In the early days of the covid 19 pandemic there was much speculation about what percent of the population would have to be vaccinated in order to achieve herd immunity )(HI). It seemed to be a matter of not if but when HI would be accomplished.
Writers made analogies to measles and smallpox for which vaccination programs achieved herd immunity and in the case of smallpox, eradication. If HI could be achieved for a very infectious disease such as measles spread by aerosols, why not covid 19?
Theoretical considerations regarding HI center on the basic reproduction number (R naught or Ro which is the average number of people that one infected person will infect. The number of people needed to be immunized in order to reach HI is 1-1/Ro.For example-if the Ro is 2 then 50% would have to become immune (either through vaccination or infection) for a Ro 0f 3 we see 1-1/3 equals 2 thirds or 67%
Several simplifying assumptions underlie the formula for HI including that all individuals have an equal opportunity to become infected. Epidemiologists disagree as to what exactly HI is and as to what the necessary assumptions are in estimating a HI vaccination level.
However the key-typically implicit- assumption is that being vaccinated ( or recovering from an infection) results in long lasting immunity.(1)
We now know that this major assumption does not conform with reality. We now know that fully immunized individuals can become infected and can transmit the disease and also that recovery from covid 19 infection does not preclude becoming infected again.
The idea underlying HI is that as individuals become vaccinated ( or recover from infection) they are removed from the susceptible pool. With covid vaccination or infection both groups seem to be removed from that pool only for a time , perhaps as short a time as 6-8 months. While infection blocking immunity (nasal mucosal immunity) wanes quickly the disease reducing immunity seems to be more durable.
When cases of abreakthrough infection became recognized as a real thing,some in the public health sector said " well, the vaccine was only intended to prevent serious disease and death".But if that were the case why was there so much talk about achieving herd immunity a concept that IMO only makes sense if the vaccine prevents spread.
Dr Jennie S Levine has been quoted that she and her colleagues talk about transient herd immunity.
My thought on that is perhaps we are seeing a transient HI for example now In Louisiana and Texas .Both states had a major Delta surge with a surge of cases and hospitalization and death and now the surge seems to have abated.Possibly as more and more people were vaccinated and more infection plus mandatory NPIs (in Louisiana ) the number of susceptible individual decreases to reach some level of transient herd immunity like condition. However, as immunity wanes again in states with a large number of unvaccinated individuals serving as a covid reservoir some level of spread could occur again for another round of surges . Are we likely to continue to see regional whack a mole events .
A much more optimistic scenario is that the booster shots will bring about a sufficiently long period of immunity so that more than a transient herd immunity could be achieved. In other words- the covid 19 mRNA vaccine should have been a three shot vaccination program all along.