Public health event
Two cases of paralytic poliomyelitis caused by circulating vaccine-derived poliovirus type 1 (cVDPV1) were confirmed in Ukraine on 28 August 2015. The cases, a 4-year-old child and a 10-month-old infant, had onset of paralysis on 30 June and 7 July respectively and the positive stool samples were collected from 5–10 July 2015. The genetic similarity between the isolates indicates active transmission of cVDPV1. Both children are from the Zakarpatskaya oblast [region], in southwestern Ukraine, bordering Romania, Hungary, Slovakia and Poland. Ukraine has been at high risk of vaccine-preventable diseases outbreaks for several years due to persistent low routine vaccination coverage.
Main conclusions
Global efforts to immunise children with the oral polio vaccine (OPV) have reduced wild poliovirus cases by 99.9% since 1988. OPV is a very safe vaccine but, because it contains live weakened viruses, it can on rare occasions mutate into circulating vaccine-derived poliovirus (cVDPV). The risk of cVDPV strains emerging from OPV is higher when polio vaccination coverage is low, as has been the case in Ukraine for several years.
The outbreak of poliomyelitis in Ukraine is not unexpected, given the low polio vaccination uptake. In fact, the overall low vaccination coverage in the country means that Ukraine is at increased risk of outbreaks of other vaccine-preventable diseases, including measles and diphtheria.
An outbreak of cVDPV in a poorly-vaccinated population is a serious public health event on a par with an outbreak of wild polio virus. The eventual size of a cVDPV outbreak depends on a number of factors, including the size and density of the susceptible population; the duration of virus circulation before the outbreak is detected; the time taken from detection to response vaccination and the potential for the virus to be transported to susceptible communities elsewhere.
It is likely that the cVDPV1 strain has been circulating for many months in Ukraine and that the virus could be found in other parts of the country. Based on experiences from other similar events in the past, we can assume that the risk of more children presenting with paralytic poliomyelitis in Ukraine is high and that it will remain high until large-scale supplementary immunisations have been implemented, in accordance with WHO recommendations for the control of polio outbreaks.
There is a risk that cVDPV will be imported and transmitted in the EU via a recently infected person shedding the virus, particularly if that person enters an area of the EU with low vaccination coverage. Sub-optimal surveillance practices increase the risk of delayed detection of the virus in both the environment and the population.
The highest risk of importation and onward transmission of cVDPV1 is likely to exist in the border areas with Ukraine, particularly in areas where under-vaccinated populations are concentrated on both sides of the border and where there is a high volume of border crossings in both directions.
However, the risk that importation of cVDPV from Ukraine to the EU/EEA would result in a case of paralytic poliomyelitis is low, given the high polio vaccination uptake in EU/EEA Member States.