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What is vaccine-derived poliomyelitis?

Poliomyelitis is transmitted from person to person through contact with feces, often as a result of poor hand hygiene or consumption of food or water contaminated with human feces. The virus initially replicates in the nose or throat, before moving to the intestines and multiplying, then entering the bloodstream and entering the central nervous system, where it can cause nerve damage and paralysis. About one in 200 people who contract the disease will suffer from paralysis; between five and ten percent of people who develop paralysis die because their breathing muscles stop working.

Vaccine-derived poliomyelitis is extremely rare. Since 2000, more than 10 billion doses of OPV have been administered to nearly three billion children worldwide, and just over 1,000 cases of paralysis due to vaccine-derived poliomyelitis have been reported in that period.

Fortunately, vaccines are very effective in preventing poliomyelitis. There are two types: The first is the oral polio vaccine (OPV). It contains a mixture of poliovirus strains that have been weakened, meaning they can still replicate but are not strong enough to cause paralysis. Since OPV is administered orally, it causes the production of antibodies both in the gut and in the blood. This means that if a vaccinated person is exposed to wild poliovirus in the future, the virus will not be able to replicate and infect other people.

The other type of vaccine is the inactivated poliomyelitis vaccine (IPV). It is made from polioviruses that have been killed so that they cannot replicate, and is injected into the leg or arm. Although it is very effective at activating antibodies in the blood, preventing the virus from traveling to nerves and causing paralysis, IPV is less effective at activating antibodies in the gut. This means that vaccinated people can still become infected with wild poliovirus and pass it on to others, even if they don’t get sick themselves.

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Historically, OPV has been more popular than IPV because it is cheaper and easier to administer, allowing large numbers of children to be vaccinated. It also protects both the individual and the community from infection – unlike IPV, which only protects the individual – which is important if poliovirus is to be eradicated.

However, because OPV contains weakened viruses that can replicate, some of them can be shed by the vaccinated child and transmitted to others, especially in areas with poor sanitation. This may be beneficial because exposure to this weakened virus also immunizes these people against poliomyelitis.

However, too much transmission of this weakened virus can be problematic. In communities where many people have been vaccinated against poliomyelitis, transmission is limited and the virus dies out quickly. But in communities with low vaccination coverage, this weakened virus can continue to circulate for many months, gradually accumulating mutations that again allow it to cause paralysis.

Vaccine-derived poliomyelitis is extremely rare. Since 2000, more than 10 billion doses of OPV have been administered to nearly three billion children worldwide, and just over 1,000 cases of paralysis due to vaccine-derived poliomyelitis have been reported in that period.

It also only occurs in under-immunized communities, which is one of the reasons why vaccinating every child is so important: if communities are fully immunized, it helps prevent the spread of wild and vaccine-derived polio.

A new OPV has also been developed: it should offer the same protection as the current oral vaccine, with less risk of mutation and paralysis. It is currently used to control outbreaks of vaccine-derived poliovirus in a small number of countries.

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Enormous progress has been made in the fight against poliomyelitis, but high use of the poliomyelitis vaccine will be needed to eradicate the disease for good.

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