One aspect of the time element is not "this much time has past" but rather "the subjects have had enough exposures to the virus". For some viruses, like HIV, th

One aspect of the time element is not "this much time has past" but rather "the subjects have had enough exposures to the virus". For some viruses, like HIV, this may take a long time to accomplish. But for COVID, in the middle of a pandemic, it doesn't take that long to accomplish.