The story so far: Earlier in September, New York declared a State disaster emergency after poliovirus was discovered in wastewater samples from three counties. Traces of poliovirus were also found in sewage water in London in June 2022. In India, vaccine-derived poliovirus (VDPV) was detected in the environmental surveillance of sewage samples from Kolkata this year, although the Health Ministry said that such an incident can occur in any country where oral polio vaccines (OPVs) are given. The Ministry also added that the last time such a VDPV was detected was in New Delhi in 2018.
The U.S. has now been put on a list of 30 countries, prepared by the Global Polio Eradication Drive, where either imported wild or vaccine-derived poliovirus, or circulating vaccine-derived poliovirus (cVDPV) has been identified. The list also includes Israel, Ukraine, Nigeria and Egypt, among many others. Afghanistan and Pakistan are the two countries where polio remains endemic, and hence do not find mention on the outbreak list.
What is polio disease?
Poliomyelitis, commonly called polio, is a highly infectious viral disease that can leave patients disabled, and in some cases, even prove fatal. The virus enters the nervous system and can cause total paralysis in just a few hours.
Initial symptoms of the infection include fever, fatigue, headache, vomiting, stiffness of the neck and pain in the limbs. According to the World Health Organisation (WHO), one in 200 infections leads to irreversible paralysis, usually in the legs. Among these, 5-10% die when breathing muscles are affected and immobilised.
There is no known cure for polio. It can only be prevented by way of vaccination.
Wild poliovirus (WPV) has three known strains – types 1, 2, and 3 – each with a slight difference in structure. Immunity to one type does not guarantee immunity to others.
According to the Global Polio Eradication Drive, only type 1 of wild poliovirus remains in circulation. Type 2 was declared eradicated in September 2015, last being detected in India in 1999. Type 3 wild poliovirus was declared eradicated in October 2019.
How does polio virus spread?
The polio virus is most commonly spread through the faecal-oral route. It can also spread through contaminated water or food. The virus multiplies in the host’s intestines.
Susceptible age groups
Most polio cases are recorded in children under five years of age, but all unvaccinated people can contract the disease.
Types of polio vaccine
Currently, there are two types of vaccines available against poliomyelitis – oral poliovirus vaccine (OPV) and inactivated poliovirus vaccine (IPV).
IPV was developed in 1955 by virologist Dr Jonas Salk. It consists of inactivated (killed) poliovirus strains of all three types. The vaccine is administered through an intramuscular or intradermal injection. It produces antibodies in the blood against all three types of the poliovirus.
OPV uses a weakened (also called attenuated) form of poliovirus, which can either be one strain or a combination, to enable the vaccinated individual to build immunity against the virus. According to the Global Polio Eradication Initiative, the attenuated viruses are able to replicate in the intestines but are “around 10,000 times less able to enter the central nervous system than the wild virus”.
Since OPVs are administered orally, they are more suitable for mass vaccination campaigns than IPVs that must be administered by qualified health workers. Although both modes of vaccination are safe, the live attenuated virus in OPV can cause paralysis in extremely rare cases. IPV, on the other hand, induces a very low level of immunity in the intestine. In such a case, if an IPV vaccinated person is infected with wild poliovirus, the virus can still multiple inside their intestines and be shed in the faeces, risking continued circulation.
In some seriously under-immunised communities, the excreted vaccine virus can continue to circulate for a longer time period, with a possibility of undergoing genetic changes. In rare instances, it can mutate into a form that can paralyse people. This is known as cVDPV – the form of virus that put the U.S. on the outbreak list.
According to WHO, it takes a long time for cVDPV to occur. Generally, the strain is believed to be in circulation in an under or unimmunised community for at least a year.
Polio immunisation in India
India committed itself to WHO’s goal of global polio eradication in 1988. The Government launched the National Immunisation Day (NID), commonly called the Pulse Polio immunisation programme in 1995, aiming to administer polio drops to all children in the 0-5 years age group. Additionally, Sub-National Immunisation Days are also conducted every year in high-risk areas.
The last polio case in the country was reported from West Bengal’s Howrah district in January 2011. On February 24, 2012, WHO removed India from its list of endemic countries with active poliovirus transmission. India, along with 10 other countries in the WHO South-East Asia region, was certified polio-free on March 27, 2014.
Should India be worried about the global resurgence of poliovirus?
India’s proximity to polio-endemic countries Pakistan and Afghanistan, and possible gaps in vaccination in the past two years due to Covid-19 are causes for concern. However, according to WHO, there is “no evidence of VDPVs circulating in the community”.
In a report outlining the investigation of the Kolkata sewage water sample, WHO said that India’s acute flaccid paralysis (AFP) surveillance, along with routine immunisation and NIDs, meets the recommended global standards. Environmental Surveillance (ES), which involves regular collection of sewage water to check for poliovirus in faecal matter, supplements the AFP.
“Genetic sequencing of this isolate (Kolkata sample) 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),” WHO said.
In 2021, India had also announced that people returning to the country from Afghanistan would be vaccinated against polio as a preventive measure.