My experience teaches me that explaining things to uninformed people could be worth the effort. Not sure the same applies to stupid people. With that proviso out of the way, let’s get to brass tacks about herd immunity.
Some individuals within a population cannot be vaccinated,
- Newborn and elderly.
- Immunosuppressed people, from transplant-associated medications or chemotherapy for example.
- Immunodeficient people, from genetic conditions for example.
How to protect them from vaccine-preventable infections present in their community? That’s where herd immunity comes in.
At its simplest, herd immunity is a function of an infection’s basic reproduction number (infectiousness).
- It represents the infection’s potential for spread.
- It is the theoretical average number of susceptible individuals who could be infected from a single infected person.
A picture is worth a thousand words or so they say so here goes with an extremely crude oversimplification.
Herd immunity: What it does
- Provides a protective barrier against infection across an entire population.
- Each individual immune either from getting infected naturally or from vaccination forms a part of this barrier.
- Limits or entirely prevents transmission of a given infection.
- More immune individuals within a population, more likely infection transmission stops before reaching all potentially susceptible individuals in it.
- Indirectly protects from infection those vulnerable susceptible individuals who cannot be vaccinated either because the vaccine hasn’t been approved for their age group or because of problems with their immune system.
- Also indirectly protects those who refuse to get themselves or their children vaccinated, the so-called free riders.
Herd immunity: What it is
Also known as community immunity,
- It is an estimate of how many need to be immune in order to protect an entire population from a specific infection.
- An infected person is more likely to spread infection earlier rather than later in an outbreak since longer it lasts, lower the number of susceptible individuals as more and more have already caught the infection.
- The herd immunity threshold differs for each infectious disease agent and depends on,
- The organism’s infectiousness: how many people can get infected from contact with one infected person.
- Duration of infectivity: length of time an infected person can spread the infection.
- Number of susceptible people in a population who come in contact with an infected person when they are infectious.
- How well a given vaccine prevents infection transmission.
- How long immunity from vaccination or natural infection lasts.
- Whether or not infection spreads from human-to-human alone or from other animals as well.
- This threshold is obviously highly dynamic.
- Since the immune and the susceptible aren’t spread out evenly across a population, herd immunity threshold can vary widely between pockets within it.
- Pockets that allow non-medical exemptions for vaccination will have much higher concentrations of those susceptible even within a larger population with fairly high vaccination levels. This explains local outbreaks such as measles breaking out within a specific geographic area. One reason measles breaks out so readily in such pockets is because it has one of the highest basic reproduction numbers of 12 to 18 and thus correspondingly has a herd immunity threshold of 92 to 94%, meaning that that 92 to 94% within a population need to be measles-vaccinated in order for the unvaccinated to remain protected.
- Unless an infection has been eradicated worldwide, neither risk of catching it nor risk of harming others who cannot be vaccinated are zero.