I was in a study. My IgG titer went up to 960, when I gave plasma twice, then 7-8 months after the infection it was below the 80 threshold to be considered positive. The weird thing was that six months in, I got tested for IgM and I was positive for that as well, which might been a sign of a second exposure. I know others in the same study who were negative a few weeks after being positive. So it might depend on the person and the test.