In the last few months, media outlets have been publishing news about the polio virus that was found in sewage systems in southern Israel. Many of us remember pictures from the 1950's depicting paralyzed children, and so many people felt afraid and frightened, and not without merit. On the other hand, the polio vaccine also attracted a wave of rumors about terrible side effects. To sort things out, let's start with a description of polio.

Polio is caused by the poliomyelitis virus. It is transmitted between humans only, and is mostly infectious through fecal matter and saliva. Three different types of viruses can infect the cells of the intestine, and penetrate the bloodstream . In most cases, the disease goes away without any complications, but one in approximately one thousand cases, the virus hurts the nervous system, causing paralysis.

This paralysis can be temporary, but sometimes it stays forever, leaving the patient with a lifelong disability. In severe cases, the paralysis causes respiratory failure due to paralysis of the respiratory muscles. Patients who have this condition need to be mechanically ventllated, once commonly performed using an “iron lung”.

The following video shows how a virus invades the body and infects the cells. To watch, click the picture and the “Play” button.

The video was produced by the American History Museum

In the video, we saw how the polio virus infects a person. It enters through the mouth, and reaches the intestines, from where some of the virions leave the body, and others penetrate the wall of the intestine. After the invasion, the virus makes the cell create building blocks for new viruses, which are secreted into the bloodstream and can spread all over the body. Alarmingly, viruses can then infect nerve cells and destroy them, causing paralysis. For further information, see this video.


Nowadays, there are two types of vaccines designed to prevent infection with the three strains of polio. The first, called IPV, is a mix of viruses inactivated in formalin (formaldehyde), meaning the virus structures were fixed and thus the virus cannot function. Importantly, most of the formaldehyde is removed from the mix during preparation and the remainder has not been found to be dangerous. This vaccine was developed in the fifties by Jonas Salk.

The inactivated virus vaccine's efficacy after two doses is 90%, and as high as 99% after three doses. This virus only presents the virus to the immune system, and so after receiving IPV, the immune response would only kick in if the virus penetrates the bloodstream. This means that people immunized with just IPV may be able to carry the viruses in their digestive systems for some time (up to several weeks) and infect other people without developing the disease themselves. IPV is administered to children in Israel as part of the pentavalent (5-component) vaccine.

The second vaccine, called OPV, is a live-attenuated vaccine, meaning the viruses were treated in ways that significantly hamper their ability to infect human cells. This vaccine was developed by several groups, one of whom was led by Albert Sabin, who went on to become the president of the Weizmann Institute in 1972-1969. This vaccine is taken orally, mimicking a small infection the immune system can handle easily. A person immunized with OPV becomes a “carrier” of the weakened polio virus for a few weeks, and develops antibodies against it. The intestines of such a person become a very hostile environment for any form of the virus. The advantage of this vaccine is that it makes the body secrete antibodies into the intestinal lumen, which destroy the virus population and prevent carrying the violent, wild virus or infecting others with it.

Although OPV has the advantage of affecting the intestines, it suffers from several disadvantages compared with IPV. First, it is simply less effective, with only 50% efficacy after the first dose, and going up to 95% only after the third dose. The second, more severe disadvantage is that approximately one in a million children who receive OPV will have an outbreak of the virus causing paralysis. A third disadvantage is that rarely, some immunized children may have an outbreak without paralysis, but the carrier may be infectious for others, for a period as long as several months.

Important and comprehensive information about the vaccines can be found on the Israeli Ministry of Health website.


So How Dangerous Is It?
At the end of 2011, about 2,576,900 children lived in Israel. Statistically, this means that if none of them were immunized against polio, and all of them received OPV, 2 to 3 children would have a violent outbreak of the virus due to the vaccine. By the same imaginary scenario, if none of the children were immunized, we would have seen an outbreak similar to what Israel saw in the 1950's.

In practice, 94-95% of children are immunized at a young age against polio with IPV, which protects them well against the adverse effects of OPV. Hence the probability of suffering from these adverse effects is close to zero. The other adverse effect, namely of carrying and spreading polio, is mitigated by herd immunity, meaning that even if one person is carrying polio in their digestive system, that person is surrounded by so many immune people that the chance of passing it on is extremely low.

One of the frequently heard arguments against this vaccine is that the person who receives it becomes a carrier and infects others with live, weakened viruses. This is true, but not a disadvantage at all. The person receiving the vaccine may infect others with such weak viruses, and may change back to the regular form of the virus, but a regular virus could also change to become more violent. Herd immunity also comes into play here, protecting sensitive individuals from these effects.

In certain countries such as India, OPV is administered without IPV, and there is no evidence of outbreaks due to the vaccine.

For more information on the importance of vaccines and herd immunity, please refer to “Vaccine Avoidance – A 'Natural' Lifestyle or Playing with Fire?”.

In India, a large country in which early-age immunization rates were relatively low, large drives were conducted against polio. Due to the law of large numbers, some children got sick from attenuated viruses given in OPV reverting to a violent strain of polio.

Nowadays, polio is long gone from most of the world and is on the way to extinction. It is still found only in certain countries of Africa and Asia, in which vaccine drives are taking place to exterminate the disease entirely. Efforts are stymied by irrational anti-vaccine movements in countries such as Nigeria.

In Israel no cases of polio have been observed since 1988. A “quiet outbreak” took place in 2013 in southern Israel due to the introduction of a wild virus, most likely from Egypt, which has also seen a similar “quiet outbreak”. The virus arrived in Egypt from Pakistan, a country with a low immunization rate (approximately 60 percent).

Recently, the attenuated vaccine, OPV, was given to many children under the age of nine, who have already received the inactivated vaccine, IPV. This decision was made in order to create herd immunity and prevent infection. This way, if the virus can't find any hosts, its life cycle will be cut short and the outbreak will stop.

The vaccine drive is not meant to protect children who have already been vaccinated. It is meant to annihilate the wild virus, stop its spread among people who received only IPV, and protect sensitive populations such as people with problems with their immune systems, babies, people who are allergic to vaccines, older people and anyone in the 1% of the population for whom the vaccine does not work. Conditions were chosen to mitigate side effects: Only children who received IPV will get OPV, which is much safer, and the chosen vaccine includes only 2 out of the 3 viruses.

Recently we have been running into rumors about alleged dangers of the vaccine, and claims downplaying the complications of polio. We invite you to point us to such arguments with references, and we will do our best to confirm or deny them based on the latest scientific literature.

Further Reading:

Israeli Ministry of HealthClinical Recommendations for People with Immunodeficiency

The World Health Organization (WHO)

The Polio Eradication Project

The Centers for Disease Control and Prevention (CDC)

Wolfson Hospital: Information about Polio Vaccines

Dr. Keren Landsman's rebuttals of polio-related rumors

Comprehensive and thorough article about the history of polio in Israel

Comment: Please read the article carefully, the links for further reading and previous comments before putting up your own question. If you cannot find a satisfactory answer, we will be happy to answer your question.

Davidson Online provides scientific information and its contents do not constitute recommendations of treatments, nor can they replace consultation with medical personnel.

Thanks to Dr. Keren Landsman for her help in preparing this article.

Thanks to Dr. Keren Landsman and Dr. Ehud Kleiner for their help in answering surfers' questions.


Dr. Erez Garty
Department of Biological Chemistry
Weizmann Institute of Science

Article translated from Hebrew by Aviv J. Sharon, M.Sc. student at the Weizmann Institute of Science.

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