The polio vaccine has and continues to be extremely important to prevent poliomyelitis polio infections worldwide. The first successful polio vaccine was developed in 1952 and since then, polio vaccines have been used to prevent millions of cases of polio. While 95% of polio infections are asymptomatic, an estimated 1 in 200 polio infections results in paralysis (usually involving a leg). It is possible for polio to result in death as well with and estimated death rate of 2-5% in children with paralysis and 15-30% in adults with paralysis. [i] Through the use of polio vaccines polio infections and its side effects have been greatly reduced. The early 1900's saw more frequent and more severe polio outbreaks in the United States. In 1952 there were close to 60,000 polio cases with thousands paralyzed and 3,000 deaths. After the widespread use of the polio vaccine in 1955 polio cases dramatically decreased with the final result being the last polio infection in the United States was in 1979. [ii] Additionally, the polio vaccine has been used to greatly reduce the incidence of polio infections worldwide. In 1988 there were an estimated 350,000 polio cases [iii] and in 2012 there were only 223 reported polio cases in the entire world. [iv] Not only has the polio vaccine saved many lives and prevented many people from living with a life long disability, but if it can also be used to eradicate polio worldwide in the next 5 years, it has been estimated to have a benefit of $47 Billion. [v] If polio vaccination campaigns stop and polio is no longer controlled it is estimated that in less than 10 years there could be 200,000 new cases of polio infections worldwide every year. [vi] As we can see, the polio vaccine has played a very significant role in our society by virtually removing a debilitating and deadly disease from the face of the early in less than 60 years. However, the polio vaccine continues to be relevant until polio can be completely eradicated from the earth, at which point, no more humans will have to die or be disabled from polio ever again.
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In 1955 the first polio vaccine was approved for widespread use. Dr. Jonas Salk developed this vaccine. In developing the vaccine Dr. Salk and his team first investigated the 125 strains of poliovirus that existed. From these 125 strains, Dr. Salk identified three different distinct groups (Type I, Type II and Type III), which all strains fell into. If a vaccine were to be effective it would have to prevent the infection from all three types of poliovirus. An additional hurdle in vaccine development was the need to produce large quantities of the virus for the vaccine. This was required because the polio vaccine Dr. Salk was developing required live poliovirus (as do most vaccines), which was then killed using formaldehyde. The killed virus would still need to invoke the bodies' autoimmune response to produce antibodies to the virus to be successful. This vaccine would later become to be known as the Inactivated Poliovirus Vaccine or IPV. In subsequent years other vaccines would be developed which helped the fight against polio, but in this paper we will concentrate on the IPV. The polio vaccine development was assisted immensely by a group of researcher's who eventually went on to win the Nobel Prize in physiology/medicine for their work. The contribution that this team made was the discovery that you could grow viruses on tissue, not only in intact organisms. This inability to grow large quantities of poliovirus had hampered previous teams working on a polio vaccine. With the important polio virus strains identified and the ability to make large quantities of the polio vaccine, in 1954 huge clinical trial involving over 1.6 million school children was conducted. This unprecedented clinical trial was a success, which showed the vaccine to be effective in preventing infection by 80-90%. [vii] After these successful clinical trials the polio vaccine was licensed for widespread use in 1955 and through massive inoculation campaigns the rate of polio infections precipitously dropped in the United States. There were 38,476 polio infections in 1954 and 10 years later in 1964 there were 122 cases (a reduction of 99.7%). [viii]
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The production process for the inactivated polio vaccine has remained roughly the same with refinements through the years. The figure below shows a modern production stream for poliovirus: [ix]
The process described in the figure above gives us the general framework for the production of the polio vaccine. The first stage is the propagation of cells. Cells are needed in order to serve as hosts for the poliovirus, which will be introduced at the end of this stage. When the virus is introduced it uses the cells to multiply itself. When the virus has multiplied to a satisfactory concentration the next stage begins which is purification. The goal of purification is to separate the virus from the host cells. From the figure above you can see that a number of purification processes are used: clarification (i.e. centrifuge), concentration (i.e. ultrafiltration), SEC (size exclusion chromatography) and IEX (ion exchange chromatography). The last stage is inactivation. In this step the poliovirus is killed by the exposure to formaldehyde. This entire process would have to be repeated for each of the three types of polio virus and then the three products from each process would be mixed together in order to produce a vaccine that is protective against Type I, Type II and Type III of polio virus.
