PRINCETON – When Rio de Janeiro was awarded the 2016 Olympic Games, the Zika virus had yet to reach Brazil. Now, after billions of dollars have been invested in preparing for the Games, Rio de Janeiro state has the second highest number of suspected Zika virus infections. Should the 2016 Summer Olympic Games be postponed or moved elsewhere?

This is a difficult decision, and the facts are still not clear enough. That’s why, last month, I joined 223 scientists, bioethicists, and public health experts in signing an open letter to Margaret Chan, Director-General of the World Health Organization. We are asking her to convene an independent group to advise both the WHO and the International Olympic Committee in a transparent process that would provide the evidence needed to balance public health against disruption to a great international sporting event.

The Zika virus is not new, but the strain that entered Brazil in 2013 is more dangerous than any known variant. A study published last month in the New England Journal of Medicine has confirmed that when the virus infects a pregnant woman, it can impede brain development in her fetus – a rare condition known as microcephaly. In severe cases, microcephaly is incompatible with the child ever living an independent life.

The Brazilian outbreak of Zika virus has been linked with a particularly devastating form of microcephaly known as fetal brain disruption sequence. Until Zika spread to Brazil, only a handful of fetuses with that condition had been recorded. Recently, a study of 35 fetuses with microcephaly in Brazil found that 11 of them had fetal brain disruption sequence.

The latest research confirming the link between Zika and microcephaly builds on a study, published two months earlier, of pregnant women in Rio de Janeiro with symptoms suggestive of Zika infection. Of those in which the infection was confirmed, 29% were carrying a fetus with a serious abnormality. None of the women in whom the infection was not confirmed had an abnormal fetus.

In adults, Zika brings a high fever and a rash; but, more worryingly, there may be an association with Guillain-Barré syndrome, a devastating and sometimes life-threatening disorder that can cause paralysis for several months or even years. The risk of developing Guillain-Barré syndrome for an adult infected with Zika remains unknown.

The WHO has declared Zika a "Public Health Emergency of International Concern,” and has advised pregnant women not to travel to Brazil. But how does staying at home help a pregnant woman if others bring the virus back from Rio? With 500,000 people expected to attend the Olympics, that is likely to happen in several countries in which Aedes aegypti, the mosquito that can transmit the virus, is present.

Two arguments will be made against this reason for postponing or moving the Games. First, the risk of infection in winter is low, as there are fewer mosquitos, and Brazil is using the military to spray areas where mosquitos breed. But, as Amir Attaran, a professor of law and population health at the University of Ottawa, argued recently in the Harvard Public Health Review, the transmission of dengue fever, a virus that is related to Zika and transmitted by the same species of mosquito, declines but does not cease in the Rio winter.

As for the spraying, Attaran casts doubt on its efficacy, given that Rio has seen a sharp, unexplained increase in dengue this year. The neighborhood of Olympic Park, where events will take place, has had more cases of dengue in the first quarter of 2016 than in all of 2015.

The second response is to say that millions of tourists visit Rio anyway, so the spread of Zika to other countries with the Aedes aegypti mosquito is inevitable; it makes little difference whether visitors who bring it back to their home countries went to the Olympics. But, as a result of the Brazilian outbreak, research into Zika has accelerated, and it is reasonable to hope that a vaccine, anti-viral drug, or other means of combating the infection or its spread will be found.

So when the infection spreads is critical. If the Olympics go ahead, visitors will come to Brazil from many more countries than would otherwise be the case. If they bring Zika back to regions with Aedes aegypti and inadequate health-care systems – West Africa or South Asia, for example – millions of infections could occur before effective means of prevention or cure are developed.

The WHO has said that canceling the Olympic Games would "compromise the huge investment that athletes and others have made in preparing for what should be a fantastic occasion.” No doubt it would, but that needs to be balanced against the risks of spreading a dangerous virus. The Olympic Charter asserts that social responsibility and respect for universal fundamental ethical principles are at the core of the Olympic movement.

It is neither socially responsible nor ethical to ignore the risks that the Zika virus poses to adults and to children yet to be born. Perhaps the risks are low enough to justify going ahead with the Rio Olympics (which in any case could be postponed rather than canceled), but perhaps they are not. Until qualified experts have laid out all the facts, the world should stay away.

Peter Singer is Professor of Bioethics at Princeton University and Laureate Professor at the University of Melbourne.

Copyright Project Syndicate.