There could be a meaningful distinction between exposure and infection. I would define the first as anyone who was exposed to the virus (or even just parts of t

There could be a meaningful distinction between exposure and infection. I would define the first as anyone who was exposed to the virus (or even just parts of the virus) and generated antibodies. The second would be an actual infection and presumably a sufficient antibody response to repel the virus and confer immunity to another infection. Really you can generate antibodies without even getting exposed to live virus, just like you can develop allergy to pollen or other foreign objects that enter your body. I would say only the second case is meaningful for public health. For that you need to look at how people do convalescent plasma. You need to test for specific antibodies that actually confer immunity.