In the last six months, there have been only eight wild-type poliovirus (type 1 and type 3) cases reported in Afghanistan and Pakistan, down from 32 cases during the same period last year. “The overall situation in Afghanistan and Pakistan has significantly improved in common corridors of transmission,” says a summary report of a April meeting of the Strategic Advisory Group of Experts (SAGE) on immunisation. Except for Pishin and Quetta, children 6-11 months old showed 95% ‘seroprotection’ in all districts during a recent ‘serosurvey’ carried out in Pakistan.

Type 1 cases in Nigeria

Between July and August 2016, three cases of wild poliovirus (type 1) were reported from Borno State, Nigeria in children between 2 and 5 years of age; two of them developed acute flaccid paralysis. This is the first time a wild poliovirus was detected in the country since 2014. However, no more wild poliovirus has been reported in Nigeria since then. But with most areas in Borno remaining inaccessible, a huge population of children aged less than five have not been immunised.

A year after 156 countries, including India, made a synchronised global switch from trivalent (containing all three strains of the polovirus — type 1, type 2 and type 3) to bivalent (only type 1 and type 3 strains) oral polio vaccine, several vaccine-derived poliovirus (VDPV) type 2 have been detected from the environment in Bauchi, Gombe and Sokoto in Nigeria in 2017.

SAGE has expressed “concern over the ongoing circulation of VDPV2 in Nigeria”. Since there is increased risk of type 2 outbreak as the immunity levels wane, SAGE had recommended that countries that have circulation of both wild poliovirus (type 1 and/or type 3) and vaccine-derived poliovirus type 2 should give “priority to stopping vaccine-derived polovirus type 2 over wild polio virus elimination”. At least two doses of oral polio vaccine containing only type 2 should be given before the next round of immunisation using bivalent oral polio vaccine, SAGE has recommended.

All the 156 countries that switched to bivalent oral polio vaccine have introduced at least one dose of inactivated polio vaccine (IPV) containing all the three strains in a killed form into their routine immunization programmes.

The last time wild poliovirus type 2 was detected anywhere in the world was in 1999. On 20 September 2015, wild poliovirus type 2 was formally declared as eradicated. But with the continued use of OPV, the live, weakened type 2 strain excreted by an immunised child can, under rare instances, turn virulent and cause vaccine-associated paralytic poliomyelitis in unprotected children. Since its eradication in the wild in 1999, all type 2 cases have been caused only by vaccine-derived polioviruses.

Polio cases, whether caused by wild type or vaccine-derived, can be eradicated only when oral polio vaccine is eventually withdrawn once the transmission of wild polio type 1 and type 3 have been eradicated.

Published in The Hindu on May 1, 2017