Polio is a crippling, highly infectious and potentially fatal disease. The virus invades the nervous system and can cause irreversible paralysis within a few hours. Polio can strike at any age, but it mainly affects children under five years old. There is no cure, but there are safe and effective vaccines. The strategy to eradicate polio is based on preventing infection by immunizing every child until transmission stops and the world is polio-free. Afghanistan, Nigeria and Pakistan remain endemic for polio. Until poliovirus transmission is interrupted in these countries, all countries remain at risk of importation of polio, especially vulnerable countries with weak public health and immunization systems and travel or trade links to endemic countries. Vulnerable countries include Cameroon, Equatorial Guinea, Ethiopia, Iraq, Somalia and Syrian Arab Republic. Circulating vaccine-derived poliovirus has caused an outbreak in Madagascar and Ukraine.
Polio is spread through person-to-person contact. When a child is infected with wild poliovirus, the virus enters the body through the mouth and multiplies in the intestine. It is then shed into the environment through the faeces where it can spread rapidly through a community, especially in situations of poor hygiene and sanitation. If a sufficient number of children are fully immunized against polio, the virus is unable to find susceptible children to infect, and dies out.
Most people (90 percent) infected with the poliovirus have no signs of illness and can spread the infection to thousands of others before the first case of polio paralysis emerges.
WHO considers a single confirmed case of polio paralysis to be evidence of an epidemic – particularly in countries where very few cases occur. One in 200 infections leads to irreversible paralysis, usually in the legs.
Polio can be prevented through immunization. Polio vaccine, given multiple times, almost always protects a child for life.The oral polio vaccine (OPV) or Sabin vaccine was developed in 1961 by Albert Sabin. It consists of a mixture of live, attenuated (weakened) poliovirus strains of all three poliovirus types. It is safe, effective, inexpensive, easy to administer, immunity after three doses is long-lasting and probably life-long and can result in the passive immunization of people who have not been directly vaccinated.
In extremely rare cases (approx. 1 in every 2.7 million first doses of the vaccine) the live attenuated vaccine virus in OPV can cause paralysis. In some cases it is believed that this vaccine-associated paralytic polio (VAPP) may be triggered by immune deficiency.
In most countries, OPV remains the vaccine of choice in routine immunization schedules and supplementary immunization activities.
Inactivated polio vaccine (IPV) or the Salk vaccine was developed in 1955 by Dr Jonas Salk. It consists of inactivated (killed) poliovirus strains of all three poliovirus types. IPV is given by intramuscular injection and needs to be administered by a trained health worker. IPV produces antibodies in the blood to all three types of poliovirus. In the event of infection, these antibodies prevent the spread of the virus to the central nervous system and protect against paralysis. IPV carries no risk of vaccine-associated polio paralysis and triggers an excellent protective immune response in most people.
As IPV does not stop transmission of the virus, oral polio vaccine is used wherever a polio outbreak needs to be contained. IPV is not recommended for routine use in polio-endemic countries or in developing countries at risk of poliovirus importations. In these countries, oral polio vaccines are used.Once polio has been eradicated, use of the oral polio vaccine will need to be stopped to prevent re-establishment of transmission due to vaccine-derived polioviruses. Switching to IPV is one option for this post-OPV era.
The Global Polio Eradication Initiative is a public-private partnership. It is spearheaded by the World Health Organization (WHO), Rotary International, the US Centers for Disease Control and Prevention (CDC), and the United Nations Children’s Fund (UNICEF) and is led by national governments. Its goal is to eradicate polio worldwide.