Two children have been paralyzed in the first polio outbreaks in Europe for five years. One child was four years old and the other was ten months old when they began exhibiting paralysis.

The outbreaks both occurred in Ukraine, where the risk of contracting polio is particularly high since the vaccine coverage is limited.

"50% of children are inadequately vaccinated, and they're now at risk of contracting this disease, and that's why it's very critical to rapidly and fully implement an outbreak response in the country," Oliver Rosenbauer, a member of the Global Polio Eradication Initiative from the World Health Organization (WHO), told IFLScience. An outbreak response will involve vaccinating possibly millions of children in a very short space of time.

There is no cure for polio and so the only way to prevent it is through extensive and thorough vaccination. These new cases arose due to inadequate vaccination in the Zakarpatskaya oblast in south-western Ukraine. But even vaccination itself can sometimes lead to an outbreak.

One type of vaccine, oral polio vaccine (OPV), contains a weakened form of the polio virus. For a six to eight weeks after vaccination, the virus is excreted but is incapable of infecting people in its attenuated state. However, in extremely rare cases, the virus can be shed for longer periods and sometimes mutate into an infective form – vaccine-derived poliovirus (VDPV).

Rosenbauer told IFLScience that any rapid outbreak response needs to be tailored to the country, but the basic premise is the same: "At its core, you need to vaccinate a lot of children, several times. And urgently."

Poliovirus can spread easily across long distances via carriers that don't show any symptoms of the virus. Vaccinating in the immediate vicinity is important, but not enough. The outbreak response needs to be broad and exhaustive. 

The virus spreading across borders is also of concern. However, in neighboring countries, for example Poland, they have relatively high levels of vaccination cover and vigilant border control. "The chances, I think, are relatively low that it travels internationally, or would cause outbreaks internationally, but I would not rule it out," Rosenbauer concluded.

"The much greater risk is that it causes more outbreaks in Ukraine."

The last cases of polio in Europe were in Russia in 2010, when the virus traveled from Tajikistan in central Asia. However, the outbreak was very rapidly stopped by a swift response. The same thing can be done in Ukraine if the rapid outbreak response is fully implemented.