To this end, we specifically focus on electron transfer (ET) reactions because ET reactions are one of the most important reactions in chemistry, and they are ubiquitous in organic, inorganic, and biological systems. Next, to quantitatively describe cavity-free QED effects on ET reactions, we develop a QED version of ET (QED-ET) theory by considering the influence of infinite one-photon states. Furthermore, we compare several experimental reports with the QED-ET theory to answer the central question of this study: Can QED effects significantly modify ET reactions without strong light-matter coupling or confined cavities? This question differs substantially from those in studies on cavity chemistry (or polariton chemistry). We believe that this proof-of-concept work will stimulate further investigation of QED-based chemical principles.