Introduction: What is polio?

Poliomyelitis, commonly known as Polio, is a disease caused by a virus, called poliovirus, which mostly affects those aged 5 and under. The virus affects the nerves that supply the muscles, often in the limbs, paralysing the muscles (flaccid paralysis is the type caused by polio). They then waste away as the child grows. For example, if one leg is affected (and the other is not), the affected leg may be shorter and weaker in comparison to the unaffected leg. This can cause significant problems with gait (walking), which persist into adulthood. Rarely, polio can be fatal if it paralyses the muscles involved in breathing. In less severe cases, polio can cause flu-like symptoms and stomach pains, or may be completely asymptomatic.

How is polio transmitted?

The polio virus is spread via the faeco-oral route. This most commonly happens via contaminated food or water. Other ways of coming into contact are by changing clothes that are contaminated, or the nappies of babies.

There are 3 strains of the wild poliovirus: type 1, 2, and 3. Types 2 and 3 have been eradicated. Type 1 remains endemic in two countries in the world: Afghanistan and Pakistan. The term “wild-type” polioviruses, which are the term used for strains which are like the “original” polio viruses, can be vaccinated against.

Polio vaccine

Whilst polio does not have a cure, it can be prevented by vaccination. The polio vaccine is part of childhood vaccine schedules in almost every country, and in many countries in the world, polio has been eradicated (eradication is defined as 3 years of no cases following the last case of the disease).

There are two types of vaccines against the polio virus. There is an inactivated vaccine which contains a dead polio virus so that when injected, it does not cause polio, but can induce an immune response. This is given as an injection in the muscle in either the arm or leg. The vaccine requires multiple doses to give full immunity. The ages exactly which they are given vary from country to country, but for example, in the UK, there are 5 doses given at 8, 12 and 16 weeks of age, a pre-school booster at 3 years old, and a second booster dose at 14 years old.

Another type of vaccine is the oral polio vaccine, which is absorbed from the inside of the cheek and enters the bloodstream. It contains a live attenuated form of the virus, meaning it is a live virus which has been weakened in some way to prevent it from causing polio. It is cheap and easy to administer, enabling its use in low-income settings. Again, exact timings vary, but generally the first dose is soon after birth, followed by 6, 10 and 14 weeks of age, then a booster around 18 months. A very rare complication (around 3-4 incidences per million) of the oral vaccine is that it can cause vaccine-associated polio. However, without the oral polio vaccine, the reduction in polio cases throughout the world would not have been possible, and both types are capable of long-term immunity.

Those who are most at risk

The areas where vaccine coverage is low are areas where people are at the highest risk of contracting the polio virus. These are usually places where the oral polio vaccine has been used, because it contains a live virus; there is a risk that it can mutate into a disease-causing form of itself. These are known as vaccine-derived polioviruses (VDPV). So, even though Afghanistan and Pakistan are the only sources currently that have the “original” wild-type poliovirus, there are many more countries in the world which have reported cases of VDPV. 

It is worth noting here that adults (or even children) who have had the full doses of either type of vaccine are generally immune to both wild types and vaccine-derived strains of poliovirus. However, even for immune adults, if they acquire a mutated wild type, they can become carriers of the virus, and then can pass it on to babies and children who are not immune. There have been instances where adults who have been travelling from country to country have passed on the virus to other people (some cases from African countries were traced back to Pakistan).

If adults are not vaccinated, they can become susceptible to acquiring polio as well. This can be especially true for those who are also immunocompromised or elderly. A weakened immune system can be a consequence of conditions such as AIDS or cancer. It can also be because of taking medications, like long-term steroids, that weaken the immune response. Immunocompromised individuals can have more severe reactions than those with normal immune systems.  

Therefore, the only way that vaccination can be fully effective is for every person to be vaccinated and immune. If there is even a small proportion of the population who are not vaccinated, a vaccine-derived form of the virus can affect them and result in polio cases. 

Those staying in areas with inadequate sanitation are also at higher risk of contracting polio. Lack of sewage systems can contaminate drinking water, such as in areas where open defecation is practised. Lack of clean water for handwashing and washing food also puts people at risk- evidence has shown that the polio virus can survive on hands, on surfaces, or in food for days to weeks. In contaminated soil or water (i.e containing human faecal matter), the virus can survive for up to months. Moisture and warmth enable the virus to survive for longer, hence why it is more prevalent in warmer climates. Overcrowded living conditions in conjunction with low vaccination coverage and hygiene can exacerbate transmission as more people are likely to be vulnerable, and it can spread easily between people. Afghanistan and Pakistan, where polio has not been eradicated, both have areas of low vaccination coverage, significant movement between the two nations (spreading the virus) and areas of inadequate sanitation. Polio has also seen a resurgence in war zones, like in the Gaza Strip, where these hostile living conditions are found. 

However, simply not having access to vaccines is not the only factor influencing low vaccination coverage. Vaccine hesitancy can influence people’s decisions over whether or not to be vaccinated (or decide for their children), and refuse vaccinations even if they have access to them. Though it has very much come to people’s attention during the COVID-19 pandemic, vaccine hesitancy is not a modern problem. Incorrect information, rumours, and mistrust in the medical system can also reduce vaccine coverage rates. The decision of even one person not to be vaccinated can have a knock-on effect on many others in the population.  

Summary

Polio primarily affects children under the age of 5 and can cause muscle paralysis in the limbs. It is caused by a virus, which is transmitted via the faeco-oral route- most commonly spread by contaminated food or water. Polio can cause the muscles to not grow as much as they should, causing limbs to be shorter and weaker. Whilst children are more likely to be affected as they are growing and their immune systems have not fully developed, adults who are not vaccinated can equally acquire polio. There are regions of Pakistan and Afghanistan where the wild types of poliovirus are still present, and people in these areas are at particularly high risk. 

With one type of polio vaccine, the weakened poliovirus that is present in the vaccine can mutate into a disease-causing virus- this is known as vaccine-derived poliovirus (VDPV). If people are sufficiently vaccinated in the population, then they will be immune towards the vaccine-derived strains also. However, if they are not immune, they are susceptible to acquiring polio from the VDPV. Whilst Pakistan and Afghanistan have the wild-type polioviruses, there are many more countries in the world where there have been outbreaks of VDPV. 

Other factors (besides vaccination status) that influence the risk of contracting polio are inadequate sanitation (i.e lack of clean water for hygiene or sewage systems to prevent contamination of food or water), overcrowded living conditions and warm climates. Those living in hostile circumstances like war zones, areas of natural disasters or refugee camps are also at risk of acquiring diseases like polio.