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.
In West Africa, a 19-month-old boy was paralyzed in Guinea on 20 July and then traveled to the Malian capital for treatment, where polio was confirmed. The type 2 VDPV is closely related to one detected in the Kankan region of Guinea in 2014, which researchers believe has been circulating undetected for 2 years. Because health systems in West Africa have been decimated and immunization rates have fallen precipitiously, the World Health Organization (WHO) deems the risk of spread high.
Ukraine, with its social unrest and decline in childhood immunization—about 50% of children were un- or underimmunized in 2014—has also been considered at high risk of a polio outbreak. Two children there, a 4-year-old and a 10-month-old, were paralyzed on 30 June and 7 July, respectively; VDPV type 1 was confirmed 31 August. The risk of spread within Ukraine is high, WHO says, although the risk of international spread is low.
The two outbreaks—polio is so contagious that a single case is considered an outbreak—underscore the urgency of all countries stopping the use of the live oral vaccine, a key strategy in the polio eradication endgame, says Hamid Jafari, who directs global polio eradication at WHO in Geneva, Switzerland. For now, whether a wild or vaccine-derived virus causes an outbreak, the response is the same: Hit the area hard and fast with massive immunization rounds and quash it within 4 months. Emergency campaigns could start as early as this week.