can trigger protective immunity. The measles vaccine is a modified live virus vaccine, though exceedingly safe. So is the oral polio vaccine, which though generally safe and highly effective has had problems in some settings with the modified live virus reverting to a more virulent form.

THE PROBLEM IS that we don't know either the virus dose for SARS-CoV-2 (the virus that causes COVID-19 disease) that triggers even an initial immune response, what exposure constitutes that lower-level exposure, whether such a low-level immune response would be PROTECTIVE against a much larger viral load exposure, and if so, whether it would be "durable" (long-lasting) protection or very transient.

Add to this the very real possibility of a phenomenon known as antibody-dependent enhancement (ADE), in which the immune response to the first virus amplifies the effect of the second type (i.e., closely related yet MUTATED form) that can be lethal and was the cause of a massive failure of a previously believed "safe" Dengue vaccine. In that case, a similar phenomenon to what happens in real-world infection was incited by the vaccine when confronted with one of the remaining three serotypes to Dengue. In this case, infection with one serotype causes a fulminant and potentially more deadly immune-incited response when infected with another. .. A first infection is rarely fatal, but a second one with a different virus type can lead to a much more serious disease.

That is likewise why throwing the gates open and assuming we can rely on "herd immunity" to get us out of this disaster is equally if not a far more disastrous approach.

Bottom line, possible, but don't depend on it! There is a lot to be determined re: human immune response. That is why rushing vaccines may be so damned dangerous if short cuts are taken.

-Laelth