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

the United States has detected the first case of poliomyelitis in nearly a decade in a young adult resident of New York City and is believed to be a vaccine-derived infection.

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

Vaccination against poliomyelitis (1962)
CDC/ Mr. Stafford Smith

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

Poliomyelitis or poliomyelitis is a disease caused by the poliomyelitis virus which 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 is infected is that the virus enters the body through the mouth and nose and multiplies in the throat and gastrointestinal tract. Then it spreads through our body through the blood and the lymphatic system to end up being excreted orally and in the stool of the infected person.

Most poliovirus infections are asymptomatic and the infection resolves without further incident. 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.


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


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

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

Salk’s intramuscular vaccine uses inactivated viruses (viruses dead which cannot infect cells or multiply). The administration of this vaccine leads, 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 effective immune response in the oral and intestinal mucosa. Therefore, if someone is infected, the virus will multiply in their gastrointestinal tract and the infected person will be able to infect other people even if they themselves do not develop the disease.


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

On the other hand, we have the oral attenuated virus vaccine. This vaccine uses an attenuated but not inactivated virus. 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 administered orally, it will reproduce the life cycle of the wild virus. This 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 and could not infect other people if infected with the wild virus. Another advantage of the oral attenuated vaccine is that vaccinated people can shed the vaccine virus and thus vaccinate other people who have not received the vaccine.

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 resulted in a significant decrease in poliomyelitis 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 pros and cons.

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 has the advantage of limiting the circulation of the virus, but it has more undesirable effects. The main one is that by using a virus that infects, multiplies and mutates, there is a chance that it can reverse itself and become virulent again. This would cause disease 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 reaction (the benefit of vaccination outweighs the risk of developing vaccine-derived paralytic poliomyelitis) that is at higher risk in immunocompromised people and in settings with low vaccination coverage .

What vaccine is currently being administered?

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

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

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

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

Polio elimination status.

So what happened in New York?

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

This case has alerted the various health authorities, who strongly recommend that unvaccinated people do so as soon as possible in case other cases arise. In fact, the poliomyelitis virus has been detected in residual water of New York a month before the case of the infected person is made public, as well as in those of London.

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

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

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

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