With the current Covid-19 pandemic, some people have been advocating allowing people to resume normal life, suggesting that at some point herd immunity will happen.
If enough people become immune to an infectious agent, the entire population (or a specific community) is protected because infectious people rarely encounter a non-immune person, so the transmission dies out.
The level of individual immunity to get to herd immunity level for a virus depends on how infectious it is - measured by R, the average number of people that each infectious person infects.
- The classic example is measles, which has an R number of around 15 and a herd immunity threshold of 95%.
- For Covid-19 the R number is about 3.5 and the herd immunity threshold is thought to be around 60 to 70%.
Herd immunity has only ever been achieved by vaccination. This is because herd immunity can only be built if the immune response totally prevents individuals from picking up and transmitting the virus. That sometimes happens but often it doesn't. While a person's immune system may stop them from falling ill if they reacquire a virus, it doesn't prevent onward transmission. The same is true of vaccines.
'Letting the virus rip' would mean letting between two-thirds and three-quarters of the population catch the virus. There are a number of issues with this proposal.
- Collateral damage: even if the death rate is 1%, letting the virus run free will hospitalise and kill millions.
- We can't take it for granted that individual immunity will automatically create herd immunity, since we don't yet know what immunity a person gets from having survived Covid-19, and how long this lasts.
- While they survive a Covid-19 infection, some people are now experiencing the symptoms of 'long covid'. These symptoms can be severe, and affect more than one of the body systems.
So, herd immunity is not the answer.
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