While the modern process above uses VERO cells that come from a cell line of monkey kidney cells, which can be propagated indefinitely, the original production process largely depended on the supply of monkey kidney cells from monkeys. For example, the Dutch government produced the entire supply of the Salk Polio Vaccine at a government research laboratory that in the 1960's required 5000 monkeys annually for polio vaccine production. When this laboratory started using "microcarriers" (also used in the figure above) the yield increased a thousand times. Due to the yield increase, the demand for monkeys actually was only 7 in 1978, which was a sustainable breeding population for the Netherlands. [x] Now with the advent of cell lines no live monkeys are needed. Another change from early vaccine production techniques is that as we saw above modern processes have multiple purification steps, the first vaccine preparations were not purified. [xi]
As stated before, we are very close to having polio go the way of small pox and be completely eradicated worldwide. Polio is only endemic to three countries in the world, Afghanistan, Pakistan and Nigeria. While there are cases in other countries, this is because polio has been reintroduced somehow to the country, but in Afghanistan, Pakistan and Nigeria polio has never been eradicated. There have been many reasons why eradication in these countries has been achieved. Most recently, in December of 2012, during a three-day immunization campaign militants in Pakistan killed eight female health care workers involved with the immunization campaign. These deaths caused the organizers of the immunization campaign to suspend operations. [xii] There is mistrust in Pakistan around the polio immunization campaign. Some think it is a western conspiracy to sterilize Muslim women or infect people with HIV while others are more suspicious after the CIA used a fake hepatitis immunization campaign in their effort to scout the compound of Osama Bin Laden. This fake campaign was trying to get a DNA sample from Osama bin Laden, even though they were unsuccessful doing this has led to increased mistrust of immunization campaigns for polio. [xiii] Violence, suspicion and mistrust are hampering efforts to stamp out polio in this difficult place in the world. In Nigeria another polio endemic country, polio immunization campaigns were stopped in October 2003 in Northern Nigeria due to fears that the immunizations campaigns were a conspiracy of the Western countries to reduce the populations of Muslims by using the immunizations to sterilize and infect the population with HIV. [xiv] The immunization campaign was restarted about a year later after much international diplomacy and face saving measures, however, polio cases rose from about 300 when the campaign stopped to over 900 in 2006 when the number of cases started dropping again. [xv] As we can see these disruptions and distractions from immunization have real effects and push back the date for worldwide eradication.
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The ultimate goal of the polio vaccine from the first time it was developed to now is to use it to eradicate polio worldwide. If this goal is achieved no longer will anyone have to die or live with a life long disability caused by polio. The polio vaccine has saved countless lives so far and improved the quality of life of millions. However the job is not done until worldwide eradication is achieved.
We have seen that until the trust of the entire world can be garnered in the effort to eradicate polio it is going to be difficult to stamp out the few remaining hot spots where polio still exists.
The story of the polio vaccine is one of great achievements and scientific advancement. From polio outbreaks, which struck fear into the whole population in the 1910's to today where, we are on the brink of eradicating polio worldwide. Throughout the story of polio scientific advances have greatly assisted in making life better for millions of people and saving many lives. The use of newly developed virus culture techniques and unprecedented large clinical trials greatly sped the development and deployment of the polio vaccine. Science continues to advance the formation and production of the polio vaccine to assist in immunization campaigns. Hopefully soon, we can say that polio has been eradicated worldwide.