Oral polio vaccine

Oral polio vaccine (OPV): Two types of polio vaccines are available – a live, oral, Sabin vaccine and an inactivated, parenteral Salk vaccine, named after their developers. In India, OPV is used under NIS.

Contents – OPV is a live attenuated, polyvalent vaccine, containing three strains of virus – I, II, and III, in concentrations of 10, l0 and 10 respectively.

Supply & storage

OPV is supplied as multi-dose vials with dropper. It is extremely heat-sensitive and maintenance of adequate cold chain is extremely vital. It should be stored at —20°C in large depots (for >6 months), in the freezer at clinic level and must reach immunization center at 2-8°C in vaccine carriers.

Each OPV vial/ampoule is marked with vaccine vial monitor i.e. a sticker with a lighter square inside a darker circle. If the color of square is darker or matching the outer circle, vaccine is probably not potent and should not be used.

Dosage & administration

 OPV is given as 2 drops orally. It is preferable to defer breast feeding for ½ hour after administration.

Oral polio vaccine

Schedule

Minimum 4 doses are recommended for primary immunization – at birth, 6th, 10th and l4 weeks, though an additional dose may be given with measles vaccine at 9 months. Two booster doses are given at 18 months and 5 years.

Additional doses of Oral polio vaccine during pulse polio campaign should not be counted for routine immunization purpose.

Protective value depends on the number of primary doses, ranging from 80%-90% after 3 and 5 doses respectively.

Side-effects of Oral polio vaccine are none, though the vaccinia virus is excreted in stools for variable period. Transmission of this virus to other susceptible children via feco-oral route is responsible for partial immunization of these children and production of Herd immunity.

Vaccine associated poliomyelitis (VAPP) is an extreme rare complication of Oral polio vaccine, seen in ½-3 million doses, due to feco-oral passage of vaccinia virus from the vaccinee to a susceptible or immunodeficient host. After multiple passages through susceptible hosts, the virus may regain its virulence to cause disease.

Contraindications are none. Unlike other live vaccines, OPV may be used in immuno-compromized children, though inactivated Salk vaccine is preferred in them.

Other important issues:

• Unlike other vaccines, Oral polio vaccine produces local gut immunity by occupying all receptor sites with vaccinia virus. Consequently on exposure, wild polio strains do not get the site to attach and proliferate in gut.

• Despite a live vaccine, OPV is given in multiple doses for primary immunization to allow uptake of all three serotypes, as only one or two serotypes are usually fixed with one dose. Otherwise, it is a highly potent vaccine, capable of producing adequate immune response even with single dose.

• Currently, monvalent Oral polio vaccine containing only first serotype (OPV1) is being used at many places in pulse polio program to avoid interference to OPV1 uptake by OPV2 and OPV3 serotypes, which are virtually eliminated from India.