A disease called COVID-19 broke out in Wuhan, China, in December 2019 and rapidly spread to other parts of China and the whole world. On February 12 2020, the World Health Organization (WHO) officially coined the term COVID-19 for the disease caused by the novel coronavirus (
1).
Due to its high prevalence, complications, and mortality rate (
2), COVID-19 and its control and treatment methods have received increasing attention throughout the world. The entry receptor for COVID-19, called angiotensin-converting enzyme 2 (ACE2) receptor, is found in many different tissues, including the lungs, the heart, and the liver, making the virus extremely harmful (
3). Since some viruses, e.g., coronavirus, poorly stimulate interferon synthesis, the body’s natural defense mechanism against such infections is poorly activated. Based on the results of in vitro and in vivo experiments, interferon can strongly prevent cell proliferation, and therefore, interferon therapy is an alternative recommended for infections with coronaviruses (
4). Given the effects of interferon on cells (interferon-alpha on all cells and interferon-gamma on epithelial and certain immune cells), its early administration can efficiently inhibit the spread of SARS-CoV-2 virus (
5). The delayed increase in interferon level in SARS-CoV-2 infection strongly activates macrophages (
5). Postmortem examination revealed extremely high levels of pro-inflammatory macrophages in those who died of COVID-19 (
6).
Type I interferons (IFN-α, IFN-β, IFN-ε, IFN-κ, IFN-ω in humans) bind to the type I interferon receptor (IFNAR) in an autocrine and paracrine manner. These receptors are found everywhere in the body. This pathway activates a powerful antiviral defense program of hundreds of interferon-stimulated genes (ISGs), which have the capacity to interfere with every step of viral replication (
7).
Interestingly, although rapid production of interferon at the onset of the disease is an important factor for increasing T cells, increased interferon in later stages inhibits T cells or deplete their lymphoid tissues. Given the role of T cells in regulating the immune system function and the severe lung damage caused by cytokine release syndrome (CRS) in patients critically ill with COVID-19, the immune suppression dysfunction appears possible at this stage (
6).
Hence, the timing of interferon administration in patients with COVID-19 appears important for the regulation of both macrophage function and lymphocyte function. Since the role of interferon administration timing has not been fully investigated, we designed a retrospective study on hospitalized patients with COVID-19 to illuminate the subject.