Herd Immunity

What is herd immunity and when will we get there?

Imagine a billiard table with balls racked at one end.  Smash a cue ball into the neat arrangement and all the balls will move.  That’s like the pandemic.  The energy spread from ball to ball is like the spread of virus from person to person.   

Now imagine the same table, except this time some of the balls are bolted onto the table so they can’t move.  Smash a cue ball into the rack and not as many will move.  The bolted balls are like people with immunity; they absorb energy and do not allow it to spread.  How many balls must be bolted onto the table to prevent spread of motion across the table?  That’s herd immunity; the percentage of people with COVID-19 immunity that it takes to stop the uncontrolled spread of virus.

Epidemiologists differ on herd immunity targets for COVID-19, with estimates varying from 60% to 90%.  But these estimates are just educated guesses.  Herd immunity is something observed, not something predicted.  When the rates of disease reach a stable low level, herd immunity has been achieved.  

Now some observations from my point of view.  Last week, without a decline in the number of tests performed, there were only 6 positive SARS-CoV-2 tests in the laboratory where I work.  Until last week, weekly positives have been in the double digits going back to April 2020, with a peak of 475 during the first week of this year.  Since the last week in February, the weekly number of positive tests have been below 40.  The curve has flattened, despite the lifting of many restrictions designed to prevent spread of the disease in Texas.  We can no longer say that there is a COVID-19 epidemic in my community.  Instead, the disease appears to have reached endemic levels here.  

I need to pause to explain what I mean.  Disease prevalence, which we’ve defined before, means the rate of disease in a population.  Prevalence can be applied to any disease, not just infectious disease; thus, we can speak of the prevalence of diabetes, of breast cancer, of heart disease, and so on.  Epidemic simply means increasing prevalence, just like acceleration means increasing speed.  Endemic means that disease prevalence is stable and not changing.  Epidemics can be local, meaning confined to a house or a neighborhood or a city or a country, or epidemics can be global, meaning happening all over the world at once.  Global epidemics are called pandemics.  The term “global pandemic” is as redundant as “unexpected surprise” or “advance warning.”

Herd immunity is achieved once an infectious disease reaches endemic levels, but what is that number?  Assuming natural and vaccine-induced immunity are the same thing, then herd immunity is the percentage of folks who have either had the infection or the vaccine when disease becomes endemic.  Today, it is estimated that 10% of Texans (2.8 out of 28 million) have had COVID-19, and that 35% of Texans have been fully vaccinated.  Therefore, my area seems to have achieved COVID-19 herd immunity at 45%.

This all sounds like great news, so why not throw our masks in the air and celebrate?  There are still unanswered questions.  Vaccine-induced immunity is not the same as natural immunity, but is it the same enough for calculating herd immunity?  How long does immunity last?  Can herd immunity be lost once achieved?  What will be the impact of emerging variants on immunity of individuals and populations?  And, most puzzling to me, why is the virus surging now in India and Brazil despite previous waves of infection?  

We still don’t know as much as we would like to believe.