Mehau Kulyk/SPL/Corbis

Six vials of smallpox virus discovered at the US National Institutes of Health (NIH) in Bethesda, Maryland, in June are marked for death by autoclave — yet they linger in a high-security freezer. Chalk it up to bureaucracy: an international agreement requires that the World Health Organization (WHO) witnesses the destruction of the samples, but the agency is overwhelmed by the Ebola outbreak in West Africa.

The delay adds another twist to the strange tale of the vials, which sat forgotten for six decades in an unmarked cardboard box. It also mirrors the fierce debate over whether to destroy the two other known stocks of smallpox in the world. That decision is on hold while the WHO determines whether smallpox virus could be synthesized from scratch by those with nefarious intent, and whether virus stocks should therefore be preserved to help develop countermeasures against a terrorist attack.

The NIH smallpox was discovered in an agency ‘cold room’ used by the US Food and Drug Administration; soon after, the vials were shipped to the US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia. That agency, and its Russian counterpart — VECTOR, in Koltsovo — are the only two laboratories authorized to work with smallpox under an international agreement that also calls for the eventual destruction of all stocks. (The virus was eradicated in the wild in 1979.)

The CDC promised to destroy the NIH samples immediately, with WHO officials present. But that has proved more difficult than anticipated. The WHO has dealt with just one other surprise smallpox discovery: viral DNA found in a South African laboratory in 2013. WHO officials were present when that DNA was destroyed in January this year — but no WHO employee is certified to enter the CDC’s high-security smallpox lab. This means that a WHO official must fly to Atlanta to witness the destruction of the virus on closed-circuit television. Arranging the trip has been made more difficult by the Ebola crisis, says Alejandro Costa, head of the WHO team in Geneva, Switzerland, that monitors smallpox issues.

The authorized stocks of smallpox at the CDC and VECTOR are also in limbo. For more than 20 years, the US and Russian governments have fought efforts to set a deadline to destroy the samples. They argue that the virus should be preserved to test vaccines and antivirals that might be needed to respond to an accidental smallpox release or a terrorist attack.

In May, the WHO’s policy-making body, the World Health Assembly, again put off a decision on the fate of the smallpox stocks. Unusually, the group did not say when it might revisit the issue.

Virologist D. A. Henderson, who led the WHO’s programme to eradicate smallpox, argues that all stocks should be destroyed, in a kind of mutual disarmament. “Anyone found with isolates of smallpox virus after point X would be guilty of crimes against humanity,” says Henderson, now at the Center for Health Security in Baltimore, Maryland.

At a meeting last week, members of the WHO Advisory Committee on Variola Virus Research discussed the risk that a bioterrorist could synthesize smallpox — a situation that might add weight to the argument for keeping the natural virus. Costa says that the WHO will postpone any decision on stock destruction until a report commissioned by the agency is released, which will be in 2016 at the earliest.

The NIH smallpox is likely to be destroyed well before then: Costa estimates by January 2015. In the meantime, experts say that the virus is safe at the CDC.