# COVID-19 - How long until herd immunity?

Everyone would like to know when this pandemic will end. Herd immunity is a possible scenario in which the pandemic ends by immunizing the population if we don't find a vaccine. But how long do we have to wait until we're there?

There seems to be only two realistic scenarios how the Corona pandemic could end.

In the first scenario, we find a vaccine and reach wide-spread immunity on a global scale. This could take one year or maybe two. Looking at the second option, this could be the better strategy:

In the second scenario, we reach herd immunity and most of the population produces antibodies for the Corona virus. The threshold for herd immunity for the Corona virus is assumed to be around 60% of the global population.

**Disclaimer:** Please note that "confirmed cases" are known to grossly underestimate the number of people with antibodies. Recent numbers (23.04.2020) suggested that up to 13.9% of NYC citizens could have antibodies although the number of confirmed cases is at around 5% of the population. In this notebook, we also plot the results for when 50% or 90% of cases remain undetected.

To estimate how much the pandemic has progressed, we calculate the percentage of the population that was already infected. Since it is often assumed that many, if not most, of the cases in many countries are not detected, we also plot the number of estimated undetected cases, assuming that these are between 50% and 90% of the total cases, either because they remain asymptomatic or because they simply not tested for.

We can estimate the time it takes to reach a 60% infection of the population by extrapolating the already observed cases into the future. This estimation is model-free: we simply take the *30 day average* of the past confirmed new cases and assume that the infection rate (or rather the case confirmation rate) stays constant. Then, we can easily calculate how long it would take for each country to reach 60% of its population.

As of 25.04.2020, this number for many, if not all, countries is absurdly high. This can be due to do with the fact the outbreak hasn't really started in most places yet since only a tiny amount of the population was infected or that testing capabilities are still very insufficient in many places. This number will change as the pandemic progresses and as we get a better estimation of the actual infection rate.