Polio: Background and perspective on how international travel can be made safe against polio

International spread of polio continues to be a Public Health Emergency of International Concern since its declaration by the World Health Organization in 2014 and its reiteration in 2024. In 2023 and 2024, two countries remained endemic for wild poliovirus (WPV) but 20 countries reported polio outbreaks due to vaccine-derived polioviruses (VDPVs) in 2023 and 10 countries in 2024 (up to mid-June). Guidelines from various agencies recommend polio vaccination before travelling to or from polio-affected countries, or attending mass gatherings anywhere in the world, particularly if the crowd is international. Immunity protects against polio paralysis but not against poliovirus re-infection, irrespective of which vaccine was involved - live attenuated oral polio vaccine (OPV) or inactivated poliovirus vaccine (IPV). Infection with WPV or circulating VDPV in the vast majority of non-immune individuals (first infection) is asymptomatic, while they are efficient virus transmitters. Re-infections in immune individuals are always asymptomatic, but they are also infectious and may act as source for further transmission, although less efficiently than the former. Thus, travellers can become transmission vectors, illustrated by many episodes of importations of WPV or VDPVs into polio-free countries in recent years. The route of poliovirus transmission remains controversial, with many believing it to be faecal-oral, but epidemiological analysis is consistent with respiratory route and not faecal-oral. Transmission occurs person-to-person during social contact. Travellers must ensure they are adequately immunised to avoid polio, and becoming vectors of virus importation. The vaccine efficacy (VE) of OPV is highly variable - high in temperate climate countries, particularly rich countries, but low or very low in tropical low-income countries. On the other hand, VE of IPV is excellent everywhere irrespective of geography.