Yellow fever and vaccine coverage
Does low vaccine coverage put a state at higher risk of a yellow fever outbreak?
The background
Yellow fever is a viral disease spread by mosquitoes. There's a safe, effective vaccine, and the World Health Organization says at least 80% of people need to be vaccinated to keep outbreaks away.
In Nigeria, coverage is very uneven from one state to the next. I wanted to see what that gap means in practice: how much more likely is an outbreak where fewer people are protected?
What I did
I built a mathematical model that splits people into groups: those who could catch the disease, those who are vaccinated, those who are infected and those who have recovered. A second part of the model does the same for mosquitoes.
I fed it three years of real vaccination data from DHIS2, Nigeria's national health information system, plus population figures from the National Bureau of Statistics. Then I compared Bayelsa, where coverage was low, with Jigawa, where it was high.
For each state I worked out R₀, the number of people one case is likely to infect, and tested which parts of the model mattered most.
How the SVIR-SI model is built
People and mosquitoes each move through their own set of boxes. Mosquito bites link the two.
The numbers that matter
average vaccine coverage in Bayelsa
R₀ in Bayelsa: one case could infect about four people
average vaccine coverage in Jigawa
R₀ in Jigawa: lower, but still above 1
In Bayelsa, infections were trending upwards and the risk of spread was high.
In Jigawa, infections were trending downwards. But even with coverage above the WHO target, R₀ stayed above 1, so the disease could still spread if a case came in.
Bayelsa needs an urgent push to raise coverage from about 33% to above 80%, along with stronger surveillance to catch cases early.
Jigawa shows that hitting 80% isn't always the finish line. My recommendation was to aim higher, towards 95%, and keep surveillance strong.
Slides and figures
These come straight from my presentations and model outputs. Tap any one to see it bigger.
