Yellow fever is a viral infection, spread by mosquito bites. People travelling to many African and South American countries need to get vaccinated against it. There is a risk of infection in many countries in central and west Africa, including Nigeria, Ivory Coast, Cameroon and the Democratic Republic of Congo, as well as Uganda and parts of Kenya. Brazil, Colombia and other South American countries are also affected. (See maps here.)

Yellow fever is prevented by a highly effective vaccine. However, it’s a live vaccine, which means that it should not be used by people living with HIV when they are unwell or have a CD4 count below 200. For HIV-positive people with stronger immune systems travelling to countries affected by yellow fever, vaccination is recommended.

A new study looked at how long the yellow fever vaccine was effective. They assessed the proportion of people with HIV who had a protective immune response in the years following yellow fever vaccination – this indicates that the vaccine could still protect them against yellow fever. Overall, this was the case for 95% of HIV-positive people one year after vaccination, 86% five years after vaccination and 75% ten years after vaccination.

Results were better for people with HIV who had an undetectable viral load at the time of vaccination. At each time point, 99% or more had a protective immune response.  The recommendation is that people with HIV should be vaccinated against yellow fever (if they need it) when they have an undetectable viral load. If they remain on HIV treatment, they can then wait ten years before having a booster yellow fever vaccination.

If people are vaccinated when their viral load is not undetectable, they should either have their immune response to the yellow fever vaccine measured or get a booster yellow fever vaccination straight away.

For more information, read the NHS’ information about yellow fever vaccination.