How is vaccine production going to look by April?
Using current rates of vaccine delivery and expected increases that have been reported, my back-of-the-envelope math estimates that ~150 million U.S. residents will have received at least one shot of the Covid vaccine by the end of March. Meanwhile, if surveys are to be believed, only about 2/3rds of American citizens actually *really* want the vaccine, and about 1/3 are either unsure or decidedly against it. This means only about 145 million Americans are both eligible to get the vaccine *and* really want to get it.
Vaccine production will still be ramping up, however. Pfizer, Moderna, and Johnson & Johnson will all still be increasing production and delivery at that time. Early April is also the earliest time at which Novavax could also see FDA approval, which would increase the pool of available vaccines still more. This means that at the beginning of April, pretty much all Americans that *want* the vaccine will already be vaccinated, *and* the supply will continue to go up. So early April will mark the transition into the "vaccine glut" phase, at least for the U.S., where everyone who wants a vaccine is going to be able to get one with relative ease. This is when the job of convincing more people to take the vaccine is going to be much more important.
What does this mean for immunity and reduction of spread of the virus?
We now know that even one shot of the mRNA vaccines starts conferring pretty good immunity after 3 weeks. So in early April, we're going to have ~130 million people with good immunity from vaccines (subtracting off the people who were immunized at the tail end of March). Further, the most vulnerable populations should have a much higher rate of immunity, and also a lot of people who are more prone to being serious spreaders should also have a much higher rate of immunity. This should be devastating to the death rates for Covid, and really bad for its spreading capability as well.
In addition to immunity from the vaccines, we'll also have a fair amount of immunity from people having been infected and recovered. Right now, there have been about 30 million known cases of Covid in the U.S., counting past and present. The unknown asymptomatic cases are, as you would expect, unknown, but a decent estimate for the size of that population is double the size of the known cases. Now, you can't just add those people into the total number of people with immunity from vaccines, because all lot of the people who get the vaccine may well have been unknown asymptomatic carriers from before as well. However, it is the case that the asymptomatic unknowns are more likely to be young and healthy, and thus less likely to have already been vaccinated by the end of March. So I think it's reasonable to add, say, 40 million people to the total number of people with some immunity to Covid by the beginning of April.
This means we'll have roughly 52% of the population with some immunity to Covid by the beginning of April, with that number rapidly increasing due to vaccines already given in late March and new vaccines given as well. What is that going to do to Covid spread?
Estimates for Covid R0 vary, but I like the number 3 as a reasonable estimate. This would be the reproductive rate *without* any social distancing measures--the "life as normal" reproductive rate. In the naïve, "perfect mixing", epidemiology scenarios, you modify the R0 by the percent of population with immunity in order to get the current effective reproductive rate. So, 50% immunity would drop an R0 of 3 down to an R of 1.5. This still gives you spread and growth if you abandon all social distancing and masking measures, but it's not such a huge rate. This is in the same ballpark as the seasonal flu.
Caveats
But there are some additional caveats we can put on these numbers. First, I expect regulations enforcing masking will still be in effect in most places throughout all of March and into April. These should continue to cut down on Covid transmissibility, and if you start with an R of 1.5, you only need an extra 33% reduction in transmissibility to keep the virus decreasing rather than increasing. Second, Covid does not spread in the "perfect-mixing, simplified" style. It seems to have a more asymmetrical style of spread than does the seasonal flu, relying more on super-spreader events than the flu does. This *should* tend to mean that those people who are more likely to get and spread Covid will have already gotten the disease, recovered, and developed immunity earlier rather than later. So those 40 million people we already added into the "some immunity" pool will more important to halting the spread of the disease than remaining people who have not yet had the disease. The fact that we have been vaccinating with an eye to the more dangerous transmission scenarios (medical staff, prison inmates, etc.) should work in the same way. How *exactly* this factor will influence Covid spread is difficult to quantify, but I think it is an important factor. Third and finally, in early April winter will be mostly over. At this point, it's pretty clear I think that there is a seasonal aspect to Covid and that it has reduced transmissibility outside of winter weather. Again it is hard to quantify exactly what effect this will have, but again I suspect that it will be substantial.
Conclusion
With all of these considerations in place, I think we can foresee April being a very tough month to be Covid in the United States. As I said, you only need an extra 33% protection against transmission to put a pandemic into subsidence if the R0 is 3-ish and you have 50% herd immunity, and I think the "extra factors" I mentioned will probably have at least that effect. If I'm right, that means that it should be possible to remove all social restrictions in the U.S. in early April *and* still see infection numbers continue to decrease. It's likely public authorities will proceed with more caution than this, and that's probably a good thing. If it were up to me, when we get to this point, I'd remove all restrictions except requirements to wear masks in public indoor locations (being an easy restriction to live with that doesn't damage the economy) and aggressively continue a public vaccination campaign with the goal of getting to 80% vaccination rate (probably the best we can do considering the amount of vaccine hesitation).
The major factor that could throw a wrench in the spokes of this happy prediction is the spread of newer Covid variants. What we really need to know to know how bad a factor this could be, is how much the newer variants escape the natural immunization from the original variant and the immunization derived from the vaccine. That is the most critical new information I am looking for right now; I know there are trials and experiments looking for this information now and I am eagerly awaiting better information coming out of these. For now, I'm retaining a cautious but hopeful optimism.
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