@baseball mom2020 posted:Herd immunity does not occur until around 60 - 70% of the population is infected. They are finding that some people who had covid and donated antibodies are now several months later antibody negative. So herd immunity may never be reached with this disease.
That 60-70 percent number is/was a guess based on a snapshot of information. There could be 20-50 percent of the population with cross-reactive T cell immunity from exposure to other viruses. That will impact the herd immunity threshold significantly.
Also, antibody negative does not mean someone has lost long term immunity as "memory" B cells and T cells remain. When these memory cells recognize the virus they produce new antibodies as needed. There are patients from the SARS outbreak 17 years ago that still have T cells for that virus.