Why officials are moving too quickly to vaccinate against polio in Gaza after just one confirmed case

Health and aid organizations have been working since Sunday to vaccinate more than 640,000 children under the age of 10 in Gaza against polio, with brief pauses in fighting between Israel and Hamas at designated locations and safe corridors to facilitate this humanitarian effort.

There has been only one confirmed case of polio in an unvaccinated 11-month-old boy from Deir al Balah, but the agencies leading the vaccinations – the United Nations Relief and Works Agency for Palestine Refugees in the Near East, the World Health Organization , UNICEF and the Palestinian Ministry of Health knew they had to act quickly. This is why.

Why is this being done now?

The 11 month old, who is paralyzedIt is the first case of polio in Gaza in 25 years. Health officials have also identified and are continuing to identify other children with suspected paralysis consistent with polio testing for the virus in stool samples. One of the most important factors in maintaining public health is sanitation. The ongoing conflict has significantly disrupted Gaza’s public health infrastructure, with reports of contaminated drinking water and areas of open, untreated sewage.

For an epidemic to occur, three factors must align: First, there is the presence of the pathogen – in this case the poliovirus. It was identified by UNICEF in sewage samples in Khan Younis and Deir al Balah, meaning someone released the virus into the sewage system through their feces, which could lead to human exposure. The second factor is a susceptible population. Before the war, in 2022, vaccination levels were the same reported at 99%. Maintaining levels of 95% or higher is ideal to reduce the risk of spread. Since the start of the war in October 2023, vaccination levels have fallen to less than 90%. This decline presents a potential opportunity for the virus to cause illness 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 war. To spread, a pathogen must circulate among a population in a suitable environment that promotes spread.

In this environment, polio is ideally suited to spread. It is an enterovirus, meaning it reproduces in the human intestines and spreads via the fecal-oral route. People become infected when they ingest the virus through food or water contaminated with human feces. Since all three factors (pathogen, susceptible individuals, environment) are now aligned in Gaza, you have the potential for the “perfect storm”: an environment ripe for the spread of disease.

Why is a single case a problem?

Polio vaccines, administered as oral drops, use weakened live viruses to confer immunity against one of the 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 comes from a live vaccine that has accumulated mutations that make the strain capable of causing disease again 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 insidious. It has variable manifestations, ranging from no symptoms (in 90% to 95% of those infected), a flu-like illness (in about 5%), all the way to the most severe signs of the disease: paralysis and possible respiratory failure (occurring in fewer than 1% of cases). Therefore, only a small minority of those who become infected actually develop a paralytic disease. The majority of people who are infected and may shed the virus in their feces do not even know they are infected. It is also very difficult for public health authorities to identify them.

This has significant public health implications, as a single case of paralytic disease, as happened in Gaza, is just the tip of an iceberg. For every identified case, there may be a hundred or more individuals in the population potentially spreading the virus. No vaccine provides 100% protection against infections and diseases, and the decline in childhood vaccination rates since the start of the conflict is creating more susceptible individuals. This is why WHO and its partners are taking aggressive action to vaccinate children.

Have warring parties paused before for public health?

Infectious diseases have proven over the centuries that they are able to take advantage of the opportunities we provide for them to spread. War is such an excellent opportunity for the spread of disease. That is why neutral parties have made attempts in the past to reduce disease through vaccination. Dr. DA Hendersonwho led the global effort to eradicate smallpox in the 1960s and 1970s, noted that the effort succeeded “despite a never-ending series of obstacles created by floods, civil war, famine and bureaucratic inertia.” One of my public health professors who worked in Africa on smallpox eradication 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 spreading live viruses. This would also include addressing the enabling environment by rebuilding the water and sanitation system – not so easy in the midst of an active conflict. Consequently, public health authorities are trying to address the only factor they can directly control at this time: increasing the immunity of the subset of the population most susceptible through vaccination.