Selective-Serotonin reuptake inhibitors increase serotonin levels in the brain improving depression and stabilizing emotions. Serotonin also has anti-inflammatory action by preventing the release of IL-6 and TNF-alpha, which are pro-inflammatory cytokines. This anti-inflammatory action  can be useful in the treatment of COVID-19, as this condition occurs as an overstimulation of the body’s pro-inflammatory action. Fluvoxamine is a SSRI and may have this anti-inflammatory activity and other possible antivirus abilities. Inhibition of the acid sphingomyelinase or ceramide system by fluvoxamine also plays a role in the infection. The use of fluvoxamine in COVID-19 has shown to decrease mortality. 
References: 
1. Oskotsky T, Marić I, Tang A, et al. Mortality risk among patients with COVID-19 prescribed selective serotonin reuptake inhibitor antidepressants. JAMA Net. 2021;4(11):e2133090-e2133090. doi: 10.1001/jamanetworkopen.2021.33090.
2. Carpinteiro  A, Edwards  MJ, Hoffmann  M,  et al.  Pharmacological inhibition of acid sphingomyelinase prevents uptake of SARS-CoV-2 by epithelial cells.   Cell Rep Med. 2020;1(8):100142. doi:10.1016/j.xcrm.2020.100142