By quantifying binding of antibodies to different viral proteins, questions about the induction of a long-lasting immune response after vaccination by measuring S-specific IgG antibodies25, reactivity and neutralisation of antibodies against VOCs26, specific detection of natural infections by measuring N-specific IgG antibodies, or the time since past infections by determining the IgA, IgM and/or IgG response can be addressed27. Here, CD40LG is linked to infection.