Abstract
Live vaccination against polio has effectively prevented outbreaks in most developed countries for more than 40 years, and there remain only a few countries where outbreaks of poliomyelitis by the wild strain still threaten the community. It is expected that worldwide eradication will be eventually achieved through careful surveillance and a well-managed immunization program. The present paper argues, however, that based on a simple stochastic model the risk of outbreak by a vaccine-derived strain after the cessation of vaccination is quite high, even if many years have passed since the last confirmed case. As vaccinated hosts are natural reservoirs for virulent poliovirus, the source of the risk is the vaccination itself, employed to prevent the outbreaks. The crisis after stopping vaccination will emerge when the following two conditions are met: the susceptible host density exceeds the threshold for epidemics and the vaccinated host density remains large enough to ensure the occurrence of virulent mutants in the population. Our estimates for transmission, recovery, and mutation rates, show that the probability of an outbreak of vaccine-derived virulent viruses easily exceeds 90%. Moreover, if a small fraction of hosts have a longer infectious period, as observed in individuals with innate immunodeficiency, the risk of an outbreak rises significantly. Under such conditions, successful global eradication of polio is restricted to a certain range of parameters even if inactivated polio vaccine (IPV) is extensively used after the termination of live vaccination.
Highlights
The World Health Organization (WHO) has a target to interrupt wild poliovirus transmission throughout the world by 2013 (World Health Organization, 2010)
The number of patients with poliomyelitis by wild-type poliovirus infection has decreased drastically due to a program using live oral polio vaccine (OPV)
1 vaccine-derived strain was reported in Haiti and the Dominican Republic in 2000 (Centers for Disease Control and Prevention, 2000), and an outbreak by a type 2 vaccine-derived strain has been reported in Egypt (Centers for Disease Control and Prevention, 2001), in Nigeria (Wassilak et al, 2011)
Summary
The World Health Organization (WHO) has a target to interrupt wild poliovirus transmission throughout the world by 2013 (World Health Organization, 2010). Nucleotide substitutions responsible for increased neurovirulence frequently occur during replication in the human gut (Poyry et al, 1988; Dunn et al, 1990; Abraham et al, 1993; Kew et al, 1998; Matsuura et al, 2000; Shulman et al, 2000) It has been reported since the 1960s that the vaccine-derived strain excreted from humans can exhibit pathogenicity The Japan Live Poliovaccine Research Commission, 1967) This suggests the possibility that vaccine-derived viruses could cause a poliomyelitis outbreak in a susceptible population after the cessation of an OPV program (Wood et al, 2000). We calculated the probability of successful global eradication, that is, the probability that the last carrier will be recovered before the population could experience an outbreak
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