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Statistical analysis that informs polio eradication

Kath O'Reilly (London School Hygiene and Tropical Medicine)

Since the declaration to eradicate polio in ‘88, vaccination has been used to protect against polio. The virus spreads largely between children, where infections are mostly asymptomatic, but those with clinical disease develop permanent paralysis, which can be fatal. It has been a long road to eradication and the outcome is still uncertain. Statistical analysis and modelling have been used to guide strategy. I will focus on 2 examples; estimating the spatial risk of wild polio outbreaks and analysis to understand the global emergence and spread of vaccine-derived polio (VDPV).

The risks of wild polio (serotypes 1 and 3) have been quantified in a spatio-temporal model with risk factors, based on previous outbreaks. The model outputs are used to provide ‘6-month ahead’ risk for each country, and high-risk countries implement preventive vaccination. To increase engagement with stakeholders, a simpler risk score was used to classify country risk, and this has been in use since ‘14. Coincident with control of wild polio in Nigeria outbreaks have sharply decreased, and wild polio has most likely been eliminated in Africa.

After elimination of wild serotype 2 in ‘99, the vaccine containing attenuated serotype 2 was removed in ‘16 through a global synchronised ‘switch’. This occurred despite a small number of VDPV outbreaks. The vaccine used to halt transmission has itself a risk of seeding new VDPV outbreaks. Since ‘16 the number of VDPV outbreaks has increased, especially in Africa, resulting in a public health emergency. There is a real need to vaccinate at-risk populations while minimising vaccine use to prevent further seeding. Here I describe a comparative analysis of wild and VDPV outbreaks to quantify the speed, direction of travel and modelling to help refine where to implement vaccination while minimising risks. VDPVs move slowly and in a consistent direction of travel, meaning that responses can be small and directed provided they occur rapidly.