By doing so, in the first study phase, i.e. 5–9 weeks after the first Belgian COVID-19 case, we report respective rates of SARS-CoV-2 IgM, IgA and IgG seropositivity of 10.6%, 10.2% and 10.4% among Belgian healthcare and non-healthcare workers. The gene discussed is CD40LG; the disease is COVID-19.