More than three decades after the eradication of smallpox, US officials say it is still not time to destroy the last known stockpiles of the virus behind one of history's deadliest diseases.
The world's health ministers meet this month to again debate the fate of vials held under tight security in two laboratories - one in the US, another in Russia.
The virus is being used for carefully limited research to create drugs and safer vaccines in case the disease ever returns, through terrorism or a lab accident or if all the world's stocks aren't really accounted for.
Although member countries of the World Health Organisation agreed years ago that eventually the last virus strains would be destroyed, they have still not agreed on when.
Some countries say it is long overdue. But the World Health Assembly, the WHO's decision-making assembly, has repeatedly postponed that step.
Today there are new generations of smallpox vaccine, and two long-sought antiviral treatments are in the pipeline, but experts are not convinced that is enough.
"Despite these advances, we argue that there is more to be done" in improving protections, Dr Inger Damon, poxvirus chief at the federal Centres for Disease Control and Prevention, wrote in the journal PLoS Pathogens. She co-authored the article with experts from the US and Brazil.
In addition, a recent WHO meeting raised a new spectre: advances in synthetic biology mean it may be possible to create a version of smallpox from scratch.
"The synthetic biology adds a new wrinkle to it," said Jimmy Kolker, health and human services assistant secretary for global affairs.
"We now aren't as sure that our countermeasures are going to be as effective as we'd thought even five years ago."
For centuries smallpox killed about a third of the people who became infected. But thanks to worldwide vaccination, in 1980 it became the only human disease so far to be declared eradicated from the environment.