3 ms·
Herd immunity varies by disease. A lot. For each disease we estimate a number called R0, which is how many people the average person will transmit it to if no
by btilly 7y ago
Herd immunity varies by disease. A lot.
For each disease we estimate a number called R0, which is how many people the average person will transmit it to if no precautions are taken.
Herd immunity sets in when only 1/R0 people have not managed to get it.
With the flu, R0 is about 1.3. Meaning that herd immunity to a particular strain starts slowing the spread after about 1/4 of us are immune. (It can continue spreading for some time after that point.) I calculated that from the fact that 1 - 1/1.3 = 0.23076923076923076...
R0 for COVID-19 is likely around 5.6 +- 0.9 per https://www.medrxiv.org/content/10.1101/2020.02.07.20021154v1.If https://www.medrxiv.org/content/10.1101/2020.02.07.20021154v... we take the middle of that range, over 82% of us have to get it before herd immunity starts to work in our favor.
- dmoy 7y agoThis is also why people refusing the MMR vaccine is really, really hurtful to vulnerable populations. Measles R0 is like 15 or something - it's crazy infectious. So one kid in a daycare or class of 20 with parents who don't believe in vaccines is enough to push you under herd immunity. Bake in the fact that MMR vaccine is only ~97% good against measles, and you end up with a bunch of dead kids. We got lucky last time around in Seattle area, in that it only cost us $1m and 0 deaths to get it under control.
- networkimprov 7y agoHerd immunity is derived from R0 and "susceptibility" percentage. The value for the latter also varies with disease and strain. If you look at the Italian data, their % positive level never crossed ~30%, and overall averaged <20%. That might indicate that susceptibility for this agent is well below 50%. https://en.wikipedia.org/wiki/2020_coronavirus_pandemic_in_Italy https://en.wikipedia.org/wiki/2020_coronavirus_pandemic_in_I...
- btilly 7y agoIf you look at the Italian data, their % positive level never crossed ~30%, and overall averaged <20%. That might indicate that susceptibility for this agent is well below 50%. The more likely explanation is that social distancing was implemented early enough, and tests were given to people who might or might not have had it. There is no reason to believe that we burned through most of the vulnerable people in the population.
- Mvandenbergh 7y agoCould you post a working version of that link? Most R0 estimates I have seen are between 2.5 and 3.5. 5.6 seems really high to me. It is worth considering that even if the % immune is lower than the herd immunity level, it still slows the outbreak and reduces Re. That gives us a much slower ramp up and gives other measures like less restrictive social distancing, contact tracing, etc. a chance to work. Even if all those together only get Re down to between 1 and 2, we still benefit since it means we might only need another 1 or 2 lockdown periods before we get an effective vaccine.
- DanBC 7y agoI think it's this link: https://www.medrxiv.org/content/10.1101/2020.02.07.20021154v1 https://www.medrxiv.org/content/10.1101/2020.02.07.20021154v...