Herd immunity normally requires a vaccine, unless the IFR is low and immunity is nearly lifelong
COVID IFR is about 10 x higher than flu
and the igG antibodies conferring immunity drop away within weeks.
Also, recent serological surveys in high COVID epicentres have shown v high levels of antibodies, suggesting herd immunity in desnely populated areas may need to be very high indeed:
- 93% in Iquitos, Peru
- 78% in some areas of New York City
- 57 % in Mumbai slums
It is thought T cells in at least some previously infected people may combat the disease,
but scientists researching these for COVID have warned they do not confer actual immunity or stop virus shed, but probably would considerably reduce severity of illness:
"Immunity" and T cells
here is a scientist specialising in vaccines & the immune system:
https://threadreaderapp.com/thread/1293344524731691008.html
Shane Crotty@profshanecrotty
1/ There are various tweets misinterpreting COVID-19 “pre-existing immunity” and making dangerous claims about herd immunity.
Since many of those claims refer to our scientific papers,
we will reiterate the facts.
2/ Our 1st scientific paper showed that ~50% of unexposed people have T cells that recognized SARS-CoV-2 already doi.org/10.1016/j.cell….
The most obvious conclusion was these were memory T cells from previous common cold coronavirus infections,
but that was not directly shown.
3/ Our 2nd paper, very recently published,
showed that these were memory T cells from previous common cold coronavirus infections
https://science.sciencemag.org/content/early/2020/08/04/science.abd3871
Five other labs have also published related findings
doi.org/10.1038/s41577…
4/ These observations about pre-existing T cell immunity (also call "crossreactive immune memory",
which avoids the word "immunity" that sometimes gets misinterpreted as “protective immunity”
[sorry, immunology is complicated] )
are important because...
5/ ...these memory cells MAY impact people’s responses to SARS-CoV-2 infection,
or COVID-19 vaccines.
6/ Therefore, we have pointed out that these coronavirus crossreactive immune memory T cells are important,
to pay attention to in human COVID-19 studies this year.
7/ We SPECULATE that it is conceivable that these T cells may potentially reduce COVID-19 disease severity,
based on things we know about flu and T cells.
8/ These are only speculations (no data)
and because of their potential importance it is key for scientists to test these ideas, as quickly as possible.
While scientists are racing as fast as possible,
sophisticated research like this usually takes a lot of time and resources.
9/ We, and other labs, are working hard on these unanswered questions.
10/ Additionally, even if our most optimistic speculations about crossreactive T cell memory were found to be correct,
it would mean that just as many people would get infected with SARS-CoV-2, but fewer would become severely ill and die from COVID-19.
11/ T cells generally don’t completely prevent infections, they limit disease
(make it shorter and/or less serious).
Thus, wearing a mask is much more effective than hoping you and the people around you have pre-existing T cell memory.
Wearing a mask stops infections.