How is it possible that the case of polio detected in the United States is a vaccine-related infection?

The United States has detected the first case of polio in almost a decade in a young adult living in New York and it is thought that it could be an infection derived from vaccines.

How is it possible that we are facing a case of poliovirus derived from the vaccine if the disease is almost eradicated and it is assumed that the main objective of vaccines is precisely to prevent people from getting sick?

Polio Vaccination (1962)
CDC/ Mr. Stafford Smith

To better understand what is happening, we will first describe what polio is and how the virus infects and multiplies when it enters our body.

Polio or polio is a disease caused by the polio virus that can affect the spinal cord, causing muscle weakness and paralysis. In most cases, the virus is transmitted by direct person-to-person contact or by the fecal-oral route.

Usually what happens when a person becomes infected is that the virus enters the body through the mouth and nose and multiplies in the throat and gastrointestinal tract. Then, it spreads in our body through the blood and the lymphatic system to end up being excreted orally and in the feces of the infected person.

Most infections caused by the poliovirus are asymptomatic and the infection resolves without further mishap. However, in about 1% of cases, the virus enters the central nervous system, where it can infect and destroy motor neurons, causing paralysis and muscle atrophy.

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In short, the virus enters through the mouth and nose, multiplies in the intestine and is transmitted by the infected person orally or through the feces (fecal-oral), being able to pass the virus to other people. In most infections there are no symptoms and the infection resolves, but in a small number of cases the virus can infect cells of the nervous system and cause the disease we all know.


Now that we know how this virus infects and how the disease occurs, let’s talk about vaccines. In short, there are two types of polio vaccines:

1) Intramuscular vaccine with inactivated virus (Jonas Salk, 1952)

Salk’s intramuscular vaccine uses inactivated viruses (viruses dead that cannot infect cells or multiply). The administration of this vaccine gives rise, among other things, to the presence of antibodies in the blood. This largely prevents the virus from invading the nervous system and causing disease.

However, this vaccine does not generate a very efficient immune response in the oral and intestinal mucosa. Therefore, if someone becomes infected, the virus will multiply in their gastrointestinal tract and the infected person will be able to infect others even if they themselves do not develop the disease.

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2) Oral vaccine with attenuated virus (Albert Sabin, 1957)

On the other hand, we have the oral vaccine with attenuated virus. This vaccine uses a virus that is attenuated but not inactivated. In other words, it is a virus that can infect cells and multiply.

The advantage of this vaccine is that, by using an attenuated virus that is administered orally, it will reproduce the life cycle of the wild virus. That is why it confers protection in the blood, but also in the oral and intestinal mucosa.

Therefore, with this vaccine, the immunized person would not develop the disease nor could he infect others if he becomes infected with the wild virus. Another advantage of the attenuated oral vaccine is that vaccinated persons can excrete the vaccine virus and thereby vaccinate other persons who have not received the vaccine.

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Polio vaccination in response to a recent polio outbreak in the Horn of Africa. ©UNICEF Ethiopia/2013/Sewunet.

Advantages and disadvantages of each vaccine

The inactivated virus vaccine was the first to be used and led to a significant decrease in polio cases. However, the oral vaccine had (and still has) a fundamental role in the control and future eradication of the disease.

Each vaccine has its advantages and disadvantages.

On the one hand, the inactivated vaccine is very safe and prevents people from getting sick, but it does not prevent the virus from continuing to circulate.

On the other hand, the attenuated vaccine does have the advantage of limiting the circulation of the virus, but it has more adverse effects. The main one is that by using a virus that infects, multiplies and mutates, there is a chance that it could reverse and become virulent again. This would cause illness in the vaccinated person or in someone else who has been exposed to the virus excreted by the vaccinated person.

This is known as “vaccine-derived poliovirus” and “vaccine-derived paralytic poliomyelitis.” This vaccine-derived disease is a very rare adverse effect (the benefit of vaccination is greater than the risk of developing vaccine-derived paralytic poliomyelitis) that presents a higher risk in immunosuppressed people and in places with low vaccination coverage.

What vaccine is currently administered?

Nowadays one or another vaccine is used depending on the epidemiological situation of the country.

The attenuated oral virus vaccine is used mainly in places where the virus continues to circulate (Pakistan and Afghanistan, for example) or where the risk of new outbreaks is higher, in order to avoid transmission of the virus.

The inactivated virus vaccine is currently used in countries where there are no cases of polio and the probability of becoming infected is very low.

In the following image we see how the number of countries where polio has been eliminated has evolved:

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Polio elimination status.

So what has happened in New York?

A person residing in New York who has not been vaccinated against polio is thought to have come into contact with a person from a country where live attenuated oral vaccines continue to be vaccinated and has become infected with vaccine-derived poliovirus. As a result, she has developed vaccine-derived paralytic polio.

This case has alerted the different health authorities, which strongly recommend that unvaccinated people do so as soon as possible in case more cases occur. In fact, the polio virus has been detected in the sewage of New York a month before the case of the infected person was made public, and also in those of London.

Finally, it is important to keep in mind that polio has not been eradicated yet. Although we no longer see cases of polio around us, the virus continues to circulate.

There is currently no cure for polio, but fortunately it can be prevented with vaccines that have been with us for more than 60 years. Therefore, the best way to protect against the disease and advance the eradication of polio is through vaccination.

A version of this article was originally published on the author’s website.

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