Polio switch strategy: An obituary in the making?

Abstract

India was certified free of polio in 2014. Until now, the oral polio vaccine (OPV) was being used in India. As the OPV is a live vaccine, vaccine-associated paralytic poliomyelitis may occur after its use. The aim is to replace the OPV with injectable polio vaccine. The Polio Eradication and Endgame Strategic Plan 2013–2018 has been made to eradicate polio. A switch from the trivalent OPV (tOPV) to bivalent OPV (bOPV) has been undertaken in all countries since April 2016. tOPV vials have been withdrawn and replaced by bOPV. In addition, the inactivated polio vaccine (IPV) has been introduced. The next step would be to remove the type 3 virus component followed by complete cessation of the OPV and a final switch to the IPV. The timeline has been fixed as 2018–2019. Replacement of a vaccine may raise fears in the community that need to be addressed. Re-emergence of circulating vaccine-derived poliovirus after withdrawal of the tOPV may occur. Proper disposal of vaccine vials needs to be ensured. Proper training of vaccinators is important. All stakeholders need to be incorporated, and focus should be more on deprived populations. The switch marks a significant step towards the final goal of polio eradication. Finally, the importance of community participation cannot be overemphasized. Sustained surveillance is the key to prevent occurrence of cases in polio-free countries through importation.

Ever since the World Health Assembly (WHA) 1988 adopted Resolution 41.28 for the global eradication of polio by the year 2000, there have been many hurdles towards achievement of the target.¹ As the target could not be achieved in 2000, the same was extended until 2012, with failure being met again. This can be attributed to global conflicts, refugee crisis, inaccessible areas, corruption in the government, poor quality of cold chain maintenance, non-involvement of the community and lack of funds.

India reported the last polio case in 2011 and was certified free of polio on March 27, 2014. However, the disease still remains endemic in two of its neighbours Pakistan and Afghanistan. Nigeria was certified as polio free in September 2015. However, polio resurged in Nigeria after two years, with the occurrence of two cases in Borno State in August 2016, followed by one more case in September 2016.² During January 2015–May 2016, five new circulating vaccine-derived poliovirus (cVDPV) outbreaks were identified in Burma (Myanmar) (two cases), Guinea (seven cases), Laos (11 cases), Madagascar (10 cases) and Ukraine (two cases), whereas cVDPV type 2 (cVDPV2) circulation in Nigeria and Pakistan decreased sharply.³

Until now, the oral polio vaccine (OPV) was being used in India for routine immunization and intensified pulse polio immunization (IPPI) owing to its low cost and ease of administration by peripheral vaccinators. However, as the OPV is a live vaccine and the only remaining source of live polio virus after interruption of wild poliovirus transmission, cases of vaccine-associated paralytic poliomyelitis (VAPP) may occur after its use as the virus may very rarely revert to the dreaded neurovirulent form. The incidence of VAPP is estimated to be 2.4 per million birth cohort.⁴ Hence, the ultimate aim is to replace the OPV in a phased manner to nullify the chances of occurrence of possible polio cases due to cVDPV.

The Polio Eradication and Endgame Strategic Plan 2013–2018 was chalked up after the WHA 2012 declared eradication of polio virus as an emergency.⁵ The ultimate aim was withdrawal of all forms of OPV, beginning with the introduction of a minimum of one dose of the inactivated polio vaccine (IPV) at least six months before the withdrawal of type 2 OPV.

Changing epidemiological pattern of polio transmission

Polio is caused by three strains of the wild polio virus (WPV)—type 1, 2 and 3. Since the year 1999, there have been no reports of transmission of type 2 WPV. Type 2 WPV was finally certified as eradicated in September 2015. Type 3 virus transmission was last reported in November 2013. Since then, only type 1 virus has been incriminated in polio transmission.

With cases of polio caused by WPV decreasing enormously, the vaccine virus has now become a major threat to the community. The OPV contains attenuated forms of all the three strains of the live polio virus. Owing to low immunization coverage in certain areas, the live vaccine–derived virus may mutate over generations and with time and acquire neurovirulent properties. As the virus gets passed on from person to person through the faecal–oral route, it may spread within the community and lead to cases or outbreaks of VAPP. Rarely, outbreaks of cVDPV may also occur when the vaccine virus strain is being transmitted between humans and gains neurovirulence after mutation. These outbreaks have been attributed almost entirely (97%) to the type 2 component.⁶ The risk of cVDPV is increased in areas where the OPV is still being used although wild polio has been eradicated, coupled with poor sanitation, high birth cohort, increasing population density, a low OPV coverage and a substantial immunity gap in the community. Hence, it became imperative to prevent cases of polio resulting from the live vaccine.

Although type 3 WPV has not been reported since November 2013, it cannot be presumed that the virus has been eradicated for sure. Continued surveillance especially in the periphery will give a final answer to this question.

The switch

To ensure a smooth switch, regional workshops were started across the country two months preceding the switch to ensure microplanning, surveillance and validation of the switch.

