Pulse Polio campaign using Oral Polio Virus (OPV) drops won all the accolades in getting India, a Polio free status in march 2014 by World Health Organisation (WHO). Now, India has all geared up to march in race against Polio in “endgame plan” as suggested by WHO. One dose of Inactivated Polio Vaccine (IPV) has been introduced in the routine immunization programme at 14 weeks of age along with the routine Oral Polio drops. This will ensure double and comprehensive protection of children from Polio.
Right now, six states, Assam, Gujarat, Punjab, Bihar, Madhya Pradesh, and Uttar Pradesh are covered in first phase. Further expansion of IPV coverage over the remaining states of the country will be done in the first half of 2016.
Inactivated Polio Vaccine (IPV) or the Salk vaccine is not a new vaccine. In fact, it was discovered earlier (1955) than the Oral Polio Vaccine (OPV) drops or the Sabin vaccine (1961). OPV replaced IPV in mass vaccination campaigns soon after its introduction due to the following reasons. It is easier to administer drops than injections, cheaper because of low production costs and is able to induce superior mucosal immunity of the intestines along with humoral immunity. It stops multiplication of wild Poliovirus inside the intestines and shedding of viruses through the faeces. It also provides life-long immunity and also indirectly immunizes others through the spread of attenuated vaccine viruses (herd immunity).
But, in rarest of rare cases ( 1 in a million case), OPV drops administration can cause Vaccine- Associated Paralytic Poliomyelitis Disease (VAPPD). OPV drops consist of three attenuated viral strains of polio, out of which the second one has shown the capability to revert back to the virulent strain, causing paralysis.
Then, why are we still giving polio drops to kids? Although, IPV provides an excellent immunity against progression of Poliomyelitis and also controls the rare events of VAPP, it has a limitation of being able to induce very low levels of mucosal immunity in the intestine. IPV mainly works on developing antibodies in the blood against polioviruses (humoral immunity). So, a person immunized with IPV, when infected with wild polioviruses, can still have viruses multiplying in the intestines and is able to shed the viruses in faeces. This leads to continued circulation of the wild type poliovirus in the community even after the good coverage of vaccination from IPV alone. OPV drops provide herd immunity against these wild viruses and protect everybody in the community, even the non-vaccinated ones. As OPV drops provide superior immunity against Polio, they are still used in the pulse polio campaigns in the developing world.
The “end game” plan of WHO is to eradicate Polio from the world altogether. The wild polio viruses are still present in the endemic countries and the vaccine derived polio viruses may arise in the Polio free countries. In order to tackle both these kinds, the ultimate need is to remove all the oral polio vaccines in phases, and incorporate IPV only in immunisation drives. Pulse Polio campaigns can not cease abruptly in India in order to maintain the herd immunity against the wild viruses. The risk of spread of Polio through the neighbouring Polio-endemic countries like Pakistan and Afghanistan is still looming. Gradual removal of Oral Polio Vaccines after successful wide coverage of Inactivated Polio vaccines in the country, is the key to achieve the comprehensive immunity against Polio.
The first step in this plan is to introduce at least one dose of inactivated polio vaccine (IPV) in the routine immunization programmes in all the developing countries by the end of 2015. The second step will be to switch from trivalent OPV to bivalent OPV drops, removing the type 2 viral component from polio immunisation programmes. The immunity against the type 2 poliovirus will be provided by IPV then. The proposed date for this step is April 2016, which is considered the “low” season for Poliovirus transmission in many polio endemic countries. The final step will be, to remove the Oral Polio Vaccine drops altogether and give only IPV injections. This can be safe only when wild type viruses are not circulating anywhere. But, this will happen only after Polio is eradicated worldwide.