In the Post-COVID-19 Era, Poliomyelitis is Making a Comeback

During the COVID-19 pandemic, measures such as “physical prevention and control” of the novel coronavirus have reduced children’s exposure to pathogens. Additionally, the pandemic has significantly disrupted routine vaccination programs in many countries. Official data released by the World Health Organization (WHO) and the United Nations Children’s Fund show that the number of children receiving vaccinations is experiencing the most significant decline in 30 years (https://www.who.int/news/item/15-07-2022-covid-19-pandemic-fuels-largest-continued-backslide-in-vaccinations-in-three-decades). In the postpandemic era, there is a significant gap in the level of immunity among children compared to prepandemic times.

Polio is an acute infectious disease caused by the wild poliovirus (WPV), which includes 3 serotypes: type 1, type 2 and type 3. It can lead to permanent flaccid paralysis of the limbs, and all reported cases of polio worldwide are currently caused by WPV type 1. As of 2022, Pakistan and Afghanistan remain the only countries with indigenous WPV epidemics (https://www.who.int/health-topics/poliomyelitis#tab=tab_1). However, it is important to note that polio is highly contagious, and until the world achieves complete eradication of polio, countries and regions that have already achieved polio-free status remain at risk of WPV importation and spread. According to WHO estimates, between 2002 and 2009, 39 polio-free countries reported WPV importations.^1–3 In the postpandemic era of 2022, Malawi and Mozambique in Africa reported their first cases of WPV infection in 30 years. In addition to the risk of WPV importation, countries such as Israel and the United States (especially New York) have reported cases of polio caused by circulating vaccine-derived poliovirus (cVDPV). In London, UK, cVDPV was detected in routine sewage monitoring despite no cases of paralysis being reported, indicating possible local transmission (Fig. 1).⁴ The re-emergence of infection in previously declared polio-free countries is a wake-up call to the Global Polio Eradication Initiative.

FIGURE 1.

A map showing the latest number of WPV1 and cVDPV cases in each affected. country from 01 Mar.2022 to 28 Feb.2023. Data are from WHO (https://polioeradication.org/polio-today/polio-now/). cVDPV indicates circulating vaccine-derived poliovirus; WPV1, wild poliovirus. 

In the postpandemic era, with the resumption of international exchanges and tourism, polio returns in ways we did not anticipate. Vaccinating against polio is the most cost-effective means of preventing and controlling the disease, and all countries must raise their vaccination coverage to 95% to block the spread of the virus. There are currently two types of polio vaccines widely used: inactivated polio vaccine (IPV), administered by intramuscular injection, and oral polio vaccine (OPV). Both vaccines have been certified as safe and effective by the WHO. However, we agree with Farahat et al.⁵ that the attenuated viruses in OPV can sometimes mutate into transmissible live viruses in areas with low vaccination rates. These viruses can be transmitted from vaccinated individuals’ feces to their environment, leading to outbreaks of cVDPV in unvaccinated populations.⁵ Therefore, under the context of immunity debt in the postpandemic era, we strongly recommend a complete shift from OPV to inactivated polio vaccine to avoid such risks. It is also worth noting that timely and strengthened environmental and wastewater monitoring should be carried out to detect early “silent” poliovirus transmission in the population rather than relying solely on clinical monitoring of acute flaccid paralysis.