Herd Immunity & COVID-19 (SARS-CoV-2 Outbreak) What you need to know
The idea of herd immunity as the solution to the COVID-19 pandemic has triggered heated debate, but what is herd immunity and how does it work?
Herd immunity happens when so many people in a community become immune to an infectious disease that it stops the disease from spreading. This can happen in two ways:
- Natural immunity: Many people contract the disease and in time build up an immune response to it
- Vaccines: Many people are vaccinated against the disease to achieve immunity
When a large percentage of the population becomes immune to a disease, the chain of transmission is broken. Hence, the spread of that disease slows down or stops.
This helps protect people who aren’t vaccinated or who have low functioning immune systems and may develop an infection more easily.
For herd immunity to take effect, a certain percentage of the population must be immunized. This threshold of how many people are required to stop a disease from spreading is different for every disease and depends on many factors, including how easily it spreads and whom it infects.
The Tricky Math
When will a disease stop spreading through a population? The formula is simple, but the variables are much more complicated.
The only thing you need to know is “basic reproduction number” of a pathogen, known as R0. It represents the number of people one contagious person can infect in a community of unprotected individuals. Once you have that, you can plug it into a simple formula for calculating the herd immunity threshold: 1 − 1/R0.
Now, let's say the R0 for Covid-19 is 2.5, meaning each infected person infects 2.5 people on average. In that case, the herd immunity threshold for Covid-19 is 0.6, or 60%. That means the virus will spread at an accelerating rate until, on average, 60% of the population becomes immune.
The more contagious disease is, the higher R0 will be, and the more people need to be immunized for herd immunity to take effect.
The bottom line
Herd immunity isn’t the answer to stopping the spread of SARS-CoV-2, the new coronavirus that causes COVID-19. Once a vaccine is developed for this virus, establishing herd immunity is one way to help protect people in the community who are vulnerable or have low functioning immune systems.
- Quanta Magazine
- National Geographic
- Health Scopemag