How “Polio” Was Found in Gazan Sewage Samples

Jessica Lan-Anh, 8/10/24

On July 16, 2024, the Ministry of Health of Gaza announced that circulating vaccine-derived poliovirus type 2 (cVDPV2) had been detected in sewage samples from six locations across Khan Younis and Deir Al-Balah, two governorates of Palestine located in the Southern Gaza Strip. A virtual meeting between the Regional Subcommittee on Polio Eradication and Outbreaks was called on July 25, 2024, with health administers across the Eastern Mediterranean Region, Global Polio Eradication Initiative (GPEI) partners, World Health Organization Regional Director for Eastern Mediterranean, and UNICEF regional directors for the Middle East, North Africa, and South Asia in attendance. The goal of the meeting had been to address the detection of cVDPV2 in Gaza and what plans of action were necessary to prevent a potential outbreak in the region. But what was so important about this meeting? What does the emergence of cVDPV2 mean? What are the effects this could have on the wider population? To better understand all this, we must take a deep dive into the history of polio- the vaccine development, the eradication programs, and the re-emergence of vaccine-derived polioviruses. 

Polio may be used interchangeably to describe a disease or a pathogen, but the distinction between the two must be made. Poliovirus is the name of the pathogen that can lead to poliomyelitis, also known as "infant paralysis" or, more commonly, "polio." So, although a person may be infected with poliovirus, that does not mean they have polio; that term is reserved for patients with a paralytic infection, the most severe symptom of poliovirus infection. Polioviruses are primarily spread through the fecal-oral route (i.e., contact with the feces of an infected person or consumption of water/food contaminated with an infected person's fecal matter). It can also spread through nasal and oral secretions, which is less common than the fecal-oral route. Out of all age demographics, children under the age of five (even more so unvaccinated children under five) are at the most risk of developing polio because they have not learned sanitary toilet training yet and often act as a source of transmission. 

There are three wild serotypes, or variants, of poliovirus (PV1, PV2, and PV3). Initial symptoms of poliovirus infection include fevers, fatigue, headaches, vomiting, neck stiffness, and limb pain. Usually, most infections are asymptomatic or only have mild symptoms, but out of 200 infections, one infection can lead to irreversible paralysis (polio, typically in the legs). Of those polio cases, 5-10% of people will die when their breathing muscles are immobilized. This can occur within hours of infection as the virus attacks the entire nervous system (the brain and the spinal cord), specifically cells that control muscle movements leading to disruption in nerve impulses to different muscle groups. 

Back before the invention of the polio vaccines, polio was rampant and caused several deaths. Many global efforts tried to research the nature of polioviruses to develop vaccines and implement immunization programs to reduce the prevalence of worldwide cases. It was not until 1955 that Jonas Salk developed the first polio vaccine, the inactivated poliovirus vaccine (IPV). The vaccine used all three inactivated (killed) poliovirus strains and was administered through intramuscular or intradermal injection. This produced antibodies in the blood for all three poliovirus serotypes but did not stop the poliovirus from multiplying in the intestinal tract meaning that while an IPV recipient would get immunity, the virus could still be transmitted. This downside to IPV was resolved less than a decade later, Albert Sabin developed the second polio vaccine, the oral polio vaccine (OPV), which contained live but weakened poliovirus. This vaccine was much easier to administer and, more importantly, prevented person-to-person spread of poliovirus. The invention of the oral vaccine played a vital role in eradicating wild-type polioviruses. Babies received two doses of IPV and one dose of OPV as well as two boosters at 18 months and five years old, which ultimately led to lifelong protection against polio. 

In 1988, the World Health Assembly adopted a resolution for the worldwide eradication of polio and launched the GPEI. The initiative was spearheaded by national governments and organizations like the United States Centers for Disease Control (CDC), UNICEF, and WHO. Through this program, the region of America was the first in the world to be certified polio-free. Many other regions followed shortly after, and as of September 2015 and October 2019, type 2 and type 3 wild poliovirus have been declared eradicated, respectively. As of October 2023, type 1 wild poliovirus is only endemic in Pakistan and Afghanistan. All in all, these vaccines have helped reduce the amount of global polio cases by 99% since 1988.

Nowadays, the OPV is the most commonly administered vaccine in countries where there is an outbreak of poliovirus. Several different types of OPVs are used- some that only protect against one serotype, two serotypes, or all three serotypes. Currently, the most commonly used OPV is the bivalent oral polio vaccine (bOPV) that protects against PV1 and PV3. Other types, such as the trivalent oral poliovirus vaccines and monovalent oral poliovirus vaccines, are only used in moderation, depending on whether they would benefit an area. 

