Stopping polio transmission will require addressing persistent challenges to vaccinating every child.
A highly infectious disease that invades the nervous system, polio can render an individual completely paralyzed in a matter of hours. Because of several global efforts, the incidence of polio has dramatically declined around the world and has even been completely eradicated in some areas, such as the United States. However, individuals who are most vulnerable continue to struggle with the disease, particularly children living in poor countries and marginalized communities.
Now, new research from the US Centers for Disease Control and Prevention (CDC) published in a recent Morbidity and Mortality Weekly Report, shows that although there were fewer cases of wild poliovirus type 1 (WPV1) reported in 2017 compared with 2016, transmission of the disease has remained uninterrupted in 3 countries: Afghanistan, Nigeria, and Pakistan. Transmission of circulating vaccine-derived poliovirus type 2 (cVDPV2) has also remained uninterrupted.
Overall, there were fewer cases of WPV1 reported in 2017 compared with 2016. A total of 37 WPV cases were detected in 2016; 13 of which were reported in Afghanistan, 20 in Pakistan, and 4 in Nigeria. In 2017, fewer cases of the disease were reported—a total of 22—with 14 reported in Afghanistan and 8 in Pakistan. Between January 1, 2018, and March 30, 2018, 8 cases of WPV1 were reported, with 7 detected in Afghanistan and 1 reported in Pakistan.
Five cases of cVDPH were reported in 3 different countries in 2016. In 2017, 96 cases of cVDPV2 were reported. Furthermore, an outbreak of disease that sprung up in the Democratic Republic of the Congo has continued into 2018, with 4 cases reported as of April 24, 2018.
“Isolation of cVDPV2 from environmental samples in Mogadishu, Somalia, in late 2017 and early 2018, and related cVDPV2 from environmental samples in Nairobi, Kenya, in early 2018, has confirmed long-term cVDPV2 transmission, in a broad area, although no associated polio cases have been detected to date,” the authors added.
According to the CDC, the strategy for eradicating polio is focused on preventing infection by vaccinating every child to stop transmission of the disease. Among infants 1 year of age, the estimated global coverage with 3 doses of the polio vaccines through routine immunization services proved to be 85% in 2016—the most recent year for which data is available.
Coverage with the third dose of the OPV (OPV3) vaccine in the 3 endemic countries was 60% in Afghanistan, 72% in Pakistan, and 49% in Nigeria. “OPV3 coverage is substantially lower in areas of WPV transmission,” the authors stressed, “where children in high-risk mobile populations or areas of conflict are repeatedly missed.” In addition, OPV3 coverage was “substantially lower” in areas with known cVDPV emergence and transmission.
Supplementary immunization activities (SIAs) have been carried out in remote areas in an attempt to vaccinate those children without ready access to the vaccine; 186 of these activities were conducted in 2016 in 5 different World Health Organization regions and about 2 billion doses of the vaccine were administered. A total of 172 SIAs were conducted in 2017, with about 1.79 billion doses administered.
“Intensified SIA schedules and efforts to reach previously unvaccinated children, along with an expansion of community-based initiatives employing local permanent vaccinators and ensuring worker safety have helped reduce the number of WPV cases,” the authors wrote. Furthermore, improvements in microplanning and SIA implementation have helped reach unvaccinated children and islands that are particularly difficult to access.
However, more work is needed in order to achieve polio eradication.
“Intensification of efforts to improve the quality of immunization and surveillance activities and to develop additional innovations in addressing persisting challenges is necessary,” the authors conclude. “Until poliovirus eradication is achieved, all countries must remain vigilant by maintaining high population immunity and sensitive poliovirus surveillance.”