A Closer Look – The Fascinating Science of Polio Vaccines
Vaccines, and the campaigns to deliver them, have done tremendous work to uplift global health and prevent the spread of disease. One example is the campaign against polio.
Polio is caused by infection with polio virus, which infects cells of the throat, intestine and nervous system. It is spread from person to person through fecal contamination of food, water or objects placed in the mouth. Sometimes it is spread through respiratory secretions of infected individuals.
The infection can cause a wide range of symptoms, including sore throat, fever, stomach pain and vomiting. In about one of every 200 people infected with polio — most of whom are children under 5 years of age — the virus replicates in the nervous system and causes irreversible paralysis. If paralysis affects the lungs, it can prevent patients from breathing under their own power. Indeed, before vaccines could prevent polio, whole hospital wards would be filled with machines, called iron lungs, noisily doing the work of breathing for patients whose lungs could no longer do so.
Polio has been a plague on human health for as long as history can remember. Pictographs from ancient Egyptian sites depict what archeologists theorize is evidence of paralytic polio affecting victims in 1400 BCE. But during the early 1900s, polio became increasingly prevalent with epidemics occurring frequently in large cities throughout the U.S., paralyzing and killing thousands of children. By the 1950s, polio had created an environment of extreme fear in the U.S. – Americans polled in 1952 expressed that the only thing they feared more than poliomyelitis was the threat of the atomic bomb. Something needed to be done, and researchers were hard at work to find a solution.
The Hunt for a Vaccine
Efforts to develop a polio vaccine began in earnest in the 1930s with researchers including Maurice Brodie in New York and John Kolmer in Philadelphia; however, they struggled to make much progress. At the time, scientists lacked the tools to grow polio virus in the laboratory, an important obstacle to learning about the virus or producing vaccines. As a result, research into polio vaccine development stalled until 1948, when a team of researchers at Boston Children’s Hospital was able to successfully grow the virus in human tissue in the lab. This opened the door for researchers around the world to more effectively study polio virus, earning the researchers — John Enders, Thomas Weller, and Frederick Robbins — the 1954 Nobel Prize in Physiology or Medicine.
The First Polio Vaccine: Hope and Horror
Thanks to the groundbreaking work of Enders and his colleagues, research into polio vaccines was reinvigorated, and by 1952, Jonas Salk and his team had developed the first polio vaccine. Salk’s vaccine was made by growing the virus in cells in the lab, then killing it with a chemical, so that the virus could not reproduce when given as a vaccine. Because of this inability to reproduce, the vaccine was dubbed the inactivated polio vaccine, or IPV for short. The clinical trial for IPV was massive. The trial, which was sponsored by what is now known as the March of Dimes, included more than 1.5 million children throughout the U.S. Called the Francis Field Trial, it was an enormous success, proving the vaccine to be extremely effective in protecting children against paralytic polio. The results were announced to great fanfare and celebration in 1955, and the vaccine was quickly licensed for distribution.
Unfortunately, the introduction of IPV wasn’t without incident. In April of 1955, horror struck when some vaccinated children started becoming paralyzed. It was quickly realized that some doses of vaccine contained live polio virus. Tens of thousands of vaccine doses had been released. More than 40,000 children developed polio. Almost 200 of them were paralyzed and 10 died. This immense tragedy was known as the Cutter Incident — named after the laboratory that produced the implicated doses of vaccine. The Cutter Incident led to the modern system of vaccine regulation in the U.S.
While IPV protected against paralytic polio, it did not protect against infection with the virus. Immunity established by IPV stopped the virus from growing in the central nervous system, where it caused paralysis, but it did not stop growth of the virus in the intestines. This meant that vaccine recipients could be infected, often without symptoms, and because the virus was in their feces, it could spread to others, such as after diaper changes or in contaminated food or water. This inability to stop the virus from spreading made another approach to polio vaccination attractive. Enter Albert Sabin.
The Next Polio Vaccine: Hope and Horror… again
Albert Sabin had been developing a polio vaccine in parallel with Salk’s efforts, but Sabin had taken a different approach. Sabin weakened, but did not inactivate, polio virus. In this manner, the vaccine virus would reproduce when it was administered. The vaccine was given either as liquid drops into the mouth or consumed on a sugar cube. Because of this delivery method, Sabin’s vaccine became known as the oral polio vaccine, or OPV.
Besides being easier to administer, OPV offered another important advantage over IPV— it was better at stopping outbreaks. Viral replication in the intestine after receipt of OPV afforded two benefits. First, although the vaccine recipients had virus in their feces, the virus was the weakened form, so others exposed to it sometimes developed immunity to polio. This is known as contact immunity. Second, when vaccinated individuals were exposed to “natural” or “wild-type” polio circulating in the community, they would not be infected because protection was present in their intestine. As such, they did not spread the more harmful, paralysis-causing virus circulating in the community. This proved immensely important for stopping outbreaks of poliovirus — something IPV had been unable to accomplish.
Unfortunately, OPV could, on occasion, cause its own type of horror, which only became clear after millions of people had received the vaccine. Specifically, about one of every 2.4 million doses cause what is known as vaccine-associated paralytic polio, or VAPP. VAPP occurs when the live, weakened virus from the vaccine reverts to its original, wild-type form, causing a paralyzing infection in the vaccine recipient or in someone who comes into contact with the reverted virus.
From Fear to a Polio-free Future
The U.S. used OPV from the early 1960s through the mid-1990s. However, between 1997 and 2000, the country transitioned to using only IPV because by that time the only cases of polio occurring in the U.S. were those caused by reversion of the virus in OPV. Each of the polio vaccines — Jonas Salk’s IPV and Albert Sabin’s OPV — played a role in our history of combatting the scourge that is polio.
Polio earned the fear it instilled by virtue of its horrible cost. Despite our advances and victories in this fight, wild-type polio virus still exists. Two countries, Afghanistan and Pakistan, have yet to interrupt transmission of the wild-type virus, and because of widespread travel, polio outbreaks continue to occur.
As we struggle to maintain trust and overcome vaccine hesitancy, the question now becomes which way will things go — toward hope or horror? Can we be hopeful in our collective ability to close the book on polio in our lifetime or will we witness horror as this disease rises again?
Related resources
Polio: The Disease and the Vaccine (webpage, VEC at CHOP)
Polio Vaccines: Oral versus Inactivated (webpage, Polio Network)