In addition, Ruggiero et al. reported a pattern of persistent anti-S IgM in 21.6% of a Pfizer-BioNTech COVID-19 mRNA vaccinated cohort of healthcare workers who had suffered previous SARS-CoV-2 infection one year prior to vaccination (i.e. the anti-S IgM was present in serum pre-vaccination after one year) [53]. Here, CD40LG is linked to COVID-19.