Read up on T Cells and immunity. They think 40-60 percent of the population may have complete immunity to it.
A July 15 study published in Nature found that there are three separate groups that showed immunity to the coronavirus. The first consisted of COVID-19 patients who have T cells—a bunch of immune cells that can kill the virus and act as a line of defense against future pathogens. The next faction included people who had been infected with the SARS virus in 2003 and also had these same T cells.
But the final group was the most surprising to researchers: 37 study subjects had the T cells despite the fact that they were never exposed to or infected with either of the illnesses. This proves that we all may have some immunity to this strain of coronavirus built into our bodies.
"A level of pre-existing immunity against SARS-CoV2 appears to exist in the general population," Antonio Bertoletti, MD, a virologist at Duke NUS Medical School in Singapore told The New York Times.