So far, only one human viral infection has ever been eradicated by vaccinations: smallpox. However, polio might be next. It has been just over a year and not one new case of wild polio has been reported in Africa. The last reported case was on August 11, 2014, in central Somalia.

"Every month would go by, and we'd think 'oh my goodness, another month,'" said Sona Bari, a member of the Global Polio Eradication Initiative from the World Health Organization (WHO), to IFLScience.

While eradication has already been achieved in many areas of the world, progress in Africa has been hampered due to poor vaccination coverage in densely populated regions, as well as difficulty in accessing remote communities that don't exist on maps. Bari described workers who would wander into the bush for days and return with data on, for example, 10 isolated but occupied huts of people that would otherwise never have received vaccinations. "It's really a tribute to the health workers of Africa," she said.

There are two main types of polio virus: wild poliovirus (WPV) that is contracted, as the name implies, in the wild. This is the strain that hasn't emerged in Africa for the last year. The other type, which has had 10 cases in Africa (Madagascar and Nigeria) in 2015, is called circulating vaccine-derived poliovirus (cVDPV). To understand how this version of the virus circulates, it is important to know how the oral polio vaccine works.

The oral vaccine contains a weakened version of the polio virus, and once gulped down, it replicates in the gut. This prompts the human immune system to start producing antibodies against it before being excreted out in the stool. This represents a source of virus that can then enter another person, providing 'passive' protection without the need for vaccination. However, the more this process repeats, the more likely it is that the virus will mutate back to a version that can cause paralysis.

Mother in a wheeling chair waiting for a polio vaccine in Begoua. hdptcar/Flickr (CC BY 2.0)

The relative risk of cVDPV is very small: In 2014, 359 people were infected with WPV globally, whereas 55 were infected with cVDPV. This is because cVDPV appears to be less transmissible than WPV. However, stamping this out will be next on the agenda. To do this, the WHO will start to replace the oral vaccine with an injected vaccine.

Despite this initial success, it's still too early to announce that polio has been eradicated in Africa. Since many families live in remote locations, it can be difficult to pinpoint an outbreak in these regions. To account for these people, the WHO requires three consecutive years of no new cases before Africa can be declared polio-free. There are also other diseases that can cause paralysis like polio does. This means that any patient with polio-like symptoms needs to have one of their stool samples tested for the virus.

However, this doesn't detract from their remarkable milestone. If Africa truly becomes polio-free after two more years of careful observation, then that will leave only two countries where polio remains: Pakistan and Afghanistan.

Bari summarized to IFLScience by saying, "This is such a moment of celebration, but it's such cautious celebration. We cannot take our foot off the pedal right now. We have to keep at this rate of activity because we've never been this close."