A baby with microcephaly, a defect linked to the Zika virus, in Brazil. Credit:AP University of Ottawa public health professor Amir Attaran was among 150 public health experts who wrote an open letter to the WHO last month calling for the Games to be moved or postponed. In a subsequent letter published in the Harvard Public Health Review, he controversially argued the influx of 500,000 people into a country where Zika was rampant could result in a global epidemic if travellers returned home with infections. Athletes including Matildas co-captain Lisa da Vanna and basketballer Andrew Bogut have raised concerns about Zika and Australian golfer Marc Leishman has pulled out in fear of passing the disease to his wife. Professor Attaran was asked two weeks ago by WHO director-general Margaret Chan to attend Tuesday's meeting on Zika, but he said the invitation was revoked when he refused to sign a clause that would prevent him from discussing the deliberations.

An email chain that accidentally copied in Professor Attaran indicated WHO officials were thrown when he crossed out of the confidentiality agreement a clause that specified the undersigned agreed "not to discuss the deliberations and decisions of the advisory process to third parties except as agreed to by WHO". He added the words: "Not applicable to inconsistency with academic freedom". A confused official forwarded the form to his colleagues, asking, "Have we ever encountered this before with another expert from Academia?". The official then drafted a reply that politely requested Professor Attaran to reconsider, given the other participants had signed and it was better for all committee members to participate on an equal footing. I know not everyone in the room would think that. Professor Amir Attaran

But a more senior colleague said the message should be stronger: "ie If you don't agree we are sorry that we won't be able to invite you to the committee". Unfortunately, the workshopping of a suitable response to Professor Attaran was included beneath the email that was ultimately sent. "Thanks for your email, and for sharing the honest deliberations of your colleagues," Professor Attaran replied. But he said he was unable to sign the clause because it effectively prevented him from repeating his own words, and could create an impression of consensus even if there was none. He told Fairfax Media he was happy to sign the clauses that undertook not to disseminate confidential or proprietary information, but covering up dissent engendered "group think".

"This one paragraph is really poisonous," Professor Attaran said. "We now have a WHO comms person saying, 'This is what the scientists think'. With all due respect, I know that not everyone in the room would think that. I know beyond doubt, but of course the people in the room could be sued for saying so." Professor Attaran disagreed with the WHO's risk assessment of staging the Olympics in Brazil on the basis that even if the risk of the virus spreading globally was small, its consequences would be catastrophic. Zika is a mosquito-borne virus that can cause birth defects in babies born to women who were pregnant when they were infected, including microcephaly, which results in an unusually small head. Among adults it can cause Guillain-Barre syndrome, an autoimmune paralysis that can be deadly, although 80 per cent of infected people are asymptomatic.

As most countries do not have the mosquito vectors that would allow the disease to spread, for the Rio games to spark a global pandemic an athlete or visitor from a relevant country would need to be bitten by a mosquito carrying Zika and then to return home and be bitten by another mosquito of the right species within 10 days. Marie Bashir Institute for Infectious Diseases virologist Grant Hill-Cawthorne said the WHO had made the right call and that healthy people who were not pregnant were unlikely to be harmed by Zika. The Olympic village would be subjected to heavy insecticides, he said. "Mosquito-borne viruses are everywhere," Dr Hill-Cawthorne said. "To have a policy to avoid any area where there's mosquito-borne disease is going to destroy tourism, destroy the economy and bring the world to a standstill. So I think we need to have a sensible approach to this."

The WHO said in a statement that the invitation to Professor Attaran was not rescinded, but it could not proceed when he did not file the necessary documents. The meetings are confidential to promote frankness among the experts, but a report on deliberations is made public a few hours after them in the interests of transparency, the statement said. On this occasion, there was consensus among the committee members on the risks of the Olympics going ahead. " The committee considers views/perspectives as well as scientific evidence," the statement said. "Decisions are based on this evidence.