Tell me again, will herd immunity work to save us all from COVID-19?

Biology/Immunity Infection and Spread Uncertainty and Misinformation

A: The quick answer: NO.

It is likely that herd immunity (when a large portion of the population has immunity to an infectious disease, thus forming a protective layer, a cushion if you will, to stop the infection from reaching people who do not have immunity) will not work with COVID-19. This is not to say that herd immunity reached through vaccination wouldn’t work, rather herd immunity reached through lots of people getting infected, will probably not work.

Quick definition of herd immunity

The key factors:

“R-naught (R0)” How many people (on average) a person with COVID-19 will infect (assuming no immunity).

”Case fatality rate (Death rate) The percentage of people who die from COVID-19.

”Immunity duration” How long a person is protected (immune) from infection after they have recovered.

What we know about the key factors?

R0: Early estimates reported the R0 to be between 2-3 (compared to the flu at around 1.3). This isn’t a constant and is influenced also by our contact with others. Important point: This disease infects fairly easily.

Quick R0 Explainers:


NY Times

Death rate: Depending on the models and the data, we may believe that the death rate is 0.5-1.1%. Even so, if we estimate that 70% of the population must become infected to reach immunity, that would mean a lot of deaths.

Even in New York, it is anticipated that perhaps 20% have been infected and recovered. Note that even at 20% our hospitals were overwhelmed and many people died. Still, its hard to pin down the true death rate (here is why). The point: Many people would die.

Immunity duration: We have posted a bit about models and some estimates of immunity. While we don’t have evidence to believe that people become reinfected with COVID-19, we don’t know how long people would be (if they are) immune. We also don’t know if people can become reinfected but asymptomatic and potentially spread disease. It is unlikely that we don’t mount sufficient immune response (like in HIV) or that the virus mutates quickly, rendering our old response inadequate (like with the seasonal flu virus), but these scenarios are less likely.

Nice summary of herd immunity

So, in sum. These three factors really influence the idea of herd immunity. Its not a great public health strategy to have many people get infected because 1) Many people will still get infected even if 70% are recovered (there will still be transmission because of the high R0), 2) Many people would die and likely overwhelm our healthcare systems and 3) We don’t know enough about immunity in COVID-19 (we need long term data to understand this) to be confident that infection confers immunity.

Here is a great summary of herd immunity with a fun modeler. Stay safe!


Link to original FB post