So this must be the end. With time, Type 1 wild poliovirus will be eradicated, and the world will celebrate being polio-free, right?Although that is the hope, the solution is not that easy. The key to eradicating polio is immunization and high vaccination coverage to prevent the spread of infection. The recommended vaccination coverage in an area to prevent the reintroduction of the virus is 95%. When the vaccination coverage gets too low, the weakened poliovirus can jump among unvaccinated populations and slowly mutate to become similar to its wild serotype that can lead to polio. While wild-type polioviruses are more widely known, these mutated forms of vaccine-poliovirus are called vaccine-derived poliovirus (VDPV). They are the main culprit that causes current polio cases in the world. 

Of the VPDVs, cVDPV2, the strain detected in South Gaza this past July, is the most prevalent in recent years, especially in African regions. As mentioned before, type 2 wild polioviruses have already been eradicated, but cVDPV2 poses a severe threat to unvaccinated children under the age of 5. An entirely new vaccine, the novel oral polio vaccine type 2 (nOPV2), was developed in 2011 to control the spread of cVDPV2 in affected countries. The administration of these vaccination programs is essential, but it is also vital to prevent the conditions that could lead to the spread of cVDPV2s. 

In regards to Khan Younis and Deir Al-Balah, it was reported by WHO Regional Director for the Eastern Mediterranean, Dr. Hanan Balkhy, that the ongoing humanitarian crisis in these areas has resulted in poor sanitation conditions that are favorable for the spread of polioviruses and other infectious diseases. Clean water shortages (both drinkable and for sanitation services), open, untreated sewages, lack of toilet paper, increased cases of diarrhea in children, and cramped living spaces are just some of the unsanitary conditions that are promoting the spread of infectious diseases in the area. Additionally, the lack of access to healthcare is another possible contributing factor that has made it more challenging to prevent the spread of the cVDPV2s. Prior polio vaccination campaigns in 2022 were launched to vaccinate Gazan children as part of the global effort to eradicate poliovirus by the GPEI. However, with the ongoing humanitarian crisis occurring in Gaza this has made access to vaccination services much harder. As of 2022, 99% of children in the Palestinian Territories got their third polio vaccination dose, which is necessary for lifelong immunization. By 2023, this fell to 89%, below the 95% recommended vaccination coverage in an area to prevent the reintroduction of the virus. 

For clarification, though, no polio cases have been reported in South Gaza; cVDPV2, the pathogen, was merely detected in the sewage. However, various tests on those samples have found that all isolates were genetically linked and likely came from a singular source. Genetic sequencing of the sample cVDPV2s has shown that it is closely related to a strain circulating in the second half of 2023 in Egypt and could have been introduced in Gaza as early as September 2023. More testing also found enough nucleotide differences to indicate that it has been circulating for some time now. So, while no polio cases have been reported, the transmission of the cVDPV2 may have already begun or become widespread with these ongoing sanitary conditions. 

From the 11th meeting of the Eastern Mediterranean Regional Subcommittee on Polio Eradication and Outbreaks, the personnel in attendance concluded that there must be intensified surveillance in the area and a mass polio immunization campaign whenever feasible. The following day, July 26, 2024, WHO director-general Tedros Adhanom Ghebreyesus announced that one million polio vaccines would be sent and administered to Gazan children within the coming weeks. Additionally, the meeting had also called for a "safe and enabling environment" to administer vaccinations "by way of a ceasefire or days of tranquility, to allow for steps to be taken to stop polio from paralyzing children in Gaza, and surrounding areas and countries". 

Polio has long been an extremely devastating disease, especially among children. So much advancement has been made to eradicate wild-type polioviruses worldwide and prevent outbreaks of vaccine-derived polioviruses. However, as stated on the WHO website, "as long as a single child remains infected, children in all countries are at risk of contracting polio." Even with the eradication of wild-type polio, the fact remains that high vaccination rates must be held in a community to ensure that vaccine-derived variants will not evolve into forms that can lead to polio. Ensuring every child is vaccinated against polio, whether they live in a polio-free country or not, is essential to eradicating polio forever. It will take collaboration between many countries, many governments, and many communities to ensure that no child will ever have to deal with the debilitating, life-long effects of polio paralysis again. 

References: 

Administrator & World Health Organization. (n.d.). Palestinian Ministry of Health launches polio vaccination campaign to boost immunity in Bethlehem and Jerusalem. Global Polio Eradication Initiative. (n.d.-a). GPEI-Variant Poliovirus (cVDPV). Global Polio Eradication Initiative. (n.d.-b). IPV. 

Global Polio Eradication Initiative. (n.d.-c). OPV. 

Pan American Health Organization. (2022, September). The history of Polio – from eradication to re-emergence. 

Roberts, L. (2024, July 22). The virus that causes polio has been found in Gaza. Here’s why that is grim news. 

Team, H. J. & Health Jade. (2018, February 20). Polio (Poliomyelitis) - How Polio Is Spread, Causes, Symptoms, Treatment. 

United Nations. (2024, July 26). WHO to send one million polio vaccines to Gaza to protect children. 

World Health Organization. (2023, October 24). Poliomyelitis. 

World Health Organization. (2024, July 26). Regional Health Ministers rally to support immediate steps to stop polio from paralyzing children in the Gaza Strip.