In opposition to a regular randomized controlled trial, in which you would vaccinate a patient and wait for that person to be exposed to the disease to see whether the vaccine would be then protective, in a challenge trial you actually squirt the virus directly into the nose of a person who's been either vaccinated or not, to see if those who are vaccinated are more protected than those who are not.
We haven't done any challenge trials in Canada thus far with any virus, but two challenge units have been built as part of a CFI, one in Dalhousie, where they are doing that first phase one study, and one at the MUHC in Montreal. Those challenge units were built to do studies on influenza vaccines after vaccination, but there haven't been any. I think that to do a challenge trial, you would need to be in at least phase two or three of a vaccine, because before that you would still be studying the safety of the vaccine and its immunogenicity, but instead of vaccinating and then just letting people be and hoping that they would be exposed soon enough to see whether that vaccine works, you would have a challenge trial.
They've also done that for norovirus vaccines.