India, along with 10 other countries in the World Health Organization (WHO) South East Asia Region, was declared polio free in 2014. No new case of polio has been reported in India since January 2011. There is no evidence of VDPVs circulating in the community.
India continues to face the threat of importation of the wild polio virus (WPV) and/or vaccine-derived polio viruses (VDPV) from countries that are still reporting cases.
To maintain its polio free status, India maintains high levels of population immunity against polio through Routine Immunization (RI) and National Immunization Days (NIDs), when all children under five years are vaccinated against the virus.
India simultaneously also conducts high-quality acute flaccid paralysis (AFP) surveillance, which meets the recommended global standards.
AFP surveillance is supplemented with Environmental Surveillance (ES), which involves regular collection and testing of sewage samples from selected sites from across the country to detect polio virus in faecal matter. This robust mode of surveillance detects polio even in people who do not present with symptoms.
A WHO team provides on-site training to health workers on sample collection during a site visit for environmental surveillance of poliovirus at Fatahpur Pumping Station in Khapar Kheri village in Amritsar, Punjab, in 2021/ FILE PHOTO (Photo: Swaran Singh/©WHO India)
The current surveillance system in India, which includes AFP surveillance and environmental surveillance, is robust enough to detect not only any wild polio transmission but also any instance of vaccine virus or vaccine-derived polio virus (VDPV) in individuals or in the environment.
One case of VDPV-type 1 was detected from environmental sewage sample collected from the Shyamlal Lane ES site in Kolkata on 25 April 2022. Genetic sequencing of this isolate established that it was not related to any of the previously identified VDPV1 viruses and was likely to be iVDPV (excreted from an immune-deficient individual).
The detection of the isolate from the sewage sample establishes that the surveillance system in India is highly sensitive and provides early warning about declining population immunity in a geographical area.
Following the detection of the isolate, a thorough, epidemiological investigation was conducted by the Government of India, with technical support of WHO, in the catchment area of the site between 26 and 28 April 2022. The activity was supported by West Bengal state government and the municipal corporation.
The entire catchment area of about 70 neighbourhoods was surveyed and information gathered on the local demography, migratory pattern, immunization coverage, and quality of AFP and ES surveillance.
The survey results show that:
- Surveillance indicators are well above the recommended global standards
- No missed AFP cases were identified in the community
- No missed AFP cases were identified in clinics and hospitals
- Data of optimal OPV coverage during pulse polio campaigns and routine immunization
Following the investigation, the state government will continue to maintain strong surveillance and immunization.
VDPVs are extremely rare. In addition to being reported among immunocompromised individuals, like in this case, VDPVs may also develop in areas with low population immunity. Hence it is extremely important for all children to receive their recommended doses of polio vaccines -- oral polio vaccine (OPV) and fractional-dose inactivated polio vaccine (fIPV) -- under RI and through special campaigns, such as NIDs.