This is a very good (and long) article which covers the ever evolving science and research going into understanding depression -- and also a possible role of inflammation in depression, which can come through an number of causes, including viral (such as Covid-19 inflammation).
QuoteDepression has often been blamed on low levels of serotonin in the brain. That answer is insufficient, but alternatives are coming into view and changing our understanding of the disease
https://www.quantamagazine.org/the-cause-of-depression-is-probably-not-what-you-think-20230126/