April 25, 2017 0627 GMT
The world is on the verge of eradicating polio, a disease that for millennia has caused paralysis and death, mostly in children, independent.co.uk reported.
Polio is now only found in three countries — Pakistan, Afghanistan and Nigeria, and just 37 cases of the disease were recorded last year.
But challenges still remain, including cultural resistance to the vaccinations in some areas, where there have been violent attackson health workers by Islamists.
In April 2016, the type of vaccine used to fight the disease was changed, to protect against the worrying phenomenon of vaccine-derived polio.
Here is everything you need to know about the switch.
How many different types of polio virus are there?
There are three different types of naturally circulating, or 'wild', polio virus. Even if you become immune to one strain of the virus, you can still be infected with the others.
The success of vaccination campaigns means only one of these strains, type one, is currently at large.
Type two polio virus was last detected in India in 1999 and was declared eradicated in 2015, while type three has not been seen anywhere in the world since November 2012.
What is the difference between the injectable polio vaccine (IPV) and the oral polio vaccine (OPV)?
The injectable vaccine, developed in the early 1950s by Jonas Salk, is made from a killed, or chemically inactivated, polio virus. It contains all three strains of the virus and is injected into the skin or muscle by a trained health worker.
Antibodies are then created in the blood that fight the infection and protect against paralysis.
While the vaccine is highly effective in preventing the disease, it does not induce immunity in the intestines. This means that the virus can still multiply in the gut and continue to circulate within a community.
The oral polio vaccine, created by Albert Sabin in 1961, contains a weakened form of the live virus. It is the most widely-used vaccine in the fight against polio worldwide, because it is inexpensive and easy to deliver by volunteers with no medical training.
What is vaccine-derived polio?
The weakened vaccine-virus in OPV can multiply in the gut, spreading immunity between people. This is usually an advantage, because it strengthens a community’s collective defenses against the disease and curbs its transmission.
However on rare occasions, when levels of routine immunization are poor, it can continue to circulate for a long time. If this is allowed to occur, the vaccine-virus sometimes mutates into a form that can cause paralysis.
These cases are unusual; last year, there were just five cases of vaccine-derived polio in the world. But to prevent any further cases, IPV is being deployed as well as OPV.
Since 2000, nearly three billion children globally have been given more than 10 billion doses of OPV, according to the World Health Organization. This has prevented more than 13 million cases of polio.
In that time, 24 outbreaks of vaccine-derived polio were detected in 21 countries, with fewer than 760 cases reported overall.