Yellow fever vaccine: study note

Vaccine:

• Live, attenuated.
• Made from a 17D strain of yellow fever virus grown on embryonated chick eggs.
• Storage: +2˚C to +8˚C and protected from light.
• Presentation: lyophilised powder for reconstitution with a diluent; thiomersal-free.
• Administration: deep subcutaneous or intramuscular injection.
• Effectiveness: A single dose correctly administered confers immunity in 95 to 100% of recipients; immunity persists for decades, probably lifelong.

Coadministration with other vaccines

It can be co-administered (on different limbs /the same limb 2.5 cm apart) with other inactivated and most live vaccines, except MMR, which should be given 28 days apart. If yellow fever and MMR were given together due to the immediate need for protection, repeat vaccine doses should be considered individually.

Who needs a yellow fever vaccine?

• Laboratory workers handling infected material
• Persons travelling to or living in countries that require an International Certificate of Vaccination or Prophylaxis (ICVP) for entry.
• Persons travelling to or living in areas or countries with a risk of yellow fever transmission.
  (even if these countries do not require evidence of immunisation on entry)

• Immunisation should be performed at least ten days before travel to an endemic area.
• It can only be given to people more than 9 months-old.
  Infants aged six to nine months should only be immunised following a detailed risk assessment.
  It is contraindicated for <6 month-olds (except when the risk of yellow fever infection is high such as during epidemics/outbreaks or based on expert opinion).
• ICVP – is valid for the duration of the life of the person vaccinated (not 10 years)

Revaccination/Booster dose

One dose confers lifelong immunity; hence routine booster is not necessary. However, it can be considered that the first dose was given.

• When aged less than two years old.
• During pregnancy.
• Whilst infected with HIV.
• When immunosuppressed.
• Before undergoing a bone marrow transplant.
• Where there is concern about a traveller’s risk of exposure to yellow fever (e.g. working/living for an extended period in a high-risk setting) – selected cases (seek advice from NaTHNaC/Health Protection Scotland).

Contraindication

• Those aged under six months.
• Confirmed anaphylactic reaction to a previous dose of yellow fever vaccine or a component of the vaccine (including egg).
• History of thymus disorder or thymectomy.
• Primary or acquired immunodeficiency (congenital, HIV*, high dose systemic steroids, immunosuppressive biologics, radiotherapy or cytotoxic drugs etc).
• Those aged 60 years or older who are travelling to areas where the yellow fever vaccine is generally not recommended by the WHO.
• Those who have a first-degree family history of YEL-AVD or YEL-AND (see below) following vaccination that was not related to a known medical risk factor.

If someone with contraindication travels to a yellow fever endemic area, they should take mosquito avoidance measures and a letter of ICVP exemption (if ICVP is required).
Minor illnesses without fever or systemic upset is not a contraindication.
If an individual is acutely unwell – the vaccine should be postponed until they have fully recovered.

* Yellow fever vaccine may be given safely to HIV-infected persons with a CD4 count greater than 200 and a suppressed viral load ; however, antibody response may be diminished.

Pregnancy and breastfeeding

YFV is not recommended in pregnant women as it is a live vaccine. It is advised to avoid travelling to a high-risk area. When travel is unavoidable, a risk-benefit assessment should be done. WHO states that in areas where yellow fever is endemic or during outbreaks, the benefits of vaccination are likely to far outweigh the risk of the vaccine. Pregnant women should be counselled, and they should make an informed decision.

Breastfeeding women – For women breastfeeding a child under nine months, expert advice should be sought from NaTHNaC/ Health Protection Scotland.

Adverse effects

• Headache, myalgia, low-grade fever and/or soreness at the injection site – in 10 to 30% of recipients.
• Rash, urticaria, bronchospasm and anaphylaxis (rare). Anaphylaxis – 0.0013%.
• Post-vaccine encephalitis – mostly in infants < 6 months old (0.05-0.4%).
• Yellow fever vaccine-associated neurological disease (YEL-AND) (0.0008%).
• Yellow fever vaccine-associated viscerotropic disease (YEL-AVD) (0.0003%).

Yellow fever vaccine-associated neurological disease (YEL-AND)

• Begins 2-56 days after the vaccine.
• Rare – 0.0008% of vaccine doses (higher in people of 60 years and older – 0.0022%)
• Almost all cases have occurred in primary vaccinees who have no underlying yellow fever immunity.
• It starts with fever and headache, leading to confusion, focal neurological deficits, coma and Guillain-Barré syndrome.
• CSF – pleocytosis, increased protein, yellow fever IgM +.
• Usually result in complete recovery.

Yellow fever vaccine-associated viscerotropic disease (YEL-AVD)

• Begins 1-18 days following vaccination.
• Rare – 0.0003% of vaccine doses (higher in people 60 years and older – 0.0012%).
• Fever, malaise, headache and myalgias that progress to hepatitis, hypotension and multi-organ failure – It resembles severe yellow fever.
• Fatality >60%.
• It occurred in primary vaccinees without underlying yellow fever immunity.
• Some cases occurred in patients with thymus disease/thymectomy.