In this small study from China involving asymptomatic or mildly symptomatic people with COVID-19, after 2 to 3 months there was a steep decline in antibodies against the SARS-CoV-2.
https://www.nature.com/articles/s41591-020-0965-6
This doesn't bode well for the efficacy of a vaccine. Darn. I've been very optimistic about a vaccine.
Oh well, maybe a vaccine will offer some protection but will have to be taken four times per year.