There are three wild types of polioviruses (WPV) – type 1, type 2, and type 3.

People need to be protected against all three types of the virus in order to prevent polio disease and the polio vaccination is the best protection. 

Type 2 wild poliovirus was declared eradicated in September 2015, with the last virus detected in India in 1999.

Type 3 wild poliovirus was declared eradicated in October 2019. It was last detected in November 2012.

Only type 1 wild poliovirus remains.

There are two vaccines used to protect against polio disease, oral polio vaccine and inactivated poliovirus vaccine.

Oral poliovirus vaccines (OPV) are the predominant vaccine used in the fight to eradicate polio.  There are different types of oral poliovirus vaccine, which may contain one, a combination of two, or all three different serotypes of attenuated vaccine.

Each has their own advantages and disadvantages over the others. 

The attenuated poliovirus(es) contained in OPV are able to replicate effectively in the intestine, but around 10,000 times less able to enter the central nervous system than the wild virus. This enables individuals to mount an immune response against the virus.

Virtually all countries which have eradicated polio used OPV to interrupt person to person transmission of the virus.

Advantages

OPVs are all inexpensive.

OPVs are safe and effective and offer long lasting protection against the serotype(s) which they target.

OPVs are administered orally and do not require health professionals or sterile needle syringes. As such, OPVs are easy to administer in mass vaccination campaigns.

For several weeks after vaccination the vaccine virus replicates in the intestine, is excreted and can be spread to others in close contact. This means that in areas with poor hygiene and sanitation, immunization with OPV can result in ‘passive’ immunization of people who have not been vaccinated.

Disadvantages

OPV is extremely safe and effective. However, in extremely rare cases (at a rate of approximately 2 to 4 events per 1 million births the live attenuated vaccine-virus in OPV can cause paralysis.

In some cases, it is believed that this may be triggered by an immunodeficiency.

Very rarely, when there is insufficient coverage in a community the vaccine-virus may be able to circulate, mutate and, over the course of 12 to 18 months, reacquire neurovirulence. This is known as a circulating vaccine-derived poliovirus.

Monovalent oral poliovirus vaccine (mOPV)

Monovalent oral polio vaccines confer immunity to just one of the three serotypes of OPV. They are more successful in conferring immunity to the serotype targeted than tOPV, but do not provide protection to the other two types.

Novel oral polio vaccine type 2 (nOPV2)

To better address the evolving risk of type 2 circulating vaccine-derived poliovirus (cVDPV2).

Bivalent oral poliovirus vaccine (bOPV)

Following April 2016, the trivalent oral poliovirus vaccine was replaced with the bivalent oral poliovirus vaccine (bOPV) in routine immunization around the world.

Bivalent OPV contains only attenuated virus of serotypes 1 and 3, in the same number as in the trivalent vaccine.

Bivalent OPV elicits a better immune response against poliovirus types 1 and 3 than trivalent OPV, but does not give immunity against serotype 2.

Trivalent oral poliovirus vaccine (tOPV)

Prior to April 2016, the trivalent oral poliovirus vaccine (tOPV) was the predominant vaccine used for routine immunization against poliovirus.

Developed in the 1950s by Albert Sabin, tOPV consists of a mixture of live, attenuated polioviruses of all three serotypes.

Also called the ‘Sabin vaccine’, tOPV is inexpensive and effective, and offers long lasting protection to all three serotypes of poliovirus.

The trivalent vaccine was withdrawn in April 2016 and replaced with the bivalent oral poliovirus vaccine (bOPV), which contains only attenuated virus of types 1 and 3.

This is because continued use of tOPV threatened to continue seeding new type 2 circulating vaccine-derived polioviruses (cVDPV2), despite the wild type 2 virus being eradicated in 1999.

Wild poliovirus (WPV) is the most commonly known form of the poliovirus. However, there is another form of polio that can spread within communities: circulating vaccine-derived poliovirus, or cVDPV. While cVDPVs are rare, they have been increasing in recent years due to low immunization rates within communities.

In communities with low immunization rates, as the virus is spread from one unvaccinated child to another over a long period of time (often over the course of about 12-18 months), it can mutate and take on a form that can cause paralysis just like the wild poliovirus. This mutated poliovirus can then spread in communities, leading to cVDPVs.

The cause of cVDPV is low immunization rates. So, the best way to prevent them and stop them when there is an outbreak is to vaccinate children.

The polio vaccine protects children whether the kind of polio is wild poliovirus or vaccine-derived poliovirus. Outbreaks (whether WPV or cVDPV) are usually rapidly stopped with 2–3 rounds of high-quality supplementary immunization activities (immunization campaigns).

The probability of cVDPV3 outbreak is low in India, but on account of our population size of 1,400 million, its impact is likely to be enormous.

India must withdraw type 3 and continue monovalent type 1 OPV, which also must be withdrawn after reaching 85-90% coverage with IPV, three doses per child.