Some countries are doing antibody testing, does this help in knowing those people who ever infected by COVID-19 are less likely to get a new infection? if the answer is "No" what are the reasons for antibody testing; if the answer is "Yes" why is not scaled up?
Please go through the article.
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31034-5/fulltext
In some cases people need to tie themselves to some hope even if it is wrong !
Forget about herd or any significant immunity after recovering from COVID-19
Although, the course of a second infection (similar to first episode, or milder, or even severe is not yet known.
To know why, read the latest article:
https://www.medrxiv.org/content/10.1101/2020.06.13.20130252v1.full.pdf
Yes, initially public health leaders of few countries thought for that and then switched to lockdown quickly.