Poliomyelitis (polio) and the vaccines used to eradicate it – questions and answers

What is polio?
Is it still necessary to vaccinate against polio?
What vaccines are available against polio?
What is IPV?
What is OPV?
Why use OPV?
Why use IPV?
Why is OPV being phased out?
What is the OPV 'switch'?
Which countries in the WHO European Region will discontinue use of trivalent OPV in April 2016?
What happens after polio is eradicated?
Could polio re-emerge?
What is polio containment?

What is polio?

Poliomyelitis (polio) is a highly infectious disease caused by a virus. It invades the nervous system and can cause total paralysis in a matter of hours. The virus is transmitted from person to person mainly through the faecal–oral route. The initial symptoms of polio are fever, fatigue, headache, vomiting, stiffness of the neck and pain in the limbs. One in 200 infections leads to irreversible paralysis (usually in the legs). Of those who are paralysed, 5–10% die when their breathing muscles become immobilized. Polio mainly affects children under 5 years of age. There is no cure for polio, it can only be prevented. Polio vaccine, given multiple times, can protect a child for life.

Is it still necessary to vaccinate against polio?

The European Region has been polio free since 2002. But no country is safe from polio until the virus has been eradicated globally. One of the three known types of wild poliovirus (type 1) is still endemic in parts of Pakistan and Afghanistan, and circulating vaccine-derived polioviruses were detected in several countries in 2015, including Ukraine. It only takes one traveller from an infected area to reintroduce the virus into a polio-free country. High levels of vaccination coverage must be maintained to stop transmission in the event of importation and to prevent outbreaks from occurring.

What vaccines are available against polio?

There are two different versions of vaccine available to protect against polio – inactivated polio vaccine (IPV) and oral polio vaccine (OPV).

What is IPV?

Inactivated polio vaccine (IPV) consists of inactivated (killed) strains of all three poliovirus types. IPV is given by intramuscular injection and must 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. In this way, IPV prevents infection, but it does not stop transmission of the virus.

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What is OPV?

The oral polio vaccine (OPV) consists of live, attenuated (weakened) poliovirus strains of one to three poliovirus types. OPV is administered orally. It can be given by volunteers and does not require trained health workers or sterile injection equipment. There are three different types of this vaccine: 

  • trivalent OPV protects against poliovirus types 1, 2 and 3; 
  • bivalent OPV protects against poliovirus types 1 and 3; and 
  • monovalent OPVs protect against either poliovirus type 1 or type 3.

Why use OPV?

The oral polio vaccine is simple to administer. It can be given by volunteers and does not require trained health workers or sterile injection equipment. The vaccine is relatively inexpensive. For several weeks after vaccination, the vaccine virus replicates in the intestine, is excreted in the faeces and can be spread to people in close contact. This means that, in areas where hygiene and sanitation are poor, vaccination with OPV can result in "passive" immunization of people who have not been directly vaccinated. After three doses of OPV, a person becomes immune for life and can no longer transmit the virus to others if exposed again. Thanks to this "gut immunity", OPV is the only effective weapon to stop transmission of the poliovirus when an outbreak is detected. 

Why use IPV?

An increasing number of polio-free countries use IPV as the vaccine of choice. This is because the risk of emergence of a circulating vaccine-derived poliovirus (cVDPV) with continued routine use of OPV (see below) is deemed greater than the risk of a wild virus importation. However, as IPV prevents infection but does not stop transmission of the virus, until OPV is completely phased out, it will continue to be used wherever a polio outbreak has to be contained, even in countries that use exclusively IPV in their routine immunization programme. Three doses of IPV confer life-long protection against the disease. National immunization programmes will continue to administer IPV for some years after global polio eradication has been declared. 

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Why is OPV being phased out?

The Polio Eradication & Endgame Strategic Plan 2013-2018 (the Endgame Strategy) set the goal of a polio-free world by 2018. Achieving this goal requires a number of steps, including cessation of the use of OPV. Although OPV is safe and effective, in extremely rare cases (about 1 in every 2.7 million first doses of the vaccine), the live attenuated vaccine virus in OPV can cause paralysis. It is believed that in some cases vaccine-associated paralytic polio (VAPP) can 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 of OPV is that, very rarely, the virus in the vaccine changes genetically and starts to circulate in a population. Such viruses are known as circulating vaccine-derived polioviruses (cVDPV). To remove any risk of polio due to vaccine-derived polioviruses, OPV will be phased out globally from April 2016. 

What is the OPV 'switch'?

As wild type 2 polio has not been detected anywhere in the world since 1999 and was declared eradicated in 2015, phasing out of OPV will start with a globally synchronized switch from trivalent OPV (containing types 1, 2 and 3) to bivalent OPV (containing types 1 and 3) in April 2016. In preparation for the switch, OPV-using countries will introduce at least one dose of IPV (containing inactivated strains of all three poliovirus types) into their routine immunization schedules, if they have not already done so. Each country that uses OPV alone or in combination with IPV will choose a single day between 17 April and 1 May on which they will switch from trivalent to bivalent OPV. Immediately after the switch, all remaining stocks of trivalent OPV will be destroyed in a planned fashion. With careful planning and thorough oversight, the switch will represent a huge achievement for the polio programme and will provide a basis for the eventual withdrawal of all OPV, following eradication of poliovirus types 1 and 3.

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Which countries in the WHO European Region will discontinue use of trivalent OPV in April 2016? 

The countries in the WHO European Region that will switch to bivalent OPV are Albania, Armenia, Azerbaijan, Bosnia and Herzegovina, Georgia, Kazakhstan, Kyrgyzstan, Montenegro, the Republic of Moldova, the Russian Federation, Serbia, Tajikistan, the former Yugoslav Republic of Macedonia, Turkey, Turkmenistan, Ukraine and Uzbekistan. Belarus and Poland will move to an IPV-only schedule.

What happens after polio is eradicated? 

Once polio is eradicated, the world can celebrate a major public health achievement that will benefit all people equally, no matter where they live. Most importantly, success will mean that no child will ever again suffer the terrible effects of lifelong polio paralysis.

Could polio re-emerge?

Once wild polioviruses have been eradicated, the only risks of the virus returning would be due to rare strains of circulating vaccine-derived polioviruses or a leak of the virus from a laboratory or vaccine manufacturer. To minimize these risks, OPV is being phased out globally, starting in April 2016, and the process of polio containment is underway. 

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What is polio containment? 

To prevent reintroduction of the virus after the phased withdrawal of oral and eventually inactivated polio vaccine, the number of certified poliovirus facilities will be reduced to the minimum necessary to perform the critical functions of vaccine production, diagnosis and research. Global efforts are therefore underway to identify, destroy or safely contain all potentially infectious poliovirus samples in all laboratories and manufacturing sites worldwide. The timelines and requirements for this process are described in the WHO Global Action Plan to minimize poliovirus facility-associated risk after type-specific eradication of wild polioviruses and sequential cessation of oral polio vaccine use.