Read on Twitter
On herd immunity.
Herd immunity is when sufficiently high % of population are immune, so that an infected individual pass the infection to <1, thus an outbreak peters out on its own.
Herd immunity is when sufficiently high % of population are immune, so that an infected individual pass the infection to <1, thus an outbreak peters out on its own.
The most basic equation is 1-1/R0, where R0 is the # infected by the infected.
For Covid-19, R0 estimate is 2~3, although it varies by density, demographics, social structure, etc.
For US, assuming R0=2, you need 1-1/2=0.5, i.e. 50% of population has to be immune.
For Covid-19, R0 estimate is 2~3, although it varies by density, demographics, social structure, etc.
For US, assuming R0=2, you need 1-1/2=0.5, i.e. 50% of population has to be immune.
If you assume IFR=1.0%, that's ~1.65 million dead. Even if you assume IFR=0.5%, ~825 thousand dead.
The unknown is over what time period.
If immunity lasts 1 yr, the #s above are "per year".
If 10 yrs, 1/10th (165k, 83k), if near permanent (100 yrs), 1/100th (17k, 8k) per yr.
The unknown is over what time period.
If immunity lasts 1 yr, the #s above are "per year".
If 10 yrs, 1/10th (165k, 83k), if near permanent (100 yrs), 1/100th (17k, 8k) per yr.
What do we know about how long immunity lasts for Covid-19?
All we know is "at least several months".
For non-SAR/MERs CVs (i.e. common cold coronavirus), the answer is a few months to a couple of years.
https://www.medrxiv.org/content/10.1101/2020.05.11.20086439v2
http://www.columbia.edu/~jls106/galanti_shaman_ms_supp.pdf
All we know is "at least several months".
For non-SAR/MERs CVs (i.e. common cold coronavirus), the answer is a few months to a couple of years.
https://www.medrxiv.org/content/10.1101/2020.05.11.20086439v2
http://www.columbia.edu/~jls106/galanti_shaman_ms_supp.pdf
Reviews on this topic. If you were to read anything from this thread, the MIT one is for non-scientists.
https://www.microbiologyresearch.org/content/journal/jgv/10.1099/jgv.0.001439 https://www.technologyreview.com/2020/04/27/1000569/how-long-are-people-immune-to-covid-19/
https://www.microbiologyresearch.org/content/journal/jgv/10.1099/jgv.0.001439 https://www.technologyreview.com/2020/04/27/1000569/how-long-are-people-immune-to-covid-19/
We don't know that much about SARS Classic, because it's not endemic. But what we know suggests the immunity lasts a bit longer than for common cold CVs.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2851497/
https://www.jimmunol.org/content/jimmunol/186/12/7264.full.pdf https://academic.oup.com/jid/article/193/6/792/1031353
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2851497/
https://www.jimmunol.org/content/jimmunol/186/12/7264.full.pdf https://academic.oup.com/jid/article/193/6/792/1031353
This is the main reason most knowledgeable people think the discussion of herd immunity is rather premature & reckless.
Are we willing to let 1.5~2mil/yr die? Or even 1/10th of that?
Just note that we have no herd immunity for many inf. diseases.
Are we willing to let 1.5~2mil/yr die? Or even 1/10th of that?
Just note that we have no herd immunity for many inf. diseases.
Another is that we don't know if immune response to SARS-CoV-2 is good. It can be bad (e.g. Dengu), although no alarming data so far.
Review on this topic here: https://www.nature.com/articles/s41577-020-0321-6
Review on this topic here: https://www.nature.com/articles/s41577-020-0321-6
This is the most simplistic version. It could be worse (overshoot, etc.) or better in many ways, but we still don't know much yet. Many nuances I didn't cover.
Along w/ the discussion of herd immunity, I hear a lot of "protect the vulnerable" talk, as if that's easy.
Along w/ the discussion of herd immunity, I hear a lot of "protect the vulnerable" talk, as if that's easy.
Where was it successful? Swedes failed. So did most everyone. The only places where nursing homes, etc. weren't hit hard were those w/ low level of infection in the first place (e.g. Taiwan, S.Korea).
Nursing homes, etc. already do a lot. Expecting much more is unrealistic, IMO.
Nursing homes, etc. already do a lot. Expecting much more is unrealistic, IMO.
Look at the death tolls in these fairly young countries. Sure, better HC in the US, but not by much.
Does anyone think we can "isolate" enough seniors to only expose the demographics younger than these countries? I don't.
Does anyone think we can "isolate" enough seniors to only expose the demographics younger than these countries? I don't.
In fact, herd immunity strategy may be a realistic option in countries w/ limited HC resources & very young demographics. But many developing countries have successfully suppressed it, so that clearly isn't the only option.
Having said that, the situation is different in developed countries like US. We just have too many seniors.
Show me a successful strategy to protect the seniors (other than lowering overall infection levels), or stop considering it a realistic option.
Show me a successful strategy to protect the seniors (other than lowering overall infection levels), or stop considering it a realistic option.
Then what?
Many countries have (or getting close to) suppressed it well enough for something resembling "normal" w/o herd immunity or vaccine.
No need to eradicate (only smallpox in our history). We already live w/ many inf. diseases by suppressing them just enough.
Many countries have (or getting close to) suppressed it well enough for something resembling "normal" w/o herd immunity or vaccine.
No need to eradicate (only smallpox in our history). We already live w/ many inf. diseases by suppressing them just enough.
Covid-19 may well become endemic in the next few decades. But, as one epidemiologist I follow said, the goal is to get there as painlessly as possible.
We're starting to see some promising treatments. Vaccine may become available in a year or two.
We're starting to see some promising treatments. Vaccine may become available in a year or two.
We're getting better at dealing w/ this everyday. Just "letting it rip til herd immunity" will be quite painful & may even be futile. We have no herd immunity for many endemic diseases, yet we manage.
Just my 2 cents on this "herd immunity" talk.
Just my 2 cents on this "herd immunity" talk.
