Health and relief agencies have been scrambling since Sunday to vaccinate more than 640,000 children under 10 years old in Gaza against polio, with brief pauses in fighting between Israel and Hamas in designated locations and safe corridors to allow for this humanitarian effort.
There’s only been a single confirmed case of polio in an unvaccinated 11-month-old boy from Deir al Balah, but the agencies spearheading the vaccinations—the United Nations Relief and Works Agency for Palestine Refugees in the Near East, the World Health Organization, UNICEF and the Palestine Ministry of Health—knew they needed to act fast. Here’s why.
Why Is This Being Done Now?
The 11-month-old, who is paralyzed, is the first case of polio identified in Gaza in 25 years. Health officials have also identified other children with suspected paralysis consistent with polio and are currently testing for the virus in stool samples. One of the big factors in maintaining population health is sanitation. The ongoing conflict has significantly disrupted the public health infrastructure in Gaza, with reports of contaminated drinking water and areas of open, untreated sewage.
For an epidemic to occur, three factors need to align: first is the presence of the pathogen—in this case, the polio virus. It was identified by UNICEF in sewage samples in Khan Younis and Deir al Balah, meaning someone was shedding virus in their feces into the sewage system and it could lead to human exposures. The second factor is a susceptible population. Prior to the war, in 2022, vaccination levels were reported at 99%. Maintaining levels of 95% or above is ideal for reducing the risk of spread. Since the war began in October 2023, vaccination levels have declined to less than 90%. This decline provides a potential window of opportunity for the virus to cause disease if a susceptible individual is exposed. The third factor is the environment. In some ways, this is the most critical, but also the most difficult to control—especially in a war. To spread, a pathogen has to be circulating among a population in an appropriate environment that is conducive to spread.
Polio is ideally suited to spread in this environment. It is an enterovirus, meaning it reproduces in the human gut, and it spreads by the fecal-oral route. People become infected when they ingest the virus in food or water contaminated with human feces. Since all three factors (pathogen, susceptible individuals, environment) are now aligned in Gaza, you have potential for the “perfect storm”: an environment ripe for the spread of disease.
Why Is A Single Case A Problem?
Polio vaccines given as oral droplets use weakened live viruses to impart immunity to one of three polioviruses that cause polio. In this situation, the cause is not an original, wild-type virus, but a variant poliovirus, called circulating vaccine-derived poliovirus type two. A cVDPV2 strain originates from a live vaccine that has accumulated mutations that revert it back to become capable of causing disease in someone who has not been vaccinated. A cVDPV2 strain can spread through exposure to feces similar to the way the original polioviruses spread.
Polio is stealthy. It has variable manifestations, ranging from no symptoms (in 90% to 95% of those infected), an influenza-like illness (in about 5%), all the way to the most serious signs of disease: paralysis and possibly respiratory failure (occurring in less than 1% of cases). Therefore, only a small minority of those who become infected actually gets paralytic disease. The majority of individuals who are infected and possibly shedding virus in their feces don’t even know they are infected. It is very difficult for public health authorities to identify them as well.
This has significant implications for public health, because a single case of paralytic illness, as occurred in Gaza, is like the tip of an iceberg. For every identified case, there could be a hundred or more individuals in the population potentially spreading it. No vaccine provides 100% protection against infection and illness, and the decline in vaccination rates in children since the conflict began leads to more susceptible individuals. This is why the WHO and partners are moving aggressively to vaccinate children.
Have Warring Factions Previously Paused For Public Health?
Infectious diseases have proven over centuries their ability to take advantage of any openings we give them for spread. War is one such prime opportunity for disease spread. Therefore, neutral parties have in the past launched efforts to reduce disease through vaccination. Dr. D.A. Henderson, who led the global effort to eliminate smallpox during the 1960s and 1970s noted that the effort succeeded “despite a never ending array of obstacles occasioned by floods, civil war, famine and bureaucratic inertia.” One of my public health professors working in Africa on the smallpox eradication effort was briefly given safe passage to enter a rebel compound and vaccinate a group of fighters.
How Can We Disrupt A Polio Epidemic?
Because we know the three factors that lead to an epidemic, we can also use that knowledge to develop countermeasures. One way to disrupt the epidemic potential is to eliminate the virus in sewage. That’s hard to do unless you know who is actually shedding live virus and would include tackling the conducive environment by rebuilding the water and sanitation system—not so easy in the middle of active conflict. Consequently, the public health authorities are trying to address the only factor they can directly control at this time: increasing immunity of the subset of the population that is most susceptible through vaccination.
