Oral polio vaccine (OPV): OPV is made up of live attenuated sabin virus strain. The administration of OPV mimics the milder form of natural infection. The immune response is same as that of natural infection in terms of humoral and local immunity. The vaccine allows the replication of the vaccine virus in the intestine, but the progress is restricted due to development of neutralizing antibody. OPV thus permits the shedding of virus in the throat as well as in the intestine and in the stool. This was beneficial in the initial phase of vaccination to increase the herd immunity through feco-oral transmission of the vaccine virus in the community, particularly in developing country. However, due to virus shedding in the stool, OPV is associated with VDPV which needs to be stopped at this juncture of polio eradication.

OPV Formulations

Trivalent OPV (tOPV) is the commonest formulation available. It contains the attenuated strains of all three poliovirus serotypes. The proportion of three serotypes in vaccine is 10:1:6, respectively. The rate of seroconversion in children to the three poliovirus serotypes (1, 2, and 3) is 73, 90 and 70% respectively after the completion of the primary vaccination.

Bivalent OPV (bOPV): This is available in the combination of serotypes 1 and 3 as these two serotypes are circulating all over after the eradication of serotype 2 in 1999.

Monovalent OPV (mOPV): Monovalent form of OPV is available for serotype 1 (mOPV1) and serotype 3 (mOPV3).

In general, the rate of seroconversion against each serotype is higher with monovalent preparation followed by bivalent and trivalent preparation (mOPV > bOPV > tOPV).

Inactivated polio vaccine (IPV): IPV mainly induces humoral immunity. As the virus strains are inactivated, they do not replicate in the intestine and thus the induction of intestinal immunity is limited. However, IPV has been seen to reduce the duration and amount of virus shedding in stool.

The seroconversion rate is near 100% for each serotype. IPV is available as stand alone vaccine or in combination with other primary immunization vaccine.

It is not associated with VDPV.

Vaccine associated paralytic polio (VAPP): Paralytic polio occurs due to oral polio vaccine administration in two settings. First, when the vaccine strain undergoes mutation during replication in the intestine and there is revert back of the neurovirulence property. Second, when the vaccine (OPV) is administered to a B cell deficient child.

In the first scenario, it occurs due to the oral polio vaccine strain which genetically gets changed during replication in the intestine and revert back its neurovirulence. It occurs in (i) the child vaccinated with the first dose of OPV and (ii) susceptible close contacts of the recipient who is excreting the virus.

VAPP occurs in 1 in 2.7 million doses of OPV administration and does not spread to cause outbreaks.

Vaccine derived poliovirus (VDPV): Vaccine derived poliovirus is a genetically different strain that is originated from the oral polio vaccine strain.

VDPV can be of three types: Circulating VDPV (cVDPV), immunodeficiency related VDPV (iVDPV) and ambiguous VDPV (aVDPV).

Circulating VDPV (cVDPV): Circulating VDPV strain emerges in poorly immunized population. A fully immunized person is protected against both wild virus and VDPV. Due to poor coverage of immunization, population remains susceptible to virus, and the excreted VDPV strain continues to transmit from person-to-person. Longer is the circulation, higher is the chance of genetic changes. It takes a few months to circulate to emerge as cVDPV.

cVDPV can lead to outbreak and has the potential to become endemic when circulates for a long period. The strain can also get transmitted to other susceptible country or locality.

Outbreak due to cVDPV can be prevented by giving 2–3 rounds of supplementary immunization.

Prevalence of cVDPV: It has caused 24 outbreaks in last 10 years, with more than 750 cases of paralytic polio, affecting 21 countries.

To prevent the emergence and circulation of VDPV, OPV has to be stopped.

Immunodeficiency related VDPV (iVDPV): Immune deficient individuals are not able to mount immune response which leads to continuous replication of vaccine virus in the intestine. These individuals excrete immunodeficiency related VDPV. The excretion of iVDPV is usually stopped within six months.

Thirty-three cases of iVDPV have occurred so far.

Ambiguous VDPV (aVDPV): These strains are VDPV that are isolated from individuals with no known immunodeficiency.

Polio end game plan: The strategy plan recommendation of polio eradication and end game strategic plan 2013–2018 of WHO is as follows:

• Vaccination with “only OPV” should be stopped.

• OPV containing P2 should not be used any more.

• At least single dose of IPV must be introduced before six months of switch over from trivalent OPV (tOPV) to bivalent OPV (bOPV) by the end of 2015.

• In 2016, switch over from use of tOPV to bOPV should be done in countries using only OPV.

• With introduction of IPV, complete withdrawal of all OPV use by 2019–2020.

Polio surveillance: Polio surveillance is done by two methods:

• Acute flaccid paralysis (AFP) surveillance

• Environmental surveillance

AFP surveillance is the gold standard for detection of poliomyelitis cases. The purpose of AFP surveillance is to isolate, identify and characterize the poliovirus isolated from the AFP cases.

This includes (i) finding and reporting of AFP case, (ii) stool sample collection and transport to designated lab for testing, (iii) isolation and identification of poliovirus in the lab, and (iv) to determine the origin of the isolated virus.

Environmental surveillance is an additional measure to detect the presence of poliovirus in the sewage and other environmental samples. Detection of poliovirus in the environ mental samples indicates the transmission of virus in absence of paralytic cases.

Amongst the Southeast Asian countries, India is the only country to have environmen tal surveillance in addition to AFP surveillance. Presently environmental surveillance system in India includes a total of 21 sites in various cities of Maharashtra, Delhi, Kolkata and Punjab.

The list of poliovirus laboratory network of Southeast Asian region is shown in Table 1.

Table1. Poliovirus laboratory network and environmental surveillance sites, Southeast Asian region

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