I've seen it mostly in the Dutch news until now. The source is the Dutch CDC (RIVM) via Marion Koopmans (Professor of virology and head of the virology department at the Erasmus Medical Center).

Article in Dutch (translated title: Mild corona virus symptoms seem to lead to lower creation of antibodies)

https://nos.nl/artikel/2329846-milde-coronaklachten-lijken-m...