Herd Immunity

Updated 18-Jun-2021

Herd Immunity is when a population has enough of its members immune from infection (either through the presence of antibodies due to previous infection or vaccination. Depending on the disease, the percentage of the population varies. For the measles, herd immunity requires 95% of a population be vaccinated. Herd immunity is immunity for the herd, not for all individuals. It does not stop or eradicate a disease, but rather prevents it from breaking out across a population.

Those who do not want to vaccinate do not necessarily stop a population from achieving herd immunity, unless they are a large enough group which then makes herd immunity through vaccination impossible.

With Covid-19 and its variants, it is still not clear what the herd immunity threshold is, or even if herd immunity is possible. And vaccination (and infection) are not the sole criteria. Social distancing and masking also have an effect on disease transmission. It turns out that herd immunity is for a specific population, and the reproductive number of a given disease (R0) is specific to that population. In a given region or country, assumptions about the heterogeneity or homogeneity of that population are important.

In addition, transmissibility is not only based on vaccination, but rather must include a model of interaction in that society which would determine how transmissible a given disease is at a given time. For example, a city that is locked down, with very limited interaction, a high level of social distancing, and comprehensive use of facemasks would have a significant impact on the R0.

The transmissiblity of the disease for something like measles is 12-18. That means one infected person infects 12-18 other people. The herd immunity for measles is 95% vaccinated. For Covid-19, the R0 may be 2 or 3. That will still travel through a population, but perhaps 60% of a population needs to be immune, in order to stop rampant community transmission. Immunity comes from vaccination or previous infection. But vaccines do not provide 100% effectiveness. Even vaccines with 90+ effectiveness, such as the mRNA vaccines from Moderna and Pfizer mean that we would need 67% of a population vaccinated. For a vaccine that is ~70% effective, 86% of a population must be vaccinated. For a vaccine that is ~50% effective, it is not possible to reach herd immunity as 120% of a population must be vaccinated.

Add to this the fact that there are currently no vaccines for those under the age of 15. In Thailand, roughly 12 million children are under 15 years of age, out of a nearly 70 million population. That is 17% of the population that is unable to be vaccinated. This means that a ~70% effective vaccination of all those 15 years and older, 103% of that population needs to be vaccinated. Otherwise stated, herd immunity needs 86% of the population vaccinated, but only 83% of the population is 15 years or older and could take the vaccine. Leaving out the vaccination of children, and allowing schools in session, is a recipe for disaster should widespread outbreaks occur (which the herd immunity of the 15+ year-old population would not stop).

Even with vaccination, a vaccine such as Sinovac with ~50% efficacy means it would be impossible to achieve herd immunity. Granted, there are more benefits than simple immunity from infection (specifically protection against severe symptoms and death for the vaccinated). However, less effective vaccines

Also, for a given country, there will be uneven vaccination rates in different locations, unless a systematic and comprehensive mandatory vaccination program were put in place. Those without proper state identification are especially at risk.

Lifecycle of Antibodies and Variants

It is unclear how long Covid-19 antibodies, due to infection or vaccination, remain present in the vaccinated/recovered individual. In one study in Manaus, Brazil there was rapid antibody decline within several months. In addition, mutation rates means that new variants are coming into being quite regularly. It is unclear to what extent vaccines will perform against current and future variants of the virus. This was the case in Manaus, which had a resurgence of Covid-19 in a second wave which consisted solely of the P.1 variant in January 2021, after 60-70% of the population had been infected with the original Covid-19 virus by June, 2020. It is highly likely that boosters or new versions of vaccines will be needed on a yearly basis.

As of early June 2021, the Delta variant from India is causing much concern, and the UK has recently gone into or extended lockdown for two weeks due to it.

Plan for the future

The point is this:

  • It is likely impossible to eradicate Covid-19 in any meaningful timeframe, and the best that could be hoped would be to keep infection and mortality down to a level that does not overburden the healthcare system, and allows economies to recover, including the important travel and tourism sectors.
  • Systematic, yearly re-vaccinations will possibly be needed going forward.
  • Highly effective vaccines, and vaccines for all ages (4-11, 12-17) are needed to reach herd immunity or something close to that.
  • Even populations with 60% infection rates are susceptible to new waves of infections (possibly caused or exacerbated by new variants, and by changes in social distancing and masking behavior), so distancing and masking is here to stay.