A critical factor that makes the elderly more susceptible to infectious diseases is what immunologists call “immunosenescence”: the decline in the immune system’s functionality as people age. This is also associated with an increase in the incidence of inflammatory diseases, because an elderly body tends to be in a state of chronic low-grade inflammation. This “inflamm-aging” is one reason why older people have tendencies to develop more severe forms of respiratory diseases.
The key problem with SARS-CoV-2 infection is inflammation in the respiratory tract, which can be exacerbated in individuals predisposed towards potent inflammatory responses.
Immunosenescence also results in diminished responses to vaccination. Indeed, annual flu vaccines are notoriously less effective in the elderly. This phenomenon is very important in the context of the massive efforts and funds being invested worldwide into the ultra-rapid development of vaccines for COVID-19.
The fact that elderly people do not respond well to immunizations has largely been ignored in most discussions of COVID-19 vaccines, despite this being the group in greatest need. Most of the scientific community’s experience with vaccine development for any disease has been focused on vaccinating the relatively young.