"Herd immunity was accepted to be 70-75%. When polls showed only about 60% were willing to get vaccinated, Fauci suddenly claimed herd immunity was 80-85% to try to trick people into getting vaccinated."
Not really interested in arguing with rich, but I did want to pull this specific sentence out and talk about why herd immunity and vaccine effectiveness is important in the context of more virulent variants like Delta
Couple of quick definitions:
- The herd immunity threshold is achieved when there is sufficient immunity in the population such that each person who acquires the infection passes it on to less than one person on average. Surpassing this threshold will result in a decline in the incidence of that infection. This number is affected by a couple of things – how virulent the infection is, and how susceptible the population is to infection. The first is a characteristic of the infectious agent, and the second is significantly affected by the efficacy and commonality of a vaccine. Population susceptibility is also affected by things like some genetic immunity, for example, or by locking down populations to reduce potential transmission events. Whether you agree with them or not, the point of lockdowns was to buy time – reduce the spread of the virus until a vaccine could be developed. I’m going to say “virus/covid/delta/etc”, even though this is applicable to all infectious diseases.
- The comment calculation for HI based only on the transmissibility of the virus is 1-1/R, where R is the basic reproduction index- a measure of how transmissible the agent is. The original Wuhan variant had about a 2.5 R, Alpha (prevalent in the US up until recently) somewhere 3-4, smallpox around the time of eradication about 4, Delta about 6, Measles about 18.
- Thus, by math, the HI for Alpha is about 75%, but the HI for Delta is about 85%. Note that this is absent calculating the effect of vaccines on the susceptibility of the population.
- The formula for having a vaccine *alone* reach herd immunity is V (target vaccination level) = (1-1/R)/E, where E is the vaccine effectiveness against transmission.
- Delta is a problem for two reasons – both in the numerator and in the denominator. The top part of the calculation is higher for Delta b/c it’s a more virulent variation. That alone would make the vaccine’s task harder, even if the existing vaccines were just as effective against Delta as against earlier versions. The additional problem is that the evidence points to the current vaccines being somewhat less effective against transmission, although they still seem very effective against the severity of infection.
- With an earlier variant that had a R of 3, and an E of .95, the herd immunity from vaccines alone was 95% of 75% or about 71%.
- With Delta showing an R of 6, and an E of (lets say) .9, HI from vaccines alone pushes to nearly 95%
Short version: more virulent variations means that the HI number is higher, and would have to be accomplished either by a nearly universal vaccination rate (like w/ have w/ measles/smallpox) or by reducing the number of transmission opportunities (social activity pattern changes).
Apologies for going off on a tangent, but this talking point has really been bugging me.