I'm getting a bit frustrated with the way the media are presenting antibodies==immunity (and conversely no antibodies==no immunity). I've done a bit of immunology in my time and its not the least bit surprising that antibodies wane over time because that's what they usually do - they are proteins (albeit robust ones) with a half life and its not a good use of a body's resources to constantly keep topping up antibodies to every virus you've been exposed to in your life, ever. With mild infections, you might not need to generate much of an antibody response in the first place. What your immune system does instead (once it's dealt with a particular virus) is create memory T and B cells that would allow you to quickly mount a response to said virus should you encounter it again, meaning it will probably be less severe second time around. Vaccines do a similar thing. Neither of these necessarily mean you can't pass on an infection, but the hope would be that yours would be less severe than it otherwise might be (which is presumably why the vulnerable groups are the priority).
It drives me crazy that antibodies are being used to calculate what proportion of the population has been exposed (or at least this is how its presented) as its scientific nonsense!