Photo: Reuters
Photo: Reuters

155 countries are switching to a new polio vaccine. Here’s why

The move is part of a final push to eradicate polio by eliminating one major threat: vaccine-derived polio

New Delhi: Between 17 April and 1 May, 155 countries, including India, will switch to a new polio vaccine as part of a final push to eradicate polio by eliminating one major threat: vaccine-derived polio.

These countries will stop using the trivalent oral polio vaccine (tOPV) which protects against all three strains of wild polio virus and switch to bivalent OPV (bOPV), which protects against only two wild polio strains—types 1 and 3. Wild polio virus type 2 was confirmed eradicated in 2015 after the last case was detected in India in 1999.

Trivalent OPV was used to reduce the cases of wild polio virus from over 350,000 in 1988 to 74 in 2015. But there was another problem. In under-immunized populations, the live weakened virus contained in OPV can mutate and lead to vaccine-derived polio viruses (cVDPV).

More than 90% of cVDPV cases in the last 10 years have been caused by the type 2 vaccine strain, notes the Global Polio Eradication Initiative (GPEI), a public-private partnership led by governments and spearheaded by the World Health Organization, Rotary International, the US Centers for Disease Control and Prevention, and the United Nations Children’s Fund.

“Withdrawing tOPV and replacing it in routine immunization programs with bOPV will eliminate the risks associated with the type 2 vaccine strain and, just as importantly, boost protection against the two remaining wild strains of the virus," said a GPEI statement.

Salk and Sabin: Understanding vaccine derived polio

In 1952, Jonas Salk and his team at the University of Pittsburgh created the first effective polio vaccine. It was made by killing the poliovirus, and it was given as a shot. This inactivated polio vaccine (IPV) carried no risk of vaccine-associated polio paralysis.

The second polio vaccine, licensed in the US was created by Albert Sabin, was an oral polio vaccine (OPV) which was created using a live weakened version of the poliovirus. The trivalent oral polio vaccine contains live, weakened virus of all three serotypes, and is administered orally and is much cheaper compared to IPV.

When a child is immunized with OPV, the weakened vaccine-virus multiplies in the intestine for a limited period, thereby developing immunity by building up antibodies. The vaccine-virus is also excreted. which can spread in the immediate community giving children passive immunity.

The problem is, if a population is highly under-immunized, an excreted vaccine-virus can circulate for an long period of time and this could allow it to mutate. The new virus is called a circulating vaccine-derived poliovirus.

In April 2014, WHO Strategic Advisory Group of Experts (SAGE) emphasized that the elimination of persistent cVDPV2 should be a high priority for the global eradication effort. It stressed the need for countries to interrupt transmission of both vaccine-derived polio and wild polio in parallel so that type-2 vaccine can be withdrawn in April 2016, which is considered a seasonally low-transmission month.

Several measures are being taken to counter risks involved in implementing such an ambitious move. The switch-over has to take place in all countries at the same time as use of the trivalent vaccine by some countries could increase the risk of type-2 virus polio in countries which have switched to bivalent vaccine.

Countries at high risk of polio outbreak, such as India, have also introduced a dose of inactivated polio vaccine (IPV) to routine immunization, in addition to bOPV, in a bid to further enhance immunity. To prevent an outbreak of cVDPV type-2 after the switch-over, a global stockpile of monovalent OPV (mOPV) type 2 is kept ready in case an outbreak occurs.

Last week, while WHO noted the strong and sustained progress toward addressing the readiness criteria for the OPV2 withdrawal, it also emphasized its concern over the global IPV supply shortage which will likely to continue in 2017 and 2018.

As of today, only Afghanistan and Pakistan remain endemic to wild poliovirus. India completed three years without a case of wild poliovirus (WPV), subsequent to which the southeast Asia region was declared polio-free by the WHO in 2014. However, since 2009, India has witnessed 41 cases of vaccine derived polio virus, including two such cases in 2014.

The final goal is to discontinue all three types of oral polio vaccines by 2020, once types 1 and 3 are certified as eradicated. “We’re closer than ever to ending polio worldwide, which is why we are able to move forward with the largest and fastest globally synchronized vaccine switch ever," said Michel Zaffran, director of Polio Eradication at the World Health Organization in a press release.

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