TLR7 and COVID-19: Furthermore, it is possible, but only speculative, that the addition of factors that deteriorate TLR7 function [e.g. circulating 25-hydroxy vitamin D [25OHD] levels decline with age (22) and may be accompanied by a lower TLR7 expression and defective function (23)] in patients with these low effect variants may be a common cause of progression to the most severe stages of COVID-19 in male.