Delayed inflammatory reaction refers to symptoms accompanied by inflammatory signs that appear 2–3 weeks after HA filler injection [1]. This side effect has gained more attention with the emergence of the COVID-19 virus, which caused a global pandemic, as cases of delayed onset edema have been reported in patients who had previously received filler injections and subsequently contracted the virus or were administered the vaccine. However, the exact cause has not yet been clearly elucidated. This chapter describes the potential causes and solutions for delayed inflammatory reaction.