccination (UKHSA, 2024a). Specific destination advice on pregnancy and insect and mosquito spread infections is available on the TravelHealthPro website: https://travelhealthpro.org.uk/factsheet/45/pregnancy; https://travelhealthpro.org.uk/news/801/insect-spread-illness-reminder-for-pregnant-travellers.
Record keeping
As with all vaccines, a traveller must be given a clear written record of any dengue vaccine given, and a record must also be kept in the traveller's medical records.
Returned travellers
UK health professionals should be alert to the possibility of dengue infection in UK residents who have recently returned from a dengue risk area who present with a fever or flu-like illness.
Clinical advice should be sought initially from a local microbiology, virology or infectious disease consultant. Health professionals who suspect dengue should send appropriate samples for testing (with full clinical and travel history) to the UKHSA Rare and Imported Pathogens Laboratory: https://www.gov.uk/government/collections/rare-and-imported-pathogens-laboratory-ripl.
The Imported Fever Service offers infection health professionals a 24-hour, 7-day a week telephone access to expert clinical and microbiological advice for patients with suspected travel-associated infections, including dengue: https://www.gov.uk/guidance/imported-fever-service-ifs (NaTHNaC/TravelHealthPro, 2024a).
Conclusion
All health professionals advising UK residents planning travel abroad should be aware of the increasing prevalence of dengue worldwide. They must demonstrate appropriate knowledge of destination-specific dengue risk and have the ability to access verified resources when advising travellers.
Competent prescribers must be mindful of current UK recommendations for the provision of dengue vaccine and understand the limitations and contraindications involved. The importance of mosquito bite avoidance strategies must be emphasised to all travellers to dengue risk regions, regardless of whether they have been vaccinated or not.
UK health professionals must be vigilant for the possibility of dengue infection in unwell returned travellers who have visited dengue risk countries. Appropriate urgent clinical advice should be sought without delay for any suspected dengue infection in a returned traveller.