1/ I have a Q about this cross-sectional study for people w/ experience in virology &/or biomed. The researchers took samples & then compared them to different samples at a later time. From this, they interpreted a decline in antibodies in the population at large . . .

2/ The decline in the cross-sectional samples seems very small: 6% - 4.4%. At the same time, the study looks very well powered, such that the samples were huge. But I don't really see much statistical analysis.
Is it normal to interpret an actual shift based on these data?

3/ links:

https://www.theguardian.com/world/2020/oct/27/proportion-of-people-in-england-with-covid-antibodies-has-fallen-study-says

https://www.imperial.ac.uk/media/imperial-college/institute-of-global-health-innovation/MEDRXIV-2020-219725v1-Elliott.pdf

4/ If this were repeated measures, I would not have the same concern. But my impression is that it's not. And so I wonder if you can interpret that as a real decline, even if you have a very large sample. My intuition is that one could not, but I'm not in biomed.