Polio is caused by a human enterovirus called the poliovirus. Wild polioviruses are those that occur naturally.
The poliovirus consists of an RNA genome enclosed in a protein shell called a capsid. There are three serotypes of wild poliovirus – type 1, type 2, and type 3 – each with a slightly different capsid protein.
Type 2 poliovirus has been eliminated in the wild – the last wild type 2 poliovirus was detected in India in 1999.
In this final stage of polio eradication, only type 1 and type 3 wild poliovirus continue to circulate in endemic areas. Both are highly infectious and both cause paralytic polio. Type 1 is the most pervasive strain of poliovirus and type 3 is at very low levels.
The development of effective vaccines to prevent paralytic polio was one of the major medical breakthroughs of the 20th century. With the development and evaluation of bivalent oral polio vaccine in 2009, the Global Polio Eradication Initiative now has an armoury of five different vaccines to stop polio transmission:
- Oral polio vaccine (OPV)
- Monovalent oral polio vaccines (mOPV1 and mOPV3)
- Bivalent oral polio vaccine (bOPV)
- Inactivated polio vaccine (IPV)
If enough people in a community are immunized, the virus will be deprived of susceptible hosts and will die out. High levels of vaccination coverage must be maintained to stop transmission and prevent outbreaks occurring. The Global Polio Eradication Initiative is constantly assessing the optimal use of the different vaccines to prevent paralytic polio and stop poliovirus transmission in different areas of the world.
Oral polio vaccine (OPV)
The oral polio vaccine (OPV) was developed in 1961 by Albert Sabin. Also called “trivalent oral polio vaccine” or “Sabin vaccine”, OPV consists of a mixture of live, attenuated (weakened) poliovirus strains of all three poliovirus types.
OPV produces antibodies in the blood to all three types of poliovirus. In the event of infection, these antibodies protect against paralysis by preventing the spread of wild poliovirus to the nervous system.
OPV also produces a local, mucosal immune response in the mucous membrane of the intestines. In the event of infection, these mucosal antibodies limit the replication of the wild poliovirus inside the intestine. This intestinal immune response to OPV is thought to be the main reason why mass campaigns with OPV can rapidly stop person-to-person transmission of wild poliovirus.
OPV is administered orally. It can be given by volunteers and does not require trained health workers or sterile injection equipment.
The vaccine is relatively inexpensive. In 2011, the cost of a single dose for public health programmes in developing countries was between 11 and 14 US cents.
OPV is safe, effective, and induces long-lasting immunity to all three types of poliovirus.
For several weeks after vaccination, the vaccine virus replicates in the intestine, is excreted in the faeces, and can be spread to others in close contact. This means that in areas where hygiene and sanitation are poor, immunization with OPV can result in the “passive” immunization of people who have not been directly vaccinated.
Although OPV is safe and effective, 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.
The extremely low risk of VAPP is well known and accepted by most public health programmes in the world because without OPV, hundreds of thousands of children would be crippled every year.
A second disadvantage is that very rarely the virus in the vaccine may genetically change and start to circulate among a population. These viruses are known as circulating vaccine-derived polioviruses (cVDPV).
In most countries, OPV remains the vaccine of choice in routine immunization schedules and supplementary immunization activities.
Where more than one type of wild poliovirus is circulating, OPV is epidemiologically and operationally the best vaccine to use because protection develops to each of the three types of polio virus.
Monovalent oral polio vaccines (mOPV)
Monovalent oral polio vaccines (mOPV) consist of live, attenuated (weakened) poliovirus strains of either type 1 (mOPV1) or type 3 (mOPV3) poliovirus only. Unlike OPV, it does not contain the other two types of poliovirus. The vaccine gives protection against one type of poliovirus only (either type 1 or type 3 depending on the vaccine).
Monovalent oral polio vaccines offer the same advantages as OPV. In addition:
In children being immunized for the first time, mOPV1 (or mOPV3) provides a much stronger immunity to type 1 (or type 3) poliovirus compared with OPV.
For the same number of doses, mOPV1 (or mOPV3) provides increased immunity to type 1 (or type 3) poliovirus compared to OPV. This is because there is no competition from the other two virus types in the vaccine.
If children immunized with mOPV1 (or mOPV3) are subsequently exposed to wild poliovirus type 1 (or type 3), they will excrete less virus and for a shorter period of time, limiting the possibility of further transmission.
Monovalent oral polio vaccines are recommended for use in supplementary immunization campaigns in areas where only wild poliovirus type 1 or type 3 alone is circulating. It is not recommended as a substitute for OPV in routine immunization programmes.
Two doses of mOPV administered within two weeks forms the basis of the new Short Interval Additional Dose (SIAD) approach, which is intended to rapidly boost population immunity in Asia.
Bivalent oral polio vaccine (bOPV)
Bivalent oral polio vaccine (bOPV) consists of live, attenuated (weakened) poliovirus strains of type 1 and type 3. It simultaneously targets the two remaining types of wild poliovirus (type 1 and type 3) and was developed to improve the efficiency and impact of vaccination campaigns in areas where both types of poliovirus co-circulate.
Bivalent oral polio vaccine was first used in Afghanistan in December 2009, when 2.8 million children under five years old received the vaccine.
Bivalent oral polio vaccine offers the same advantages as OPV. In addition:
• For both types 1 and 3 polio, bOPV is more effective than OPV and almost as good as the monovalent vaccines, yet in a package that delivers both at once.
• bOPV allows countries to simplify vaccine logistics and optimize protection.
• In areas where access to children is limited, using bOPV helps maximise the impact of each contact with a child.
It is recommended that bOPV is introduced as rapidly as possible into supplementary immunization activities (SIAs) of all endemic countries and in some outbreak settings. The vaccine is recommended for use in SIAs to complement the ongoing large-scale use of trivalent OPV for routine immunization and SIAs, and the use of monovalent OPVs for SIAs.
Inactivated polio vaccine (IPV)
Inactivated polio vaccine (IPV) was developed in 1955 by Dr Jonas Salk. Also called the “Salk vaccine”, IPV 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.
The inactivated polio vaccine 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.
- As IPV is not a ‘live’ vaccine, it carries no risk of vaccine-associated polio paralysis.
- IPV triggers an excellent protective immune response in most people.
- IPV induces very low levels of immunity in the intestine. As a result, when a person immunized with IPV is infected with wild poliovirus, the virus can still multiply inside the intestines and be shed in the faeces, risking continued circulation.
- IPV is over five times more expensive than oral polio vaccine.
Administering the vaccine requires trained health workers and sterile injection equipment and procedures.
An increasing number of industrialized, polio-free countries are using IPV as the vaccine of choice. This is because the risk of paralytic polio associated with continued routine use of oral polio vaccine (OPV) is deemed greater than the risk of imported wild virus.
However, as IPV does not stop transmission of the virus, oral polio vaccine is used wherever a polio outbreak needs to be contained, even in countries which rely exclusively on IPV for their routine immunization programme (e.g. the polio outbreak in the Netherlands in 1992).
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 – either trivalent, bivalent or monovalent, depending on local epidemiology – 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.