Not time to drop guard

New Delhi, March 17: India has recorded its first case of polio caused by a vaccine-derived poliovirus (VDPV) this year in a five-month-old child in Murshidabad district of Bengal but the country remains free of the wild poliovirus.

A polio surveillance laboratory in Calcutta has found that the child from Lalbag block in Murshidabad was infected by VDPV, which occurs when the weakened virus in the oral polio vaccine (OPV) mutates over time, and regains the ability to cause paralysis.

Surveillance experts say VDPVs are extremely rare and typically occur in children with immunodeficiency or in populations with low levels of immunity. The child in Lalbag has had repeated episodes of infections, indicating the possibility of immunodeficiency, the polio surveillance programme run by the Union health ministry and the World Health Organisation said in an update on VDPVs.

The Institute of Serology, Calcutta, detected the VDPV on March 2 this year, about a week after the WHO deleted India from the list of polio-endemic countries after more than a year free of the wild poliovirus.

Public health experts say India remains free of wild polio, but the local response to VDPVs should be similar to the management of wild poliovirus outbreaks — rapid implementation of local mass immunisation campaigns.

Last year, the surveillance programme detected seven cases of VDPVs — one in a child with congenital immune deficiency in Dhamtari, Chhattisgarh, and the other six from areas with low routine immunisation coverage — Udaipur in Rajasthan, Ghaziabad and Badaun in Uttar Pradesh, Barnala in Punjab, Vidisha in Maharashtra, and Jajpur in Odisha. Rapid immunisation helped prevent the circulation of VDPVs that emerged in India during 2010 and 2011.

A VDPV is a risk associated with OPV that contains live, weakened vaccine virus. A child vaccinated with OPV excretes the vaccine virus for six to eight weeks and, rarely, a vaccine virus may genetically mutate and cause disease in vulnerable children.

“But even one case of VDPV should be viewed as the tip of an iceberg,” said Thekakarra Jacob John, former head of medical virology at the Christian Medical College, Vellore, and former member of a technical group that advises the government on polio.

“Each VDPV is a signal that the vaccine virus from OPV has become wild-like — and the rule in polio is that for every one infection detected, there are perhaps up to 1,000 silent infections,” John told The Telegraph.

The emergence of VDPVs, some of which have circulated and caused outbreaks, in several countries over the past decade, has prompted the WHO to rethink the polio eradication strategy and consider the option of introducing an inactivated polio vaccine (IPV), which contains killed viruses, and thus does not carry the risk of VDPV.

“There’s no alternative — the IPV will have to be introduced to intercept the emergence of VDPVs,” said John, who had in the late-1990s predicted the risks of trying to eradicate wild polio with OPV alone.

A report on VDPVs from the US Centres for Disease Control released last year said more than 400 cases of VDPV had been detected from countries in Africa, Asia and Europe between July 2009 and March 2011.

The Union health ministry has specific guidelines on following up each case of VDPV that include investigations on immunological conditions of each patient, efforts to determine if the VDPV is circulating in the community, and assess the population immunity in the immediate vicinity of the VDPV case.

The National Polio Surveillance Project says the OPV is a very safe and effective vaccine that is protecting millions of children from paralysis caused by the wild virus. The agency said the chance of an importation of the wild poliovirus remains “the larger threat to the children of India”.