Outbreak risks, cases, and costs of vaccination strategies against wild poliomyelitis in polio-free settings: a modelling study

BackgroundPolio eradication was threatened in 2022 by importation of wild poliovirus serotype 1 into Malawi with subsequent international spread, representing the first wild polio cases in Africa since 2014. Preventing importations and spread of wild poliovirus is critical, and dependent on population immunity provided through routine immunisation and supplementary immunisation activities (SIAs). However, the scale of preventative SIAs has reduced in recent years due to financial constraints. MethodsWe developed a mathematical model of polio transmission dynamics to evaluate the probability of an outbreak, expected number of poliomyelitis cases, and the costs associated with vaccination delivery through routine immunisation (RI), outbreak response SIAs (oSIAs) and preventative SIAs (pSIAs). Across varying levels of routine immunisation coverage, we explore three key strategies: RI+oSIAs, RI+oSIAs+annual pSIAs, and RI+oSIAs+biannual pSIAs. ResultsThe annual pSIA strategy (RI+oSIAs+annual pSIAs) had higher costs but greater probability of no outbreaks: under our model assumptions, annual pSIAs result in 80% probability of no outbreaks when routine immunisation coverage [≥]50%. The biannual pSIA strategy (RI+oSIAs+biannual pSIAs) costs less and averts more outbreaks than RI+oSIAs, but RI coverage [≥]65% was required to achieve equivalent risk of no outbreaks. The strategy with no pSIAs (RI+oSIAs) had the lowest costs but required [≥]75% RI coverage to achieve equivalent risk of no outbreaks. ConclusionPrioritisation of pSIAs must balance outbreak risk against implementation costs, ideally favouring the smallest manageable outbreak risk compatible with elimination. We infer that there are few short-term risks due to population immunity from RI, but without pSIAs, long-term risks accumulate and can result in outbreaks with potential for international spread. We do not consider the costs of further delaying the eradication timeline or societal implications of outbreaks, both of which emphasise the need for pSIAs.