In the fight against polio, two types of vaccine are used. One is the inactivated polio vaccine (IPV); the other is the oral polio vaccine (OPV). What is the difference between them?
IPV and OPV were developed by Jonas Salk and Albert Sabin. IPV came into use in 1955 and OPV in 1961. The difference between the two vaccines is the manner in which they operate. IPV involves an inactivated poliovirus administered via injection whereas OPV involves a weakened poliovirus which is orally administered.
There are two forms of OPV: trivalent (tOPV) and bivalent (bOPV). tOPV targets all three types of the wild poliovirus, whereas bOPV targets types 1 and 2.
A potential side-effect of OPV – albeit rare – is that the strain of poliovirus used in the vaccine can mutate and lead to the patient contracting vaccine-derived poliovirus (VCPV) – occasionally resulting in paralysis (known as vaccine-associated paralytic poliomyelitis, or VAPP).
This is rare. According to the World Health Organization (WHO), OPV causes just three cases of VAPP for every million doses provided. However, there is the potential for circulating vaccine-derived poliovirus (cVDPV). This is where vaccine-derived polioviruses spread among communities, in areas with low rates of immunisation.
Instances of this are rare. According to the Global Polio Eradication Initiative (GPEI), more than ten billion doses of OPV were administered globally between 2000 and 2011. In the same period, just 20 outbreaks of cVDPV were reported. These outbreaks resulted in 580 cases of polio. Without administration of the OPV, paralytic poliomyelitis would have affected six million children.
As huge swathes of the world continue to live in the post-polio age, however, the rise of cVDPV outbreaks is one health agencies are trying to minimise. To this end, the WHO issued a twofold recommendation in 2012
- Transitioning from the tOPV to the bOPV. Almost all cases of cVDPV are caused by poliovirus derived from vaccine type-2. As type-2 wild poliovirus was eradicated globally in 1999, the risk due to the tOPV was considered to outweigh its benefits. This necessitated a transition to the bOPV, which still targets types 1 and 3 of the wild poliovirus
- Introduce the IPV in countries only using OPV, with all countries having introduced at least one dose of IPV into immunisation programmes by the time of the switchover from tOPV to bOPV
The switchover from tOPV to bOPV was accomplished in April 2016 across 155 countries and territories, with the accomplishment representing an enormous public health achievement shared in by governments, NGOs, international agencies and regional health workers alike.
Just Afghanistan and Pakistan remain polio-endemic nations to date. Worldwide, a virus which caused untold misery and pain to millions has become a thing of the past for most nations, including India, which once saw as many as 400,000 polio cases every year. That the country can call itself polio-free is a testament to the success of the polio eradication campaign so far and the monumental achievement that was the development of the polio vaccines.