When a population has a level of lifelong immunity to a certain disease such that an epidemic of that disease cannot occur, the population is said to have herd immunity.
I am still very new at this, so I might be missing something glaringly obvious from people more experienced with epidemiology.
What stood out for me from the part that I inserted into my post is the life long immunity that is required for herd immunity to work. Each disease has a different herd immunity threshold. Measles is a disease that is often spokeמ about. So, the herd immunity threshold for measles is 83 – 94% immunity. I am interpreting this to mean that 83 – 94 percent of the population (from birth through to old age) is recquired to be immune – not just 83 – 94% of children are required to be vaccinated.
Why? In this article, I found it interesting that in the absense of circulating disease, immunity only last for 25 years. By having the disease circulating within the population, the population is exposed to regular ‘boosters’. Remove the disease, and the whole population is left at risk, unless they are kept up to date with vaccines.
What is potentially dangerous with this effect of waning immunity in adults is that adults can have much more serious complications from diseases like measles, mumps, and chickenpox. Essentially a large part of the population could potentially be at risk if they are relying on vaccine induced immunity for their immunity, and not up to date with boosters.
Another piece of the puzzle is that mothers pass on passive immunity to their infants through the placenta (I have read conflicting articles about whether breastmilk also confers passive immunity or not). And babies are protected from the disease of measles with this passive immunity. Again, from what I am understanding, passive immunity is not conferred when the mother relies on vaccine immunity, only if she herself contracted measles is the immunity passed to her infant. Which, theoretically could leave infants more at risk with most new mothers today not having had wild measles.
It is all theoretical. I have not heard of infants or adults coming down with measles.
I just am really struggling to get my head around these issues. I do not know if the scientists are factoring in waning immunity when they calculate the threshold herd immunity, but I am guessing they are not. I am guessing that it is assumed that the vaccine is fulfilling it’s potential to rid the human population of measles.