A recent article in the Journal Lancet, Long-term immunity against yellow fever in children vaccinated during infancy: a longitudinal cohort study, calls into question the 2013 World Health Organization (WHO) decision that a single dose of Yellow Fever vaccine is required for life-long immunity. Until 2013, the requirement was for a dose every 10 years of the Yellow Fever vaccine. However, given new data and lowering prevalence, it was felt that such a change to the vaccination schedule could be made. While many, including myself, were happy about this decision as we are now set for life, there has been some data that has called this decision into question.

As outlined in the paper above, a primary strategy to combat outbreaks, as have been going on in parts of Africa, is to vaccinate the very young. These children are the most at risk for severe infection so stopping them from ever having the disease can be lifesaving. However, they are also receiving multiple vaccines and may receive them at the same time. We now know from other recently published data that wild type measles infection can reduce the immune memory for other diseases and for vaccinations. Should this same phenomenon be seen in measles co-vaccination, this could cause issues with immune memory and response toward the disease in question later in life.

This paper utilized titers to prove vaccination and immune status. While this is a frequently utilized and routine process in the US and throughout the world, the evidence that certain titer levels and strict cut-offs for said titers is lacking. So, what does it all mean?

We are unlikely to know for sure for several years. It could mean:

1. We are dramatically underdosing infants in Africa for vaccination of a deadly disease.

2. We need to separate out the Yellow Fever vaccine from other vaccines, especially of the live virus type.

3. Our current system of titer values may not tell the whole story when it comes to immunity and protection from infection.

How will we know? Unfortunately, we are unlikely to know until we see a rise, or lack thereof, in Yellow Fever infections within the population who have been immunized as infants. Given the relatively few travelers who (vaccinated or not) contract yellow fever as tourists, it will be difficult to know for sure unless a drastic change is seen. It is also unlikely that given political and financial concerns with vaccination programs in Africa (and elsewhere) that giving more than the single dose will change unless there is also drastic evidence to do so. This may seem harsh but countries struggling to feed their people have to make hard decisions. Hopefully, the world’s medical community will continue to watch and wait and be ready to intervene when and how we are able.

I would love to hear your thoughts in the comments. Let me know if you have any questions or thoughts for future posts.

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