So, I'm an economist and not a public health expert, but I still like to try and game out how things might go in the future just for fun. For the record, I've pretty consistently underestimated the severity of the virus, and it just always seems to turn out a lot worse than I ever thought realistically possible. That said, I'm not currently as worried about what happens once we get the vaccine as I was before, and here is why.
The first thing I was slow to realize is that a certain percentage of the public will have immunity because they already had the virus and have now recovered. In NY, they estimated this to be around 20% of the population based on serology tests, which is about what you get for the entire US right now if you multiply the number of confirmed cases by 10. By the time we get a vaccine, this number could be around 30% with immunity if we continue to get about 40,000 new cases a day.
Infectious disease experts seem to say that we need to get 60% to 70% of the public to be protected in order to get herd immunity, so if 20-30% have immunity from having had the disease already, that means we need about half of those remaining to be protected from the virus. Once we have a vaccine, this level seems possible to achieve even without everyone getting vaccinated. If the vaccine is say 70% effective, then only about 70% of the public would need to get vaccinated in order to reach herd immunity. If we reach this point, then great, we have herd immunity and the virus should quickly fade away entirely. However, if the vaccine is less than 70% effective or less than 70% of the public get vaccinated, then we might not have herd immunity.
If we end up in a situation where there is not herd immunity, I see three possible outcomes. First, we could see the number of new cases decline substantially as vaccination rates go up, only to have social distancing efforts get relaxed once everyone thinks we're in the clear. Once we attempt to go back to normal, then we realize we haven't actually reached herd immunity and the cases start increasing again. This will cause us to cycle between the two modes of high restrictions and declining cases and loose restrictions and increasing cases.
The second possible outcome is that once we realize we don't have herd immunity we make a big push to increase vaccination rates, perhaps through social pressure, financial incentives, or as a last resort mandatory vaccination rules. In this case, we might not get herd immunity right away, but do get there a little bit later through increased vaccination rates.
The third possible outcome is that as we cycle back and forth between tight restrictions and loose restrictions more and more people will get and recover from the disease which then gives them some immunity after they recover. Even if we assume a worse case scenario, where say we start with 25% immunity, the vaccine is only 50% effective, and only 50% get the vaccine, then we will still have 50% of the public protected and it might only take an additional 10% of the public to get the disease to finally achieve herd immunity. This is less than ideal, but not necessarily horrific either.
My best guess is that the number of new cases will start to go down as more and more people get vaccinated, and this will spur us to open up too quickly. Once cases start rising again, we will realize that we don't have herd immunity and more people will feel inclined to get vaccinated, especially since there will have already been an initial round of vaccinations, so we will have a good sense of whether or not the vaccine is safe. Even if we don't get that, then we just cycle back and forth for a bit until enough people recover from the disease to push us up to herd immunity, but that won't require too many to get infected since a large portion will be protected already, either from a vaccine or having had the virus already before the vaccine was distributed.
Of course, the key is to get as many people vaccinated as possible, and there will be a great political divide over this question, but my takeaway is that even if we don't get everyone vaccinated, the consequences won't be too dire. It is still perfectly possible to achieve herd immunity without perfect vaccination rates, and even if we only get close, it won't take that much to push us over the top. That's a lot more encouraging than apocalyptic visions of a new intense political conflict over vaccines that makes the debate over masks look like child's play. Herd immunity might not happen right away, but we will eventually get there, which gives us some hope that this pandemic really is a temporary phenomenon.
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