It's official. Xenotransplants from non-human primates would expose “recipients, their close contacts, and the public at large . . . to significant infectious disease risk” (see page 549). This strong language comes from the US Food and Drug Administration (FDA), which just three years ago wanted to leave the regulation of xenotransplantation to local institutional review boards. In private, some FDA officials now also argue that the question of whether to proceed with human trials of living parts of any animals is so important that a federal policy ruling is needed.

That it has taken the FDA until now to decide the obvious says much about the reluctance in the United States to regulate if this can be at all avoided. The potential benefits of using primates are tiny — breeding enough clean monkeys or baboons to substantially shorten organ waiting lists is impracticable. The potential cost is the creation of more AIDS, Ebolas or Marburgs.

It would be a mistake to conclude that the FDA's exclusion of primate donors means that other animals, such as pigs, are safe. The benefit side of the equation is better with pigs, but the risk side is largely unknown. The US Department of Health and Human Services is setting up a federal advisory committee on xenotransplantation, to include experts, representatives of other stakeholders and the inevitable bioethicists. The US administration should put human trials of pig organs on hold at least until this committee has had its say.

But it would be unfortunate were the prospect of a moratorium to have the perverse effect of deterring researchers and funders from supporting continued research on xenotransplants outside the clinic. Indeed, perhaps a rule needs to be invented, which says that whenever a government calls a moratorium, it should also make sufficient funds available for research to fill the gaps in knowledge that made it necessary in the first place.