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In the US? We can roughly calculate that by working backwards from the Infection Fatality Rate (the percent of people who die after infection, taking into account all the asymptomatic cases we’re missing). The IFR has been calculated at 0.65% for every study I’ve seen since May and the CDC supports that figure as well.

As of now, we have 292,748 COVID-19 deaths. There are another likely 30,000-40,000 that happened in April/May primarily in New York and New Jersey that were missed because there was no testing available and few doctors knew what to look for at the time. If you look at the excess mortality figures, you see a sudden unexpected spike WAY over the normal 5-year averages that begins in March and ends in April for multiple causes of death known to have a very high risk of COVID-19 complications (including dementia, heart issues, and pneumonia).

If we exclude the excess deaths and assume the only people who’ve died are reflected in the official numbers, we have 292,748 / 0.0065 or 45 million people in the US who have been infected. If we add the up to 40,000 additional excess deaths from the start of the pandemic, we get 51 million. Now add the people who are currently infected, but have not yet died or beaten the infection and you get an additional ~9 million.

Altogether, we’re in a range of ~49 million to 60 million Americans who have been or currently are infected. A vaccine likely does nothing for any of them at this point (with maybe a single-digit number of exceptions?).

Now the next logical thing to ask is: how many NEED to have antibodies? For that we need to know the Herd Immunity Threshold, which is the number of people who must be immune in a population before the reproductive rate of the pathogen drops below 1 (i.e. each infected person infects fewer than 1 other on average). To get this, we take the R0 (i.e. how many people does an infected person infect - on average - in a population where no one has immunity) which is 2.5 for SARS-CoV-2, and plug that into the equation 1-1/R0. This comes to 0.6 or 60%. So once 60% of the US population is either vaccinated or has been infected, the disease will slowly begin to die out. This isn’t sudden and it’s subject to local conditions (hitting 60% nationally doesn’t mean anything if a particular city is at 30%), but it’s at least a starting point. For the US, with a population of 330 million, that’s 198 million.

So from where we stand today, we’re ~138 million - 149 million people away from reaching the point where the infection numbers begin to consistently decline. To really see big drops, we probably need more like 260 million - 280 million people with antibodies, or around 215 million Americans. At that point, I would expect to see huge drops in infection rates even if all the masks and social distancing were completely gone. The virus runs out of people it can infect. The sick get better and the virus is gone. With the vaccines we have about to be approved, we can hit that by the end of summer 2021. With a couple additional vaccines available, we can hit that much sooner, maybe by March/April of 2021.

Hope that helps!