Tremendous progress has been made since the 1988 resolution by the World Health Assembly to target poliomyelitis for eradication and this achievement is closer than ever to becoming a reality.
From an estimated 350,000 cases in 125 countries 28 years ago in 1988, widespread trivalent oral polio vaccine (tOPV) has reduced polio to a new low of 74 cases worldwide in 2015. This was a 78% decrease from 359 wild poliovirus (WPV) cases in 9 countries in 2014, Approximately 2 billion doses of tOPV were administered in 2015 alone to achieve this. By January 2016, endemic transmission of WPV persisted only in Pakistan and Afghanistan, with Nigeria removed from this short list in September, 2015.
Of the three types of WPV (1,2, and 3), wild WPV type 2 has not been detected anywhere since 1999 (in India), and the Global Commission for Certification of the Eradication of Poliovirus in September 2015 declared WPV type 2 eradicated. It exists now only in laboratories and in attenuated form in tOPV. In addition, the last case of polio due to WPV type 3 occurred in Nigeria in November 2012, even though it has not yet been formally certified as eradicated. For the past 3 ½ years the sole circulating WPV in the world is WPV type 1.
OPV has been the true cornerstone for these great advances in control of paralytic polio, but unfortunately it is very rarely associated with paralytic cases of vaccine-associated paralytic polio (VAPP) or circulating vaccine-derived polioviruses (cVDPVs). These cases occur when an attenuated vaccine strain mutates to become neurovirulent and/or transmissible like WPV. Paralytic polio from cVDPV is a risk particularly in those areas with low OPV coverage, with 32 cases of cVDPV reported from 7 countries in 2015.
Since 2006, 97% of cVDPV outbreaks and 26-31% of VAPP cases have been caused by the type 2 component of OPV. Thus, because WPV type 2 clearly is eradicated, continued routine use of type 2 containing OPV is no longer justified. In 2013, the World Health Assembly approved phased withdrawal of OPV and global introduction of inactivated polio vaccine (IPV) into immunization schedules, to help achieve complete eradication of all paralytic cases, including those caused by WPV and by VDPV. Removal of the type 2 component of OPV is therefore important.
From April 17 to May 1, 2016, a very complicated coordinated global switch from tOPV to bivalent OPV (bOPV) lacking the type 2 Sabin vaccine component in 154 countries and territories was achieved. This will markedly reduce the risk associated with type 2 VDPV emergence and transmission. Preparations for the switch have included aggressive tOPV immunization programs anywhere cVDPV2 has been detected, additional large scale tOPV campaigns just before OPV2 withdrawal, insuring adequate supplies of IPV for at least 1 dose to all children to provide (without risk of mutation) some immunity to type 2 poliovirus, destruction of WPV2 and withdrawn OPV2, and establishment of a global stockpile of monovalent OPV2 to control any WPV2 outbreak in the event it occurs. The lessons learned from this historic switch will be used during the planned withdrawal of all OPV worldwide in 2019-2020.
In the United States (US), OPV has not been licensed or available since 2000 mostly because of the risk of VAPP associated with OPV. The last reported WPV cases in the US occurred in 1979 in an outbreak of 1o paralytic cases in an unvaccinated population. Immunization schedules in the US and other areas without polio risk have included 4 doses of IPV for all infants and children, and 99-100% seroconversion is achieved after the initial 3 dose IPV series early in life.
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