COVID-19 epidemiology and natural history

The COVID-19 pandemic caused an estimated 15 million excess deaths. As SARS-CoV-2 becomes endemic, it remains a major cause of disease worldwide. The main way the virus spreads is through direct airborne transmission, mostly via close person-to-person contact. The R0 is estimated between 2.2 and 2.7. The disease progresses through three stages with three severity levels: Stage I involves viral multiplication with mild symptoms; Stage II features pneumonia with varying inflammatory responses and hypoxia. A small number of patients progress to Stage III, the most severe phase, characterized by a hyperinflammatory syndrome, severe hypoxia, and extrapulmonary effects. Preventive measures include both pharmacological strategies like vaccination and non-pharmacological approaches such as mask-wearing, ventilation, hygiene, diagnostic testing, and isolation. Vaccination remains the primary prevention method at individual and community levels. Although the emergency phase has ended, SARS-CoV-2 continues circulating and is likely to become endemic. Complete elimination seems unfeasible in the short term due to the high transmissibility of the virus, incomplete and temporary immunity, and the ongoing emergence of variants with partial immune escape. Going forward, the incidence of COVID-19 will depend on the levels of post-infection or vaccination immunity in the population and the rate at which new variants emerge.