HERD IMMUNITY
and COVID-19

June 2020 - THE THEORY...

Herd immunity and COVID-19? You may have heard this theory in regard to the novel coronavirus pandemic. It goes something like this... If everyone gets infected, we’ll eventually develop herd immunity against COVID-19 and everything will be OK.

Unfortunately, this postulation is only partly true. We might get herd immunity. But it certainly won’t be OK.

Herd immunity happens when enough people in a population are immune to a specific disease and therefore protect everyone else who is not immune. The premise is based on the facts that viruses need us to survive. They copy themselves by using the machinery of our cells and have to jump from person to person in order to keep reproducing.

So, if you are somebody who is not immune to the virus, but you are surrounded by people who are immune (either through prior infection or a vaccine), then this herd of people are essentially serving as a shield around you, so the virus can’t get to you.

DOES HERD IMMUNITY WORK?

Make no mistake. Herd immunity really does work. The problem is that you need a lot of people to be immune for it to take hold. For example, polio needs upwards of 80 percent of the population to be immune for herd immunity to work. Pertussis and measles require more than 90 percent.

Practically speaking, you can never achieve 100 percent immunity because some people cannot get vaccinations either because they are too young or have compromised immune systems or have allergies. But if you can keep the immunity rates above that critical threshold, then the herd keeps them protected. If you can maintain herd immunity for long enough, the virus will eventually disappear. Smallpox was eradicated that way. There is talk that polio may soon follow.

WHY DOESN'T IT ALWAYS WORK?

Given the number of people that have been infected by COVID-19, but with the limitations of testing, many people, especially early on in the pandemic, did not get tested. Also, many people tend to have no or very mild symptoms and may not have ever realized that they were sick.

So in reality, the number of people who have contracted COVID-19 is certainly much higher than the number of people who have tested positive and much higher than the numbers reported on the news.

So, even if we accept that the vast majority of cases of COVID-19 are undiagnosed, we are nowhere near achieving rates that would result in herd immunity.

COULD HERD IMMUNITY
WORK FOR COVID-19?

The most important point is that herd immunity needs the majority of the population to be immune. What that threshold is for COVID-19 is not entirely clear, but it is a fair estimate that it is likely to require 60 to 70 percent of the population to be immune, based on what is known about other viruses.

ANOTHER PROBLEM WITH
HERD IMMUNITY

Unfortunately, every so often people start thinking they don’t need to be vaccinated, and then immunity rates fall below the critical threshold. In Britain, a drop-off in whooping cough vaccination led to an outbreak just as waning measles vaccinations in the United States led to measles outbreaks.

Another problem with the herd immunity theory is that if in the absence of a vaccine the vast majority of people need to get infected to achieve herd immunity, even if we assume a very low mortality rate of less than one percent, it still means hundreds of thousands of people need to die. That is too high a price to pay.

Sweden tried a more relaxed form of social distancing in an attempt to achieve herd immunity and found itself with a surge in deaths compared to neighbors like Norway and Finland. It would be to our advantage that we not follow suit.

HOW TO PROCEED...

Social distancing for the rest of the time recommended by scientists and health experts still appears to be the best way we know of to contain the spread of this virus.

A slow reopening, based on recommended practices, and extra caution taken for the most vulnerable of our citizens until a reliable vaccine can be made available looks to be the ‘new normal’ for our world.

Stay safe and stay healthy.

Much of the information in this article was found on the Office for Science and Society website at McGill University and used with written permission.

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