[BostonCard]

The formula for herd immunity is derived from the fact that the Rt = R0*(1-i)  where i is the proportion of the population that is immune.  Since you need RT to be less than 1 to stop spread of the virus, you can set h as the proportion of I that leads to RT being less than 1.  Thus R0 * (1 - h) < 1, so solving for h gives you h > 1 - 1/R.  This makes sense, as the higher the reproductive number, the more people have to be immune in order to get herd immunity.  I have seen estimates of R0 for COVID-19 in the 2.5 range, so if you plug it in, you need 60% of the population to be immune in order to get herd immunity.  If R0 is more like 2, then you need 50% of the population immune, which is where the guideline that to get herd immunity we would need 70% efficacy of a vaccine with 70% of the population taking it (gives you 49% immunity).

Social distancing measures in place right now have driven R(effective) down to less than 1.5, so in those getting to 33% immune is enough to get you herd immunity.

Note that once you get up to 20% immune, that probably is enough to have a measurable effect on how fast the virus propagates.  Think about it, that’s 20% of potential transmission chains snuffed out by immunity.

BC

[BostonCard]

The formula for herd immunity is derived from the fact that the Rt = R0*(1-i)  where i is the proportion of the population that is immune.  Since you need RT to be less than 1 to stop spread of the virus, you can set h as the proportion of I that leads to RT being less than 1.  Thus R0 * (1 - h) < 1, so solving for h gives you h > 1 - 1/R.  This makes sense, as the higher the reproductive number, the more people have to be immune in order to get herd immunity.  I have seen estimates of R0 for COVID-19 in the 2.5 range, so if you plug it in, you need 60% of the population to be immune in order to get herd immunity.  If R0 is more like 2, then you need 50% of the population immune, which is where the guideline that to get herd immunity we would need 70% efficacy of a vaccine with 70% of the population taking it (gives you 49% immunity).

Social distancing measures in place right now have driven R(effective) down to less than 1.5, so in those getting to 33% immune is enough to get you herd immunity.

Note that once you get up to 20% immune, that probably is enough to have a measurable effect on how fast the virus propagates.  Think about it, that’s 20% of potential transmission chains snuffed out by immunity.

BC