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New cases of Polio – should you be concerned?

In July a case of polio in an unvaccinated adult male was reported in New York; the first case in the US since 2013. Increased detection of poliovirus in the wastewater in London has prompted public health authorities to recommend an additional booster shot in children. In early 2022 the first case in 5 years of wild type poliovirus type 1 infection was reported in Africa. The case was in Malawi and is genetically linked to isolates from a polio endemic country. Is this trend in polio cases something we should be concerned about? Is it relevant to the world beyond New York City and London? What does it mean for Africa? As always, the English phrase ‘stay calm and carry on’ is appropriate. But I would adapt it to a Southern African version which would be ‘stay calm, carry on, make a plan, and be prepared’.

My generation of physicians studied polio as a disease of a previous era and a favorite of board examinations. We understood that there were three types of polio virus (type 1, 2, and 3), and it was an RNA virus belonging to the enterovirus family. Polio was transmitted by the fecal-oral route making its way from the gastrointestinal tract via nerve cells to the spinal cord where it could target and kill nerve cells that control motor function. Like so many viruses, not all people that are infected have the same clinical course. Most individuals with polio are asymptomatic, but a few individuals will have a typical viral illness with headache, sore throat, fevers, muscle aches, nausea, vomiting, and a smaller minority develop meningitis with headache, neck pain, stiffness. An even smaller proportion will develop a motor neuron disease when the virus infects nerve cells. Infected nerve cells die and no longer stimulate muscle cells leading to the classic flaccid paralysis that characterizes acute paralytic poliomyelitis. Depending on which nerve cells are affected this may lead to the inability to walk, move upper extremities, or failure to use the muscles that assist with breathing or swallowing.

The development of the first polio vaccine in the 1950s was transformative. Prior to the vaccines, countries such as the United State experienced outbreaks in the summers. Thousands of children would develop symptomatic disease, and some would develop long lasting physical disabilities. One of the most famous victims of polio was Franklin Delano Roosevelt who contracted polio in adulthood and founded the National Foundation for Infantile Paralysis that later became known as the March of Dimes. This became one of the most successful philanthropic health organizations in history with grassroots fundraising that supported vaccine development research. By the 1960s there were two vaccines that were effective against polio - an inactivated poliovirus vaccine (IPV) and a live attenuated oral poliovirus vaccine (OPV).

The attenuated vaccine is made by introducing attenuating mutations that weaken the virus’ ability to cause disease without affecting its ability to induce a strong immune response. OPV is given as drops in the mouth. The drops reach the intestine where the virus in the vaccine replicates and stimulates an immune response. Some vaccine virus is shed in the poop of the vaccinated individual. In very rare cases (approximately 2 in a million) changes can occur in the virus. If these changes occur during replication in the intestine the vaccine virus can mutate and become virulent leading to disease known as Vaccine Associated Paralytic Polio (VAPP). In extremely rare circumstances the virus that is shed in the poop can circulate in a population with a low vaccination rate. This virus as it passes through many people without immunity can lose the attenuating mutations and become virulent causing disease. These new strains are known as circulating vaccine derived polio virus (cVDPV).

The original OPV vaccine was a trivalent vaccine containing all 3 strains of poliovirus. As cases of wild type polio throughout the globe decreased, the rare new polio cases were vaccine associated cases. Most of these were due to type 2 poliovirus. To reduce this risk, the type 2 strain was removed from the vaccines and bivalent vaccines (bOPV) consisting of only type 1 and 3 polio viruses were introduced in 2016. As wildtype polio was eradicated or the risk of imported cases fell, some countries transitioned to exclusive use of the inactivated vaccine (IPV). This happened largely in high income countries. Countries that were polio-endemic or at risk of importation were encouraged to introduce at least one dose of IPV as part of the three dose bOPV series. This would ensure that adequate protection against type 2 polio virus would be induced with the IPV vaccine while having the benefits of the gut immunity induced by the OPV.

However as wild type polio was gradually being eliminated the risk benefit analysis favored transitioning to an IPV vaccine. But, this would take time. There are currently only 8 manufacturers prequalified by the WHO to manufacture IPV vaccines (Sanofi, GSK, Bilthoven, Sinovac, Beijing Institute of Biological Products, Serum Institute of India, AJ Vaccines and LG Chem); four of the manufacturers were prequalified in the last 3 years. Due to manufacturing constraints the transition to providing more IPV vaccines may not happen for a few more years. In addition, the Covid-19 pandemic may have introduced new challenges.

In many parts of the world Covid-19 related lockdowns disrupted vaccination programs, and a significant number of children did not receive their routine infant immunizations. To prevent sustained transmission of poliovirus, 80-97% immunity may be required. Polio vaccination is typically delivered as a 3 or 4 dose schedule and currently global 3rd dose polio coverage is estimated to be 80%. In the WHO Africa region, it is 70%; this dropped from 74% in 2019 to 70% in 2021. This low coverage rate poses risks for communal transmission of polio virus. The number of cases of cVDPV tripled from 2019 to 2020, most of the countries reporting cases were in Africa, most of these cases were type 2 poliovirus cases. A new monovalent attenuated vaccine with improved genetic stability known as novel OPV2 (nOPV2) has been developed and is ideal for boosting responses in outbreak situations. However, manufacturing supply concerns may limit its availability.

Until polio transmission is interrupted in all countries, all countries are at risk. Polio vaccination works and has been effective in almost eliminating the disease globally. All children must be vaccinated, and catch-up vaccinations made available for those that missed doses during the pandemic. Most adults who have been vaccinated in childhood do not need additional doses except for those in high-risk professional situations or those traveling to endemic regions.

Should we be concerned? Yes, but there is no need to panic, we need to be prepared and vaccinate. Polio like all recent viruses will expose the weaknesses of our public health systems and highlight our capabilities in early detection through to effective access and deployment of medical countermeasures such as vaccines. Strengthening early detection systems and effective outbreak response mechanisms is critical in all parts of the world. Ensuring access to vaccines to address outbreaks and provide supplemental vaccination is essential. Protecting our own communities without protecting all communities does not work. Pathogens are smarter than us, they do not discriminate and do not need passports or visas to travel!

*A. Tariro Makadzange is the author of this blog. She is an infectious disease physician and viral immunologist focused on tackling healthcare programs in Africa.

*Nyasha Elose contributed research and editorial assistance.

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