Yellow Fever and Vaccination
Travellers often express concerns about health and sanitation, especially when visiting rural or developing areas. To ensure a safe journey, it is important to seek information and advice from healthcare professionals well in advance. We recommend booking a travel health consultation at least 8 weeks before departure, as some vaccines require time to become effective.
Understanding Yellow Fever
For comprehensive information on yellow fever, including its causative agent, clinical features, mode of transmission, incubation period, and management, please refer to the Centre for Health Protection's webpage at CHP Yellow Fever page. Yellow fever is the only disease specified in the International Health Regulations (2005) (IHR (2005)) for which countries may require proof of vaccination from travellers as a condition of entry under certain circumstances.
Areas with Risk of Yellow Fever Transmission
A list of countries at risk for yellow fever transmission and those requiring yellow fever vaccination is available on the World Health Organization (WHO) website. The list can be accessed at Countries with risk of yellow fever transmission and countries requiring yellow fever vaccination. Click here for a simple guide on how to interpret the list.
The Centers for Disease Control and Prevention also provides maps of yellow fever affected areas.
- Avoid Mosquito Bites: Wear loose, light-coloured, long-sleeved tops and trousers, use insect repellents containing DEET, and stay in air-conditioned or well-screened rooms.
- Vaccination: WHO recommends immunization for travellers aged 9 months and above going to at-risk areas, except for those with contraindications to the vaccine.
Why it's important?
- Personal Health Protection: Travellers aged ≥ 9 months who are travelling to or living in areas at risk for yellow fever transmission in South America and Africa should be vaccinated. The vaccine provides effective immunity within 10 days for 80–100% of people vaccinated, and within 30 days for more than 99% of people vaccinated. If a traveller is medically contraindicated from receiving the yellow fever vaccination, they should obtain an exemption letter from a medical practitioner, which is valid for their current journey. This is recommended only when vaccination is a requirement for entry and there are no plans to visit areas at risk of yellow fever. Unvaccinated individuals are advised against travelling to regions with ongoing yellow fever transmission. Acceptance of an exemption letter is at the discretion of the destination country.
- Entry Requirements: Some countries require proof of vaccination for entry, in the form of an International Certificate of Vaccination or Prophylaxis. The certificate becomes valid 10 days after the vaccine is administered. The amendment to the IHR (2005) on 11 July 2016 changes the period of validity of the international certificate of vaccination against yellow fever, and the protection provided by vaccination against yellow fever infection under the IHR (2005), from 10 years to the life of the person vaccinated. Accordingly, as of 11 July 2016, for both existing or new certificates, revaccination or a booster dose of yellow fever vaccine cannot be required of international travellers as a condition of entry, regardless of the date their international certificate of vaccination was initially issued.
In Brazil, for instance, entry into the country does not require proof of yellow fever vaccination, but cities like Rio de Janeiro, São Paulo, and Salvador still have a risk of yellow fever transmission. This often leads travellers to underestimate the health risks associated with these areas.
Vaccination Schedule and Safety
- Schedule: The vaccine, administered as a single subcutaneous injection from the age of 9 months onwards, offers lifelong protection. Travellers heading to countries where there is a risk of yellow fever transmission are advised to get vaccinated against yellow fever at least 10 days prior to their arrival. Yellow fever vaccine should be administered either simultaneously or 28 days apart from other live injectable viral vaccines. Some data indicate that it may be preferable to administer yellow fever vaccination and MMR vaccine at least 30 days apart, if possible.
- Vaccination centres: The Travel Health Centres of the Department of Health are the designated centres in Hong Kong providing yellow fever vaccinations. For contact details and further information about the Travel Health Centres, please refer to the 'Centre Information' page.
- Adverse Reactions:
- Localized reactions at the injection site, such as pain, swelling, redness, a lump, and warmth, may persist for up to a week after the yellow fever vaccination.
- Systemic reactions, experienced by 10%–30% of those vaccinated, are usually mild. The most common symptoms include a low-grade fever, mild headache, and muscle pain, starting within the first 2 to 3 days post-vaccination and lasting for 5 to 10 days.
However, serious adverse events are rare but include:
- Immediate hypersensitivity reactions or anaphylaxis.
- Yellow fever vaccine-associated neurotropic disease (YEL-AND), which can manifest as meningoencephalitis, acute disseminated encephalomyelitis, Guillain-Barré syndrome, and in rare cases, cranial nerve palsies. YEL-AND is serious but seldom fatal and usually leaves no long-term effects. It may occur up to 30 days post-vaccination but typically arises within 5 to 10 days.
- Yellow fever vaccine-associated viscerotropic disease (YEL-AVD), resembling naturally acquired yellow fever or presenting as nonspecific multiorgan system failure, can be life-threatening. It has so far only been reported in first-time vaccine recipients, mostly within 5 to 10 days after vaccination.
- There is evidence suggesting an increased risk of relapse in patients with multiple sclerosis.
For travellers of any age, revaccination, when necessary, has not been linked to either YEL-AND or YEL-AVD.
YEL-AND and YEL-AVD are rare adverse events. In the US, YEL-AND has been reported at a rate of 0.8 cases per 100,000 yellow fever vaccine doses distributed, while YEL-AVD occurs at a rate of 0.3 cases per 100,000 doses.
- Precautions: Travellers with the following health conditions may not be suitable to receive yellow fever vaccination. Before receiving the vaccine, travellers should consult doctor's advice:
- Having a febrile illness
- Persons who are allergic to eggs or to any of the vaccine components
- Pregnant and breastfeeding women
- Infants under 9 months old
- Persons aged 60 years and older
- Persons on immunosuppressive treatment or having radiotherapy
- Persons with immunosuppressive diseases such as lymphoma, thymoma, leukaemia, Hodgkin's disease, hypogammaglobulinaemia and HIV infection
- Received other live attenuated viral vaccines within the past 4 weeks
- Special Notes for Travellers Aged 60 Years and Older: Travellers who are aged 60 years and older are at increased risk for yellow fever vaccine associated adverse events, and this risk appears to increase with increasing age.
- The risk of YEL-AND increases to 2.2 cases per 100,000 vaccine doses for travellers aged 60 years and older.
- For YEL-AVD, the risk rises to 1.2 per 100,000 doses in those aged 60 and above, and further increases to 4 per 100,000 doses for individuals aged 70 years and older. The global case fatality rate for all reported cases of YEL-AVD is around 50%.