The severity of COVID-19 infection is not uniform across the population, demonstrating a profound dependence on a person’s age. Tracking hospitalization rates by demographic group became a fundamental practice during the pandemic to gauge the burden on healthcare systems and inform public health policy. This metric helps authorities allocate resources, such as specialized medical equipment and staffing, to the communities and age groups facing the highest risk of severe illness.
How Hospitalization Rates Are Measured
The definition of a “COVID hospitalization” is fundamental to interpreting the resulting data, though it has evolved and varied across public health agencies. The U.S. Centers for Disease Control and Prevention (CDC), through its COVID-19-Associated Hospitalization Surveillance Network (COVID-NET), monitors cases confirmed by a laboratory test for SARS-CoV-2 in a person admitted to a hospital. This metric generally includes anyone hospitalized within 14 days of a positive test, or who tests positive upon admission.
The challenge lies in distinguishing between patients admitted for COVID-19 illness and those admitted with COVID-19, often referred to as “incidental” cases. Incidental cases involve a patient hospitalized for an unrelated issue, such as a broken bone or heart condition, who tests positive during routine screening. Governmental and national health agencies, like the CDC and the World Health Organization (WHO), use this broader definition for surveillance to capture the full scope of the virus’s presence in healthcare settings. However, this can sometimes overstate the burden of severe COVID-19 disease itself. More refined metrics, sometimes adopted by hospitals, focus on clinical severity markers like the need for supplemental oxygen or low blood oxygen saturation levels, to better reflect admissions directly caused by the virus. The hospitalization rate is calculated by dividing the number of hospitalized individuals with a confirmed infection by the total population estimate for a defined surveillance area.
Statistical Breakdown of Risk by Age Group
COVID-19 hospitalization rates reveal a stark, non-linear increase in risk that accelerates dramatically with advancing age. Data consistently show that the risk of severe outcomes, including hospitalization, is lowest among children and young adults. Compared to the 18- to 29-year-old reference group, hospitalization rates are significantly lower for children aged 5 to 17 years.
The risk begins to climb steadily in middle age, becoming elevated in the 50-to-64 age bracket. Adults aged 50 to 64 years have historically faced a hospitalization risk about four times higher than the youngest adult cohort.
The most dramatic disparity is observed in the oldest cohorts, where hospitalization risk is exponentially higher. Individuals aged 65 to 74 years have shown a hospitalization rate approximately six times greater than the reference group. The risk for people aged 75 to 84 years increases to about nine times higher, and for those 85 years and older, the risk is about 15 times greater than that of 18- to 29-year-olds. This exponential trend is so pronounced that the incidence of COVID-19 hospitalizations has been shown to double with every 16 years of age.
Biological Factors Driving Age Disparities
The pronounced age-related disparity in severe COVID-19 outcomes stems from several interconnected biological changes that occur with aging. One primary factor is immunosenescence, which describes the gradual deterioration of the immune system’s function.
This decline affects both the innate and adaptive immune responses, making it harder for the body to mount an effective and timely defense against the SARS-CoV-2 virus. The thymus, which is responsible for producing new T-cells, undergoes involution, meaning it shrinks and produces fewer functional T-cells over time. Since T-cells are crucial for recognizing and clearing virus-infected cells, this reduced capacity directly correlates with a higher probability of severe disease and hospitalization given an infection.
Another factor is “inflammaging,” a state of chronic, low-grade, sterile inflammation that is associated with aging. Older adults often start with elevated levels of pro-inflammatory markers, such as specific cytokines, even before infection. When SARS-CoV-2 infects the body, this pre-existing inflammatory state can be exacerbated, leading to a dysfunctional and hyperactive immune response that contributes to tissue damage and the severe clinical manifestations of COVID-19. The increased prevalence of chronic underlying health conditions, or comorbidities, like heart disease, diabetes, and chronic lung disease, also significantly contributes to the risk of severe illness in older adults.
How Prior Immunity Changes Risk Across Age Groups
Acquired immunity, derived from either vaccination or a previous SARS-CoV-2 infection, fundamentally alters the baseline hospitalization risk across all age groups. While the underlying biological vulnerabilities tied to age remain, prior immunity acts as a protective layer, significantly reducing the probability of developing severe disease that requires hospitalization.
This immunity works by preparing the adaptive immune system to quickly recognize the virus, leading to a faster, more effective response that limits viral replication and prevents the uncontrolled inflammation that characterizes severe COVID-19.
For younger age groups, initial vaccination or infection provides robust protection against severe outcomes, often reducing their already low baseline risk to a minimal level. The benefit of this acquired immunity, however, is not static and tends to wane over time, necessitating booster doses to maintain high levels of neutralizing antibodies.
The protective effect against severe outcomes for older populations is equally important, but the decline in immunity can be more pronounced or occur faster due to their underlying immunosenescence. Older adults require booster strategies to ensure their immune defenses are continually topped up against severe disease.
Though prior immunity is highly effective, the absolute risk of hospitalization for an older, vaccinated individual may still be greater than that of a young, unvaccinated person, simply because of the overwhelming impact of age-related biological factors. Therefore, public health recommendations emphasize continued vaccination and boosters, especially for the most vulnerable elderly cohorts, to mitigate the age-driven risk of severe COVID-19 disease.