~Written by Theresa Majeski (Contact: theresa.majeski@gmail.com, twitter: @theresamajeski)
Polio may become the second viral human disease (following smallpox) ever eradicated from this planet. Like smallpox, polio can be prevented with vaccination and, perhaps most importantly, polio relies solely on person-to-person transmission for survival. This means that polio does not require any vectors like mosquitos or snails for its life cycle, so humans are the only ones infected by poliovirus. Polio is usually spread through a fecal-oral route, meaning that the virus is in the stool of an infected person and could come in contact with the mouth of an uninfected person through contaminated foods, hands, utensils, etc. If we can interrupt transmission through vaccination, poliovirus will be unable to find someone unimmunized to infect and will be eradicated.
Polio has a long history within the human population. Only sixty years ago, polio was a feared disease in the United States. Summer time brought with it polio season and public facilities such as swimming pools were shut down. This reaction was not unwarranted. In 1952 almost 600,000 children were infected with the virus. More than 3,000 died and thousands were paralyzed. Iron lungs were used to keep children alive as the paralysis left them unable to breathe on their own.
Photo of children in iron lungs. Photo courtesy of the National Museum of Health and Medicine.
President Franklin D. Roosevelt contracted polio as an adult, years before taking office. He made fighting polio a national priority and established the March of Dimes to encourage everyday citizens to fund polio research. Jonas Salk created the first polio vaccine, approved in 1955; Alfred Sabin created a second polio vaccine, approved in 1963. Through the use of these two vaccines, the United States was able to eradicate polio by 1979.
Despite this, it wasn’t until the 1970’s that polio was recognized as a serious problem in developing countries. Once polio was identified as being prevalent in developing countries, routine immunization campaigns were implemented worldwide which helped bring polio under control in many countries. In 1988, when the Global Polio Eradication Initiative began, more than 1000 children worldwide were paralyzed by polio every day. Since then, the global incidence of polio has decreased by 99 percent.
Countries that have achieved elimination have not done so without challenges. The two polio vaccines have benefits and drawbacks. The Salk vaccine, also called the inactivated polio vaccine (IPV), contains chemically inactivated polio virus. It stimulates a strong systemic immune response but because it is an injection, it does not cause a strong mucosal immunity. Without a strong mucosal immune response poliovirus can replicate in the intestines of immunized people without causing symptoms and can then be contagious. On the other hand, the Sabin vaccine, also called the oral polio vaccine (OPV), is a live-attenuated vaccine. This means that the vaccine contains live, weakened poliovirus. The Sabin vaccine is given orally so it stimulates mucosal immunity and systemic immunity. However, because it is a live vaccine, it can revert to a virulent form and cause vaccine-derived polio infection. The Sabin vaccine is easier to administer because it doesn’t require syringes, and it provides longer immunity than the Salk vaccine, however, it requires strict transport conditions because it is live. The World Health Organization is advocating for countries to move towards the IPV and phase out use of the OPV, to help prevent vaccine-derived polio cases. The Global Alliance for Vaccines (GAVI) recently announced that they will be helping Pakistan introduce the IPV as part of the Polio Eradication and Endgame Strategic Plan 2013-2018.
Currently, polio is endemic in only three countries: Nigeria, Pakistan and Afghanistan. In order for a country to be declared free of polio, three years must pass without a case of endemic (wild type) polio. Nigeria achieved one year without an endemic polio case on 24 July 2015. This means that only two more years remain before the African continent may be declared polio-free. India was removed from the list of polio-endemic countries in 2012, and in 2014 India achieved polio-free status. India, long considered the country facing the greatest challenges to eradication, demonstrates that global eradication is possible. However, some countries that had achieved elimination are experiencing outbreaks of polio, partly due to political instability, which has impacted vaccination rates. For example, Syria was polio free from 1999 to October 2013 when imported cases of polio closely related to strains circulating in Pakistan were confirmed in Deir ex-Zor and Aleppo. This demonstrates the importance of maintaining a high vaccination rate in every country until the disease is eradicated.
Current polio distribution around the world. Graphic courtesy of Global Polio Eradication Initiative.
Achieving eradication through eliminating polio in the last few countries will not be easy. The areas with endemic polio transmission face several challenges including conflict and political instability, hard-to-reach populations, and poor infrastructure. Furthermore, community workers trying to administer polio vaccine are being attacked by groups who oppose polio vaccination. The CIA providing vaccinations as a cover for searching for Osama bin Laden certainly eroded trust between health workers administering vaccines and community members, making poliovirus vaccination campaigns that much harder. However, focusing on strengthening all routine immunization delivery, helping locals take ownership of polio eradication in their communities, working directly with community members and leaders, and building trust by keeping a lower profile on international deadlines may help overcome the remaining challenges.
Humanity is on the cusp of another great infectious disease achievement, eradicating polio by 2018. Polio eradication is achievable; however continued focus and resources are required to interrupt transmission in the hardest to reach places. Eradicating polio will save many lives and prevent countless children from paralysis; a goal that can be achieved in our lifetime.