I guess this would have been deemed utterly unremarkable pre-COVID.

"... memory B cell responses do not decay after 6.2 months, ... individuals who are infected with #SARSCoV2 could mount a rapid and effective response to the virus upon re-exposure."https://t.co/kuB5NlPzej

— Prof Francois Balloux (@BallouxFrancois) November 5, 2020