04-04-2020, 10:56 AM
(04-03-2020, 11:17 PM)danbrotherston Wrote: The answer to the first part of your question is, no, it doesn't have to remain in circulation.
SARS (v1?) died out, because everyone who was sick with it got better or died, and there were no new patients, thus the virus died out (of the human population anyway). The key to doing this is controlling the R0. This is much harder for a virus like SARS-CoV-2 because it's R0 is much higher to begin with (for various reasons including pre- and a-symptomatic transmission), but it is still possible to get R0 under 0 and thus starve the virus of new hosts. China has already (or at least claimed to have) done this in specific areas.
If you can do this in a local area then you can shrink the infected population then reduce restrictions while controlling new cases through tracing, testing, travel restrictions, and quarantining, you can be free of a virus, even without herd immunity. In theory at least--I think we're yet to see if this is possible in practice with this virus but the short answer is no, it's not necessarily the case that everyone must get the virus before herd immunity is acquired.
It's also not necessarily the case that "everyone getting it" would actually provide protection either, this virus does mutate, and while I think experts haven't seen it mutate sufficiently to avoid new immunity (nor do experts even know how long or effective acquired immunity is for this virus), it's entirely possible that it could mutate in that way in the future, just as the flu and cold viruses do every year. In fact, again, I'm not an expert, but I believe that everyone getting it, would make it more likely that it would mutate in that way.
I am curious though, why you believe that a vaccine that is available in 18 months would take 2 years to give us herd immunity? I believe the 12-18 month estimate is for broad availability of the vaccine--and we're already 2 months into that timeline.
I understand that getting the ratio down to close to 0 is possible, but the possibility of doing that here in Ontario seems remote. Further, even if we could, we will remain open to other jurisdictions to some extent (I would say a large extent, depending on the perspective you take). There is no chance that we will close our borders completely. The deprivation wouldn't be worth it by anyone's estimation, I don't think. While there are cases somewhere, there is a chance that they will spread here. And that's even if
My understanding was that 12-18 months was an extremely ambitious timeline, possible but never before achieved, so this is why I think projecting the presence of the virus for two years seems logical. But I was under the impression that the timeline was ambitious for having the vaccine acceptable, not broadly available. If 18 months is the timeline for a vaccine to be validated and administered broadly, then I guess that's the timeline for the virus to be eradicated.
(By the way, both 18 months and two years are very long lengths of time to have anything like the kinds of controls and shutdown we're experiencing now...)