With the aforementioned theme in the centre, a switch from the trivalent OPV (tOPV) to bivalent OPV (bOPV) has been undertaken in all countries using the OPV across the globe. In India, the switch has come into effect from April 25, 2016 (National Switch Day), wherein all tOPV vials have been withdrawn from the entire cold chain and replaced by the bOPV so that even a drop of the tOPV is not available. The month of April has been chosen for the switch as it marks the ‘low’ season for poliovirus transmission in endemic regions.⁷

The stocks of tOPV collected from the cold chain have been destroyed. Type 2 polio virus has been destroyed in all Indian laboratories as well; less the National Institute of Virology (NIV), Pune. Consequently, India has been validated to be free of tOPV on May 9, 2016. The bOPV thus introduced is meant for routine immunization as well as National Immunization Days (NIDs) and Subnational immunization Days (SNIDs) to minimize the risk of importation. The bOPV has the added advantage of being more efficacious than its vis-à-vis, the tOPV, as it has done away with the aforementioned pitfalls of using a vaccine containing the already eradicated type 2 virus.

In addition, the IPV has been introduced in all states and union territories in a three-phased manner, based on risk assessment and operational feasibility. Regional differences regarding the number of doses and route of administration are at variance. Although majority of states have opted for a single-dose intramuscular injection of IPV, others have chosen two fractional doses intradermally, eight weeks apart, as the same has proved to be more immunogenic.⁸ The rationale behind using the IPV in conjunction with the bOPV was that the IPV will reduce the risk of paralytic polio after exposure to type 2 virus after oral poliovirus vaccine type 2 (OPV2) withdrawal and would also reduce transmission of a possibly reintroduced type 2 virus by rapidly closing any remaining immunity gaps, boosting antibody titres, and induce mucosal immunity, which would serve as a barrier to community transmission.

This initial work completed; the next step in the switch chain would be to remove the type 3 virus component because no case of polio infection caused by this virus type has been reported since November 2013.⁹ This will be followed by complete cessation of the OPV and the final switch to the innocuous IPV. The target for the final phase in which the bOPV will be withdrawn and replaced completely with the IPV has been fixed as 2018–2019.

Challenges

Proper disposal of vaccine vials needs to be ensured after the switch, especially in rural and inaccessible areas. The harmful effects of unethical shortcut methods such as dumping vials in drains need to be explained to healthcare workers. For example, a total of 14 sewage samples collected between January 2015 and May 2016, from different parts of the country, had been tested positive for VDPVs. However, no children have been found to be affected by the detected VDPV isolate in the nearby areas.¹⁰

Another area of concern is that the old and new vials and boxes are almost identical. Store holders have been instructed to mark the old vaccine vials with an ‘X’ and tag them for disposal. This requires close scrutiny. Until now, PPI was being carried out using the OPV, which could be carried out by even less educated volunteers across the countryside. With the introduction of the injectable version of the vaccine, proper training and retraining of vaccinators, besides ensuring a good education level of vaccinators, is of prime importance.

Presently, the production of the IPV will not be able to meet the requirement globally.¹¹ The inability to meet the requirement is because of failure of the existing IPV production sites in industrialized countries to scale up production of the IPV in their manufacturing units as was expected and as promised by vaccine manufacturers to the government.¹² Efforts need to be put in place involving all stakeholdersfunding being a priority area. A polio catch-up campaign using the IPV in a mass immunization setting is required in limited geographic areas of endemic countries, namely, Pakistan and Afghanistan, where IPV supply is a crucial link. Special focus should be given on refugees and those children staying in conflict-affected zones as they are likely to miss the doses owing to poor coverage. We must learn from the experience of small pox eradication when in Bangladesh, a natural calamity in the form of flood led to re-emergence of the disease.

There is a possibility of re-emergence of VDPV type 2 virus after withdrawal of the tOPV, consequently decreasing the immunity level against the virus.¹³ To prevent such an eventuality, a dose of IPV at 14 weeks has also been included in the Universal Immunization Programme (UIP) along with the routine bOPV doses to boost immunity against type 2 virus. However, in case an outbreak of cVDPV2 occurs, to cater to this eventuality, we should have a readily available stock of monovalent OPV2 (mOPV2), which as of now is not available in our country.

Similarly, accidental polio from laboratories is also a real possibility. Hence, setting of complacency needs to be guarded against.

Besides, a cohort of 27 million children is born each year, which becomes an additional pool of potential polio cases.¹¹ Finally, as a consequence of several hurdles that have been discussed, we still do not know as to how much and when the polio switch strategy will succeed. We also cannot comment as to what further changes/modifications will be required to be made in the current strategy to overcome these hurdles.

The switch to a bOPV that contains types 1 and 3 took place in 155 countries and territories.¹⁴ Notwithstanding the aforementioned challenges, the PPI campaign has been successful in averting about 3.94 million paralytic polio cases, 393,918 polio deaths and 1.48 billion Disability Adjusted Life Years (DALYs).¹⁵

The switch marks a significant step towards the final goal of polio eradication. Political will and involvement of local bodies, religious leaders, professionals and all other stakeholders is the need of the hour.

Conclusion

An Emergency Preparedness and Response Plan has already been chalked out by our national government, wherein rapid response teams have been constituted across the country to tackle an emergency situation developing in case of suspected importation of the polio virus. Finally, the importance of community participation cannot be overemphasized.¹⁶

Polio still remains a ‘Public Health Emergency of International Concern’ as per the World Health Organization guidelines.¹⁷ Sustained surveillance is the key to prevent occurrence of cases in polio-free countries through importation.

Disclosure of competing interest

The authors have none to declare.