That's how antibodies always work - there have been a few taking advantage of the publicity a report on declining antibodies creates but it's not news and it shouldn't be scary.
I detail how the body deals with fighting a virus earlier in this thread and it's the best way it's been explained to me. Think of a virus as a particular type of enemy tank for which we have no useful method of attack. Once we encounter it, we develop a new type of shell that can destroy the tank and start manufacturing them in large quantities. We win the war and the tanks go away. Over time, we allow the supply of shells (antibodies) to decline as we don't want all of our munitions dumps full of shells we're not using. We do, however keep the machines and moulds (B cells) for quickly remaking a supply of those shells should we ever spot that type of tank on the horizon.
A test for antibodies doesn't really tell us much - antibodies decline over time as otherwise (again, explained to me by someone who does this for a living) our blood would just be a gloop of antibodies. Testing for the B and T cells will tell us much more - that's what makes us immune over the longer term, and there's no good reason to think this virus is any different.
I'd like to be clear, I'm very much in support of mass vaccination - it's absolutely vital the the health of any population to have a strong vaccination programme in place. In the absence of vaccinations, "natural" immunity (quoted because vaccinated immunity is also natural) is the only other form of long term protection.
