He gives it even odds that H5N1 mutates into a form easily transmitted between people.
As a general rule, virologists are rather reluctant to attempt to make projections beyond what they can support with good evidence, and I think Dr. Webster's statements illustrate why; though the wording he chose suggests that he was speaking somewhat off-the-cuff, his remarks are subject to over-interpretation, and have been widely broadcast. As one of the world's leading authorities on influenza, I'd expect him to readily admit, if pressed, that not much is known about precisely what mutations the virus would need in order to achieve efficient, sustained human-to-human transmissibility; certainly not enough to support the assigning of any particular number to the probability of such an event occurring. In addition, the omission of a time frame seriously compromises the value his attempt to do that. Within five months? Five years? Fifteen years? Makes a lot of difference.
He says society may have to face the possibility of fifty percent of the population dying.
I was surprised to see this, since it's more than three times the next highest estimate I've heard from a source I regarded as knowledgable -- and
that was maybe twice what most epidemiologists are willing to go (at least when speaking publically). I can only assume that Dr. Webster was including a rather pessimistic outlook on the possible effects of social disruption, though I can't imagine why he would feel that his expertise would make his guesses about that any better than anyone else's.
Is there some sort of mainstream scientific opinion about this virus?
The standard approach is to extrapolate from previous pandemics. Because of the surprisingly high case fatality rate among those humans who have been infected with H5N1 in its current form (actually just a bit
over fifty percent), epidemiologists as a whole feel that this virus must be treated as having at least the same potential as the H1N1 virus which caused the Spanish flu pandemic of 1918 -- which had an attack rate (the number of people infected) of about 30 percent, and a case fatality rate of about 2.5 percent. The fact that the human population has no immunological memory of this virus doesn't mean that everyone will be equally
susceptible though, and while higher attack rates are possible, the likelihood goes down the higher you go. The main reason that the case fatality rate is expected to be about what it was for the Spanish flu is simply that we haven't encountered an influenza virus with a higher rate before; there isn't any specific scientific basis for assuming that the (very) high CFR won't be maintained in a pandemic strain.
One thing that could affect attack rates considerably is the degree to which people are prepared to practice "social distancing". If individuals and families buy groceries on the same "just-in-time" basis that modern businesses use in aquiring the goods they need, folks will be out and about rubbing elbows with each other a lot more, and a lot more of them will catch the bug. Another thing Webster mentioned is that he has stored a three-month supply of food and water at his home in case of an outbreak.
