What is Herd Immunity?

I keep hearing this concept of herd immunity and how many scientists disagree on how effective this can be to combat the coronavirus.

First, what is herd immunity? According to Google “Resistance to spread of disease within a population based on pre-existing immunity of a high percentage of individuals based on prior infection or vaccination” So basically, the immune are people who have been exposed to the virus and developed an immune response and have no symptoms or been exposed to a vaccine.

I reviewed a lecture by Dr. Paul Fine (Infectious Disease Specialist) from London, England and he explained the concept of herd immunity in detail. A concept to understand before we discuss the theory is Ro = the average number of transmissions per infected person to a totally susceptible population. So, if everyone is susceptible to an infection and I am infected; how many people on average will become infected because of me? Early this year the Ro for coronavirus was thought to be 3 (Ro=3).

What happens if some of the population isn’t susceptible (they are immune to the disease)?

The concept of herd immunity comes into play if some people are not susceptible. Let’s say 2 out of 3 people are immune?

As the graph shows, the spread of the disease goes down significantly!

A consensus among infectious disease specialists is that the level of the population that is required to be immune to achieve herd immunity is 60-90% depending on the disease.

Is it that easy?

Unfortunately, these calculations rely on certain expectations:

#1 The population randomly mixes.

#2 Immunity is long lasting.

Lastly, what is the threshold SPECIFIC to coronavirus that provides herd immunity? 60% or 90% or somewhere in between.

Nobody really knows these answers!

Can a vaccine cure our problem?

Epidemiologists and Infectious Disease specialists agree that vaccines are not 100% effective AND the people who are the most at risk and would benefit most from the vaccine are often less likely to receive the treatment.

What does the future hold?

Whether the concept of herd immunity takes precedence when a vaccine becomes available is unknown. One thing is for sure, we know very little about this virus and what we think we know can change rapidly. Hopefully, with time our understanding will grow and we can initiate treatments that help people become immune and allow our society to open up and return to some form of normality.

Paul Saiz, MD

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