CD40LG and COVID-19: We included single-arm prospective observational studies that met the following criteria: (1) their population is breastfeeding women who were vaccinated or decided to be vaccinated against COVID-19 during pregnancy or breastfeeding by any available vaccine and with any number of doses; and (2) studies that measure COVID-19-specific antibody levels (IgA, IgG, or IgM) in milk and their persistence throughout time.