Incidence of circulating vaccine-derived poliovirus (cVDPV ) decreased during the period from January 2015 to May 2016, but cases were reported in a larger number of countries, according to a report from the Centers for Disease Control and Prevention.

Seven countries reported cases of cVDPV, up from four during the previous reporting period. Seven cases were reported in Guinea, 11 in Laos, 10 in Madagascar, 2 in Myanmar, 1 in Nigeria, 2 in Pakistan, and 2 cases were reported in the Ukraine. Case incidence in Nigeria and Pakistan was significantly less than in the previous reporting period.

Immunodeficiency-associated VDPVs were reported in 21 countries during the study period, up from 8 countries in 2014; however, this was due to a newly implemented intensive surveillance program. Ambiguous VDPVs were reported in 19 countries.

“The expansion of environmental surveillance in countries at high risk has increased the sensitivity of poliovirus detection. However, detection of polioviruses from sewage presents logistical and technical challenges, including determination of VDPV genetic signatures. Determination of epidemiologic linkages from sequence data in environmental isolates represents an additional challenge,” the CDC investigators said.

Find the full report in Morbidity and Mortality Weekly Report ( doi: 10.15585/mmwr.mm6530a3 ).