Several factors determine whether COVID-19 stays mild or turns dangerous, and most of them interact with each other. Your age, underlying health conditions, immune status, vitamin D levels, genetics, and even how quickly you start treatment all influence how sick you get. Understanding these factors helps you gauge your personal risk and take practical steps to protect yourself.
Age Is the Strongest Single Risk Factor
Age drives COVID-19 severity more than any other variable. CDC surveillance data from the 2023–2024 respiratory season found that adults 65 and older accounted for 70% of all COVID-19 hospitalizations among adults, despite being a much smaller share of the population. Adults 75 and older were hospitalized at 24 times the rate of adults aged 18 to 49. They also accounted for roughly half of all in-hospital deaths.
This isn’t just because older adults have more chronic diseases, though that contributes. Aging itself weakens the immune system’s ability to mount a fast, coordinated response to new infections. The result is a slower initial defense, which gives the virus more time to replicate and spread deeper into the lungs before the immune system catches up. When it does respond, it often overreacts, causing the kind of inflammatory damage described below.
Chronic Health Conditions Multiply the Risk
Among adults hospitalized with COVID-19 during the 2023–2024 season, 80% had multiple underlying medical conditions. The specific conditions that matter most have been studied extensively.
Chronic kidney disease stands out as one of the most dangerous comorbidities. A nationwide cohort study found that people with kidney disease had 5.6 times the odds of developing severe COVID-19 or dying compared to those without it. Type 2 diabetes raised the odds of severe illness by about 43%. Both conditions impair the body’s ability to regulate inflammation and fight infection, and both are associated with blood vessel damage that compounds the lung injury COVID-19 causes.
Obesity, heart disease, chronic lung conditions like COPD, and immunosuppressive conditions (from organ transplants, cancer treatment, or autoimmune medications) all increase severity through overlapping mechanisms. Obesity, for instance, promotes chronic low-grade inflammation even before infection, meaning the immune system is already primed to overreact. It also reduces lung capacity and makes ventilation harder if breathing becomes compromised.
How the Immune System Turns Against You
The paradox of severe COVID-19 is that much of the damage comes not from the virus itself but from your own immune response. When SARS-CoV-2 infects lung cells, your immune system sends waves of defensive cells to the site. In a healthy response, these cells kill infected cells, clear the virus, and stand down. In severe cases, the signaling molecules that coordinate this response, called cytokines, spiral out of control.
This “cytokine storm” creates a destructive feedback loop. Immune cells flood the lungs and release inflammatory signals, which recruit more immune cells, which release more signals. One key player is IL-6, an inflammatory molecule found at significantly higher levels in patients who need ventilation and in older hospitalized patients who die compared to those who recover. Other inflammatory markers, including IL-1β and IL-8, are also elevated in critically ill patients at levels exceeding those seen even in severe bacterial pneumonia.
The cascade doesn’t stop at inflammation. It triggers blood clotting pathways, which is why severe COVID-19 often involves dangerous clots in the lungs and elsewhere. Immune cells in the lungs also release web-like structures called NETs that trap pathogens but simultaneously kill the delicate cells lining the airways and blood vessels. The result is fluid-filled, damaged lungs that can no longer exchange oxygen efficiently.
Genetics Can Quietly Stack the Deck
Some people with no obvious risk factors still end up critically ill. Research published in Science identified a reason: rare genetic mutations that cripple the body’s first line of antiviral defense. About 3.5% of patients with life-threatening COVID-19 pneumonia (aged 17 to 77) carried mutations in genes responsible for producing type I interferons, the signaling molecules your cells release within hours of detecting a virus. Without a strong interferon response, the virus replicates unchecked during the critical early window, and the immune system is forced into the kind of massive, delayed overreaction that causes severe disease.
A separate finding was even more striking: roughly 10% of patients with severe COVID-19 pneumonia had autoantibodies that neutralized their own interferons. Their genes were fine, but their immune systems were essentially blocking their own antiviral alarm system. This helps explain why some otherwise healthy people deteriorate rapidly, and it suggests that interferon status is one of the most important biological determinants of outcome.
Low Vitamin D Levels and ICU Outcomes
Vitamin D plays a role in regulating immune function, and its deficiency has been consistently linked to worse COVID-19 outcomes. A study of ICU-admitted COVID-19 patients found that all 40 had low vitamin D levels at admission, with a median of 12 ng/mL (healthy levels are generally considered above 30 ng/mL). Patients who died had even lower levels, with a median of 9.6 ng/mL compared to 13.3 ng/mL in survivors.
The cutoff that best predicted death was 9.9 ng/mL. Patients admitted to the ICU with vitamin D levels at or below that threshold had a 5.6-fold higher risk of dying. For context, COVID-19 outpatients who never needed intensive care had significantly higher vitamin D levels, averaging 22.5 ng/mL. This doesn’t prove that taking vitamin D supplements will prevent severe COVID-19, but it does suggest that being deficient removes one layer of immune protection your body relies on.
Vaccination Status Still Matters
Among adults hospitalized with COVID-19 during the October 2023 to April 2024 surveillance period, 88% had not received the most recent updated COVID-19 vaccine before hospitalization. Vaccination doesn’t eliminate the risk of infection, but it primes the immune system to respond faster and more effectively, reducing the window in which the virus can replicate and reducing the likelihood of the exaggerated inflammatory response that leads to severe disease.
Secondary Infections Compound the Damage
About 21% of COVID-19 patients develop a bacterial co-infection on top of the viral illness. Critically ill patients are especially vulnerable, particularly those on ventilators. The most common culprit in ventilated patients is Pseudomonas aeruginosa, found in 38% of ventilator-associated pneumonia cases. These secondary infections compound the lung damage already caused by the virus and the immune response, leading to longer hospital stays, higher fever, and increased mortality. Bacterial co-infection has been a major driver of death in every respiratory virus pandemic, and COVID-19 is no exception.
Treatment Timing Makes a Major Difference
One of the most actionable factors in COVID-19 severity is how quickly antiviral treatment begins. The oral antiviral combination of nirmatrelvir and ritonavir (commonly known as Paxlovid), when started within three days of symptom onset, reduced the risk of hospitalization or death by 89% in high-risk, unvaccinated adults. That’s a dramatic reduction, but it depends entirely on speed. The drug works by blocking the virus from replicating, so it’s most effective before the viral load peaks and before the immune system’s inflammatory cascade is fully underway.
If you’re in a high-risk group and develop COVID-19 symptoms, getting tested and starting treatment within that first three-day window is one of the most important things you can do to keep the illness from becoming severe. After the virus has already triggered widespread inflammation, antivirals have much less to offer.
Current Variants and Severity
The dominant variants circulating in 2024, including JN.1 and its descendants, do not appear to cause more severe disease than earlier Omicron subvariants. Symptoms remain similar: sore throat, congestion, fatigue, body aches, and sometimes gastrointestinal issues. The shift toward less intrinsic severity in newer variants is real, but it doesn’t eliminate risk for vulnerable people. The same factors that made earlier waves dangerous, including age, chronic disease, immune suppression, and delayed treatment, continue to determine who gets seriously ill.