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An incident that took place at a Dutch polio vaccine production facility late last year is a critical reminder of a major challenge the world faces if and when polio eradication is completed: How do we keep polio from re-establishing itself, given that laboratories and vaccine manufacturers in numerous countries will need to continue to work with the viruses?

An employee of Bilthoven Biologicals in the Netherlands somehow became infected with type 3 polio, one of the two strains of polioviruses that have been eradicated. The company, owned by the Serum Institute of India, makes inactivated polio vaccines, using polioviruses that are killed in the production process.


The employee had been previously vaccinated and did not develop paralysis. But the person shed infectious polioviruses in his or her stool for weeks, posing a risk to others. The person lived in a part of the Netherlands where the vaccination rate for polio is less than 90%, exacerbating the risk.

The individual agreed to go into isolation in a special residence provided by the company, a period that lasted 33 days. The case was reported Thursday in the journal Eurosurveillance, which is published by the European Centre for Disease Prevention and Control.

The event was initially undetected; it only came to light when routine wastewater surveillance — which the country’s national authority for containment of polioviruses requires be conducted around facilities that work with polioviruses — detected viruses in a sample collected on Nov. 15.


“This event shows that incidents that lead to a breach of containment and even an infection can remain unnoticed and not reported if routine monitoring is not in place,” the authors wrote. “We believe our environmental surveillance strategy has proven very valuable and strongly propose that other countries implement a similar system.”

The event — which involved a public health investigation, the testing of dozens of employees, and prolonged isolation of one of them — is a harbinger of the efforts and expense the world is going to have to undertake to ensure that polioviruses do not escape and resume transmission once eradication is achieved, said Kimberly Thompson, a polio expert who is president of the nonprofit organization Kid Risk.

“Containment is a much bigger issue than people realize,” Thompson told STAT. She noted the last known case of smallpox — the only human disease to date that has been eradicated — was in a British lab worker, before smallpox samples in laboratories were rounded up and destroyed.

The viruses found in the sewage at Bilthoven had mutations that suggested they had been shed by an infected person, and had not been released into wastewater by accident, as happened at a GSK vaccine production facility in Belgium in 2014.

Fifty-one employees were identified as having had access to the type 3 viruses. All were asked to provide stool and blood samples. Blood testing showed that one person had signs of a recent infection; that individual’s stool samples tested positive. None of the other employees tested positive.

The paper gives no indication of how the individual became infected.

Because the vaccination rate where the individual lived was below 90%, it was proposed the person go into isolation in a specially designed residence owned by the company in a place where the vaccination rate was above that threshold. The person had to use a special toilet unconnected to the sewer system for the duration of the isolation and all potentially infectious waste materials from the stay were incinerated.

The individual, who spent Christmas and New Year’s in isolation, was allowed to meet other people in outdoor settings, as long as there was no physical contact. The person was released from isolation after three consecutive stool samples tested negative.

The authors of the Eurosurveillance article noted the extended period during which the person shed polioviruses — 51 days — was unusual. They could detect no reason for it. But they found no evidence the individual infected anyone else, having tested stool samples from the people the individual was in close contact with in the period from Nov. 15 to Dec. 8, when the first positive test results came in.

A paper Thompson co-wrote in 2018 reported on six previous releases of eradicated polioviruses — types 2 and 3 have been eradicated — from vaccine production facilities.

“Ongoing [inactivated polio vaccine] production and storage and use of [live polioviruses] in facilities will imply ongoing risks of potential future poliovirus infections and spread in populations,” Thompson and her co-authors wrote in the article, published in the journal Future Virology. “Managing containment risks represents a critical component of the polio endgame and risk management efforts will increase prices for [vaccine] production.”

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