Countries in Ebola-ravaged West Africa are on high alert after a case of polio was confirmed in a toddler in Mali on 7 September, just a week after two cases were reported in southwestern Ukraine. The outbreaks are unrelated, but in both instances the culprit is a so-called vaccine-derived poliovirus (VDPV). These rare VDPVs arise when a virus used in the live vaccine reverts from its weakened form and regains its virulence—a danger when vaccination rates are low, as they are in both places, allowing the vaccine strain to circulate and accumulate genetic mutations.
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