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Two polio cases have been reported in Ukraine, where some parents are fearful of vaccinations. Above: A child receives the diphtheria, whooping cough and tetanus vaccine in a children's hospital in Kiev.

This month, three new cases of polio, all caused by a strain derived from the vaccine itself, have struck two children in Ukraine and an infant in Mali, the World Health Organization (WHO) has confirmed. Both countries had previously been polio-free, which leads to questions about how and why these outbreaks occurred, and how concerned we should be.

To help sort through the issues, we consulted Oliver Rosenbauer, communications officer from WHO's Global Polio Eradication Initiative; Leilia Dore, communications officer for polio at WHO; and other resources available from WHO.

How can a polio virus migrate from a vaccine to infect another person?

The answer is that orally given polio vaccines contain a weakened live virus, and those who receive the oral vaccine can shed the virus in their stools. If the virus — which though weakened is still alive — then comes into contact with someone who has not been vaccinated, two things can happen. In some cases, the virus will act as a vaccine in its own right to passively confer immunity. More than 99.9 percent of the time, this is a good thing. The reason: The more people who are immunized, the less possibility that the virus will spread, because there will be fewer people for it to spread to. The bottom line, after all, is that the only way to interrupt the person-to-person spread of the polio virus, and to eradicate the disease, is to immunize everyone.

So what is the rare bad thing that can happen?

Over the past 10 years, more than 10 billion doses of OPV [oral polio vaccine] have been administered to more than 2.5 billion children, preventing more than 650,000 polio cases every single year. In that same period, only 21 cVDPV [circulating vaccine-derived poliovirus] episodes are known to have occurred, resulting in 622 cases. This is extremely rare.

And this can happen when not everyone in a population is immunized?

When "gaps" in immunization occur, the virus can continue to circulate from person to person. In addition, since the virus can be spread through feces, if there's inadequate sanitation, sewage mixed with the water supply might be another possible route for transmission. Meanwhile, the longer the virus remains in circulation, the greater the risk that this very weak vaccine virus will genetically mutate over time to the point where it regains the ability to cause paralysis. That takes about 12 months and occurs very rarely, but that is what happened in these cases.

How low are the immunization rates in Ukraine and Mali, and why?

In Ukraine, as many as 50 percent of children are under-immunized, not having received full vaccine series. Although Mali's national rates as a whole are relatively high at 83 percent, in the capital of Bamako it is only 63 percent. In Ukraine, vaccine supply and logistics — getting the right number of vaccines to the district level — have been an issue. In addition, a large minority of people in Ukraine have lost trust in vaccines, due to a high profile case from 2008 when a boy died who had recently received the measles and rubella vaccine. His death was not caused by the vaccine but the media reported it as if it had.

In some areas in Mali, infrastructure issues have made it difficult to get vaccines to children. There is also cause for concern in Guinea. The child who contracted polio in Mali was from Guinea, where surveillance and immunization rates dropped due to Ebola. This means that the virus could be circulating in Guinea, too. Either way, we need to fill the gaps.

So even if the risk is rare, why use the live vaccine rather than the inactive vaccine, which is given by injection?

Not only is the oral vaccine both easier and less expensive to administer, it also confers an intestinal immune response that is more effective in guarding against transmission.

What will be the response in Mali and in Ukraine?

The solution is the same for all polio outbreaks: to immunize every child several times with the oral vaccine to stop transmission. At least three doses of OPV are needed to stop transmission. But when an outbreak occurs, five large scale campaigns are planned. The more times we can vaccinate a child, the stronger their immunity will be. In both countries, large-scale outbreak response is being planned: The main components are to rapidly immunize as many children as is possible; to put the surveillance system on high alert, training health workers to identify cases and testing the contacts of the paralyzed children as well, so that we will understand exactly where the virus is: and to generate momentum through political commitment and community outreach so that everyone understands the importance of the repeated campaigns.

What do you say to someone who is afraid of vaccinations, possibly because of these latest cases?

It is precisely because of these cases that vaccination is so important. There is no cure for polio. Once one has it, one has it for life, and it is a devastating and painful disease causing lifelong paralysis. The only way to protect yourself is to ensure you are vaccinated.

Copyright 2015 NPR. To see more, visit

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