On October 21, 2019, the World Health Organization (WHO), United Nations Health Agency, and Zambian Health Minister Dr. Chitalu Chilufya reported the first local polio case in Zambia since 1995. The case is a 2-year old boy residing in Chienge district, Luapula Province, who presented with weakness in both lower limbs on July 16, 2019 [1,2]. The polio infection was derived from the polio vaccine [1,2]. Currently, ten African countries have cases of vaccine-derived polio, including the neighboring country of the Democratic Republic of the Congo (DRC), where 37 vaccine-derived polio cases have occurred this year . The boy who contracted polio in Zambia has a parent who resides in the DRC, but the UN is reporting no link between the Zambia case and the outbreak in the DRC [1,2]. The last imported case of polio in Zambia occurred in 2002 .
Polio and Poliovirus Vaccines
Polio, or poliomyelitis, is an infectious viral disease that is caused by poliovirus . Polio is highly contagious and spread person-to-person . Poliovirus infiltrates the nervous system and can cause irreversible paralysis in a short amount of time following infection . No direct treatment exists for polio, but the symptoms can be treated or alleviated . Vaccination is currently the best method to try to lower, and eventually eradicate, the global burden of polio. There are three types of wild poliovirus, and as of October 17, 2019, the WHO reported global eradication of type 3 . Wild poliovirus type 2 was eradicated in 2015 but poses enough of a threat to still be included in the oral vaccination . The cases across Africa, including the Zambia case, are type 2 polio . Although rare, the type 2 poliovirus in the live oral vaccine has the capability to mutate .
Oral polio vaccines (OPV) contain weakened, or attenuated, poliovirus in order to activate an immune response . While rare, the virus in the vaccine can circulate in under-immunized areas for long enough to allow the vaccine-virus to mutate into a form that can cause paralysis . Since 2000, over 10 billion OPV doses have been administered, and as of 2017, only 760 cases of vaccine-derived polio have been confirmed . There are six types of polio vaccines, with five containing live-attenuated virus: inactivated polio vaccine (IPV), Trivalent oral polio vaccine (tOPV), Bivalent oral polio vaccine (bOPV), and Monovalent oral polio vaccines (mOPV1, mOPV2 and mOPV3) . Both the IPV and tOPV immunize against all three types of poliovirus, the bOPV protects against type 1 and type 2 poliovirus, and each of the mOPV’s protect against one type of poliovirus, indicated by the number in the vaccine name . The World Health Assembly completed the Polio Eradication and Endgame Strategy plan in 2012, and in 2016, the OPV switch occurred, where tOPV use was replaced by bOPV use in countries that use the oral vaccine . The Polio Eradication and Endgame Strategy also hopes to phase out all use of the live OPV and utilize only the IPV instead .
Thirty-four stool samples were collected from healthy-presenting contacts of the case and tested at the Zambian Ministry of Health University Teaching Hospital virology lab [2,4]. Two of the thirty-four tested positive for poliovirus type 2, and the positive samples were genetically linked to the case . Around 90% of infected people do not present with symptoms or present mildly . Health Minister Chilufya is urging Zambian residents to take precautions against infection and continues to urge child vaccination . The UN Health Agency is warning of need for increased surveillance as there are international ramifications and concerns as poliovirus is highly contagious [1,4]. The Zambian Ministry of Health has implemented surveillance in health facilities, communities, and points of entry to mitigate future cases .