Last month, Canadian professor Amir Attaran and more than 200 colleagues wrote an open letter to WHO, accusing it of shirking its responsibilities by not considering whether to recommend delaying or canceling the Rio de Janeiro Olympics.
He then received an invitation from WHO Director-General Dr Margaret Chan to sit on WHO's independent committee of Zika experts.
That committee is meeting Tuesday and is expected to announce the results of the meeting later in the day.
WHO then sent him a number of forms to sign - including one with a clause that deems the committee deliberations, including his own comments, to be secret. Attaran signed and returned the forms, but crossed out that particular clause.
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WHO spokesman Christian Lindmeier said today that because Attaran did not agree to the standard confidentiality form required of all experts, he was not issued a formal invitation and, thus, there was nothing to rescind.
"The confidentiality agreement is to hold confidential deliberations and decisions of the committee. That's very important to allow for open and unbiased discussion," Lindmeier said.
"But the transparency is assured afterwards by making all public what needs to be made public."
Attaran said that barring scientists from repeating their own comments made during the emergency committee deliberations was unjustified and that there were no such requirements tied to the initial offer he received to participate.
WHO's emergency committees are convened according to the International Health Regulations, known as the IHR, a binding international treaty that sets out protocols for how to fight outbreaks.
Lindmeier said the open letter authored by Attaran and others would be part of the background materials considered by the Zika expert meeting.