Update 9/27/15: WHO has just removed Nigeria from the list of polio-endemic countries, leaving just Pakistan and Afghanistan.
(This piece, written by me, was originally posted on This Week in Global Health www.twigh.org)
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 use 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 comes 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.
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 eliminate polio in 1979.
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. The global incidence of polio has decreased by 99% due to vaccination.
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. The Salk vaccine 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 spread to other people. The Sabin vaccine also called the oral polio vaccine (OPV), on the other hand, 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, the Sabin vaccine, because it’s a live vaccine, can revert to a virulent form and cause disease. This is what is being referred to when you hear of vaccine-derived polio infection. The Sabin vaccine is easier to administer because it doesn’t require syringes, it provides longer immunity than the Salk vaccine, but it requires strict transport conditions because it is live. The World Health Organization is advocating for countries to move towards the IPV vaccine and phase out use of the OPV vaccine, 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 in 3 countries in the world, 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, marking one year since the last case of endemic polio was recorded on the entire African continent. Only two more years to go before Africa 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 have achieved elimination but are experiencing political instability, which causes a decrease in vaccination rates due to health infrastructure challenges, are having outbreaks of polio. 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. Maintaining a high vaccination rate in every country is important as long as polio is circulating anywhere in the world.
Achieving eradication through eliminating polio in the last few countries will not be easy. The areas with endemic polio transmission are experiencing conflict and political instability and have 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. Polio eradication by 2018, which is the goal timeframe, is achievable; however continued focus and resources are required to interrupt transmission in the hardest to reach areas. Eradicating polio will save many lives and prevent many children from paralysis; a goal which is worthy of our focus and can be achieved in our lifetime.