Covid patients may be protected for up to four months, according to study

Yet another study that doesn't take into account cellular immunity (T-cell based).

Also, antibodies aren't always present in the bloodstream: memory B cells can start producing them when needed.

That said, the study provides interesting findings, from a quick look. In particular, there is no cross-reactivity of antibodies against other coronaviruses. This is the exact opposite of what was observed with cellular responses, which IMO means that cell-based immunity plays a larger role with coronaviruses. It's a pity the discussion section of the paper doesn't mention this.