ERADICATION is the final frontier in suppressing disease. Once truly eradicated, a disease is no longer a threat to people because there are simply no reservoirs of “wild” viruses left to infect anyone. If it were an organism, the virus would be called extinct in the wild. Eradication is extremely ambitious. Only two diseases have ever been globally eradicated by humans — smallpox in 1980 and rinderpest (a disease of cattle) in 2011 — but finally polio is on the brink.

Over the past 40 years, a worldwide public-health campaign has administered billions of doses of oral polio vaccine, bringing the number of cases of wild poliovirus down from about 400,000 cases per year in 1980 to just 33 last year.

In 2018, Pakistan and Afghanistan were the only two countries in the world to report cases. The final stages of eliminating a disease are the hardest and require the most intense effort because stopping the last few infections is crucial to stop the disease bouncing back. Understandably, near-elimination is also the time when people are most likely to take their foot off the gas, because the disease is no longer such a noticeable problem.

In the case of Afghanistan there are issues with people refusing the vaccine due to mistrust of vaccination campaigns. This mistrust was given a solid basis by the CIA. In 2011 it launched a campaign to immunise children against hepatitis B as part of its attempts to gather information on the compound agents believed Osama bin Laden was hiding in. The lasting damage to perceptions of vaccination campaigns has been huge.
 
However, something much scarier than vaccine refusal threatens the global polio-eradication effort. The oral polio vaccine, which is the cheapest and most widely used method to vaccinate against the polio virus, is what is known as an attenuated vaccine. It contains an extremely weakened copy of the polio virus, against which the body develops antibodies. These antibodies will protect people against the real virus if they are infected in the future.

Very rarely, the weakened virus can still cause a proper polio infection. This occurs in about one in every 2.4 million vaccinations and is more common in people with existing immunodeficiencies. If a high enough proportion of the population are vaccinated, then there will not be enough susceptible individuals around to keep polio transmission going and it will eventually be eradicated.
 
After taking the oral polio vaccine, people expel copies of the weakened polio virus which, if there is poor hygiene and sanitation, can go on to infect other people. Unexpectedly, this is usually beneficial, since the weak infection gives immunity to other people just as if they had been vaccinated themselves.

However, the polio virus mutates fairly quickly. The danger comes when some of these random mutations turn the weakened virus back into a dangerous disease. This becomes more likely the longer a weak vaccine-derived polio strain is allowed to circulate between people. Alarmingly, vaccine-derived polio strains have already emerged around the world, even in countries that have eradicated the wild type of the disease.
 
In fact, 2018 was the first year in which the number of reported cases of vaccine derived polio virus was greater than the number of wild polio virus cases. Fortunately the same vaccines work against both kinds of the disease. The problem is simply that not enough people are being vaccinated to stop circulation of vaccine-derived polio in places such as the Democratic Republic of Congo, Nigeria, Somalia and Papua New Guinea.
 
Thankfully the polio strains that have mutated from the heavily weakened virus are usually less transmissible than their wild counterpart, meaning eliminating them should be easier.  However, they must not be ignored. Eradicating polio will involve tackling the vaccine-derived strains. This will only be possible if countries can double down on vaccination programmes that are missing too many children. Often this is caused by more immediate concerns, such as ongoing armed conflicts in the Democratic Republic of Congo or having to focus on stopping deadly outbreaks of Ebola.
 
In Britain, and in other rich countries, we use an alternative vaccine. It is injected instead of swallowed and uses an inactivated poliovirus instead of merely a weakened one. This means the recipient will develop antibodies to fight future polio infection just like the oral vaccine, but there is no risk of them infecting anyone else and potentially creating vaccine-derived strains.

The injection requires a medical professional to administer it and because it cannot be passed on requires a higher level of vaccination for community protection, making it a more expensive, lower-risk programme. 

It has taken 40 years of concerted effort and globally available high-quality vaccines to get polio down to the current level. Other diseases which we cannot vaccinate against will be even more difficult to eradicate.
 
The burden of infectious disease mirrors the wide gaps in equality that we see across the world. Disease prevention campaigns require a huge funding and organisation, which is not possible in countries ravaged by colonialism and imperialist wars. But the eradication campaign shows the potential for success where there is the political will.

The first country to launch a nationwide polio vaccination campaign was Cuba. In 1962, shortly after the communist revolution, Fidel Castro launched the campaign. Within one year nearly 90 per cent of the population aged between one and 14 had been vaccinated.

The campaign was so successful that there were no recorded deaths from polio after 1962 and Cuba was declared polio-free by the World Health Organisation in 1994. This example shows that even a country with modest resources, if it is genuinely run in the interests of its citizens, can tackle some of the hardest challenges facing our world.