How many Americans have had Covid-19? The answer explains why the infection rate will have to start dropping in the coming weeks.

As of yesterday 14.76million Americans had had Covid-19 as confirmed by testing and the numbers continue to spike due to holiday gatherings. That is only four percent of all Americans who have been confirmed to have had Covid-19 according to testing data so far. However, most cases of Covid-19 never get tested. The CDC estimated that about 16% of Americans had actually already had it by the end of September using a modeling estimate. They estimated that eight times more Americans had had the disease than had been confirmed by testing.

As of September 30, America only had about half as many confirmed cases as we do today, so if the CDC’s ratio keeps up, already today about 32% of Americans may have already had the disease. And at the present rate of infection, over 50% of Americans may well have been infected by the end of January. However, as the percent of the population that is immune rises, the rate of infection will drop, so we may not get to 50% that fast.

But that was just a modelling estimate and an empirical estimate based on antibodies in blood samples is likely to be better. One antibody sample from September found that less than 10% of Americans had antibodies and so it is possible that only less than 20% of Americans have had Covid as of today.

Another antibody study from mid-July found that about 9% of Americans had had the disease at that point. That is about ten times more people than had tested positive. If today the ration had held steady at ten times more Americans who have had the disease than who have tested positive, then 45% of Americans would already have had it!

The point is, we are galloping towards herd immunity at the infection rate we are currently seeing. It is very unlikely that we are already at 45% immunity because the infection rate would have to be slower than the current torrid rate as only about half of the population would be available to become new victims, but it is entirely plausible that we could already be around 20%-33% immunity and as we get closer to herd immunity, the infection rate will have to peak and begin to fall even without any change in behavior.

So I’m an optimist about the spring. We will have started distributing the vaccine by then and as the weather warms, that will also slow transmission once again. We’ll also know even more about what kind of activities are the most dangerous and have better data about how to mitigate risks. It is likely to be darkest just before the dawn.