The union health ministry on Tuesday sought to assuage concerns after a two-year-old child from Delhi was diagnosed with a rare vaccine-derived polio virus type 2 (VDPV2) in the first week of November, in Harsh Vihar area in Shahdara.
"One should not be worried about the polio case as there has been as many as 44 cases till now. Such cases are medicine-generated and we do not count it among polio cases. There have been 44 similar cases and the health department has taken report of it and is dealing with it," The Times of India reported quoting health minister JP Nadda.
In a statement, the ministry said, polio virus surveillance, including environmental surveillance, through sewage sample testing had not revealed any polio virus circulation in the area.
The ministry added that in line with WHO guidelines for response to VDPVs, the union health ministry conducted a polio round from 6 November to immunise all children under five years, in the vicinity of the area where the case was detected.
According to the World Health Organisation, VDPV, a very rare strain of the polio virus, occurred in children with immunodeficiency or in populations with low immunity. The emergence of vaccine-derived polio viruses was very rare but not unexpected and the programme had the knowledge and experience to deal with them.
"The virus was isolated from a 25-month-old girl child. The case had onset of paralysis on 1 October 2015. The case has been investigated and found to be immunodeficient," the WHO said in a statement.
The oral vaccine contains live but attenuated (or highly weakened) forms of the three strains of the polio virus - P1, P2 and P3, which can be just recongised by the immune system, The Indian Express reported. The body then develops its defences against the virus antigens. The body creates antibodies specific to the antigens of the three virus strains as it bolsters its immunity.
Trace amounts of the attenuated virus are also excreted by the child, even as his or her body develops immunity. In rare cases, even as the antibodies are developing, the attenuated form of the virus mutates to a stronger form - and starts actively circulating in the vaccinated child, which ends up causing the very disease it was meant to prevent.