Aging & COVID-19 susceptibility, disease severity, and clinical outcomes: The role of entangled risk factors.

The emergence of Severe Acute Respiratory Syndrome Corona Virus 2 (SARS-CoV-2) in late 2019 has been associated with a high rate of mortality and morbidity. It has been determined that the old population are not only at an increased risk for affliction with COVID-19 infection, but also atypical presentations, severe forms of the disease, and mortality are more common in this population. A plethora of mechanisms and risk factors contribute to the higher risk of infection in the old population. For instance, aging is associated with an increment in the expression of Angiotensin-Converting Enzyme-2 (ACE-2), the receptor for SARS-CoV-2 spike protein, which precipitates replication of the virus in the old population. On the other hand, immune dysregulation and changes in gut microbiota as a result of aging can contribute to the cytokine storm, one of the main indicators of disease severity. Decrement in sex steroids, especially in women, as well as growth hormone, both of which have crucial roles in immune regulation, is a key contributor to disease severity in old age. Senescence-associated oxidative stress and mitochondrial dysfunction in both pneumocytes and immune cells contribute to the severity of infection in an exacerbative manner. In addition, lifestyle-associated factors such as nutrition and physical activity, which are compromised in old age, are known as important factors in COVID-19 infection. Aging-associated comorbidities, especially cardiovascular diseases and diabetes mellitus, also put older adults at an increased risk of complications, and disease severity.