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Priming the polio fight -R Prasad
-The Hindu

The global vaccine 'switch', from trivalent to bivalent OPV, has been recommended by the Strategic Advisory Group of Experts as a critical component of the polio endgame strategy

Beginning Monday, April 25, India will stop using the oral polio vaccine (OPV) with all three strains of the poliovirus (type 1, type 2 and type 3) and instead use OPV vaccines with only two strains — type 1 and type 3. The global vaccine “switch”, from trivalent to bivalent OPV, has been recommended by the Strategic Advisory Group of Experts (SAGE) as a critical component of the polio endgame strategy.

In what has been labelled the largest, globally coordinated withdrawal of one vaccine and the roll-out of another into a routine immunisation programme in history, 155 countries currently using trivalent OPV will make the switch. It began on April 17 and runs till May 1.

Though polio cases caused by wild virus have reduced by 99.9 per cent since 1988 through immunisation using OPV, the vaccine can, under rare instances, cause polio in unprotected children. This because OPV contains live, weakened (attenuated) virus, which, on rare occasions, can turn virulent, continue to circulate in the community and cause vaccine-derived poliovirus (VDPV).

The Global Polio Eradication Initiative says wild poliovirus type 2 was eradicated in 1999. Since then, all type 2 polio cases have been caused only by vaccine-derived polioviruses. In fact, type 2 strain in the trivalent OPV has caused over 90 per cent of VDPV cases in the world in the last 10 years.

Polio, including vaccine-derived polio, can be eradicated only when oral polio vaccine is eventually withdrawn after wild polio transmission has been stopped. The withdrawal of type 2 strain from OPV in April by switching over to a bivalent OPV is the “first major step” of this withdrawal process.

But what is most important is that the switch has to be globally synchronised. Else, there is a risk of importing type 2 VDVP from another country that continues to use trivalent OPV. One or two cases of type 2 VDPV outbreaks are to be “expected within a year after the switch”, says SAGE.

It is to mitigate this risk that the inactivated poliovirus vaccine (IPV) was introduced in a phased manner into the routine immunisation programme in India from November 30, 2015, well before the switch is made. As IPV contains all three polio strains in a killed form, it is safer than OPV. “From 1996 onwards I have been saying that those countries using OPV have to go through two phases to eradicate polio. [The] First phase is to eradicate wild polio through OPV and the second phase is to eradicate vaccine-derived poliovirus using IPV,” says Dr. Jacob John, formerly with the Christian Medical College, Vellore, Tamil Nadu.

First introduced in six States in November last year, immunisation using IPV has been expanded to all States except Chandigarh, Maharashtra, Odisha, Tamil Nadu and West Bengal. According to Dr. Pankaj Bhatnagar, National Professional Officer – Immunization, WHO Country Office for India, preparations are ongoing in the remaining five States for the launch of IPV in the coming weeks.

Vaccine shortage

The “rapid scale-up of IPV production required has encountered multiple challenges, leading to a global shortage”, according to the Global Polio Eradication Initiative.

In order to stretch the vaccine supply to cover more children, India has adopted a two-pronged vaccination strategy. Children in Puducherry and seven States (Andhra Pradesh, Karnataka, Kerala, Maharashtra, Odisha, Tamil Nadu and Telangana), will get two doses of 0.1 ml each of IPV given at 6 and 14 weeks and administered intradermally. In the rest of India, it will be a dose of 0.5 ml IPV at 14 weeks administered intramuscularly.

Besides stretching the supply of vaccine, “better protection” is conferred when two doses of 0.1 ml each of IPV are given intradermally than a single full dose of 0.5 ml given intramuscularly at 14 weeks, says SAGE.

“There is better seroconversion and higher antibody level when two doses of IPV are given intradermally,” says Dr. John. “The antigen presenting cells are more in number in the intradermal region of the skin than subcutaneous or muscle tissue. So, even when a tiny dose is given, more antigen presenting cells present pick up the antigen and carry it over to the nearest lymph node where it is presented to the T cells. The effect gets enhanced as it is given in two doses.”

The reason why Puducherry and the seven States were chosen for intradermal administration was because they have good immunisation coverage, well developed infrastructure, and human resources to give IPV twice to each child, says Dr. Jyoti Joshi Jain, Deputy Director, Immunization Technical Support Unit, the Public Health Foundation of India, Delhi.

“[The] Risk of drop-out between these doses exists. However, if a child misses the second dose of IPV at 14 weeks, he/she can receive it anytime with the Pentavalent 3 vaccination dose up to one year of life,” says Dr. Jain.

Dr. Bhatnagar adds: “Various studies have shown that second dose of IPV given up to one year of life provides better immunity.” But if the child takes only the first dose at six weeks then the vaccine will only have some priming effect.