What Are the Risks of COVID-19 When Unvaccinated?

Unvaccinated individuals face roughly two and a half times the odds of dying from COVID-19 compared to vaccinated individuals, according to a meta-analysis of U.S. patient data, and their risks extend well beyond mortality into prolonged hospitalization, intensive care, long-term neurological problems, and a higher chance of spreading the virus to others.¹ The gap in outcomes between vaccinated and unvaccinated people has been studied across dozens of countries, multiple viral variants, and nearly every organ system the virus can affect. What follows is a walk through the specific dangers, who they hit hardest, and where the evidence gets complicated.

Mortality and Severe Disease

The most direct measure of risk is death. A meta-analysis pooling U.S. studies found that unvaccinated patients had an odds ratio for mortality of 2.46 compared to vaccinated patients, meaning they were nearly two and a half times as likely to die once infected.¹ Studies from individual hospital systems tell a consistent story. In a Romanian tertiary hospital that compared outcomes across both the Delta and Omicron waves, unvaccinated patients had significantly higher rates of severe or critical illness, longer stays, greater need for intensive care, and higher death rates, even after researchers adjusted for age and preexisting conditions.²

Intensive care data from Lombardy, Italy, illustrate a related point: among patients sick enough to be admitted to the ICU with COVID pneumonia, about three-quarters were unvaccinated.³ That proportion is striking considering that, at the time of the study, a substantial share of the population had already been vaccinated. Being unvaccinated didn’t guarantee critical illness, but it dramatically raised the probability of ending up in the most resource-intensive hospital setting.

What the Virus Does to the Lungs

Understanding why unvaccinated infections tend to be worse requires knowing what SARS-CoV-2 does to the body when the immune system hasn’t been primed. The virus targets cells in the airways, the tiny air sacs of the lungs (alveoli), and the blood vessels running through them. In severe cases, it triggers diffuse alveolar damage, a pattern of widespread destruction in the lung tissue that disrupts the barriers between air and blood.⁴ The result can be acute respiratory distress syndrome, or ARDS, where the lungs fill with fluid, oxygen transfer collapses, and the patient needs mechanical ventilation to survive.

One distinctive feature of COVID-related ARDS is that patients sometimes develop dangerously low oxygen levels before they feel short of breath, a mismatch between how bad the lungs look on imaging and how the person reports feeling.⁵ This “silent hypoxia” delayed hospital presentations early in the pandemic, and it remains a hazard in unvaccinated patients whose immune systems are encountering the virus for the first time with no template to mount a rapid, targeted response.

Age, Obesity, and Other Risk Amplifiers

Not every unvaccinated person carries the same level of risk. Age is the single strongest predictor of bad outcomes. Among unvaccinated adults hospitalized for COVID, those 46 and older were roughly two and a half times more likely to need supplemental oxygen and about four and a half times more likely to require a higher level of care or worse outcome at discharge compared to those under 46, even after adjusting for other health conditions.⁶

Obesity stands out as an independent danger, especially for people under 65. A Hungarian hospital study found that obesity was the single most significant risk factor for both ICU admission and death in that younger age group, with adjusted odds ratios of about 3.5 for ICU admission and roughly 3 for mortality.⁷ Cardiovascular disease, cancer, chronic kidney disease, and metabolic conditions like diabetes all independently raise the odds of severe disease. Male sex was also identified as an independent risk factor in that same study. The practical implication is that an unvaccinated 55-year-old man with obesity and high blood pressure faces a profoundly different risk profile than an otherwise healthy unvaccinated 25-year-old woman.

How Variants Changed the Picture

The danger of being unvaccinated has shifted as different viral variants have emerged. Among unvaccinated patients in Japan, the risk of severe COVID was highest during the Alpha and Delta waves and fell substantially during the Omicron wave compared to the original wild-type virus.⁸ A Spanish study quantified the difference more precisely: among unvaccinated people, Omicron carried a 72% lower probability of hospitalization than Delta, and Alpha carried a 43% lower risk than Delta.⁹

That sounds reassuring, and on a population level Omicron has indeed produced milder illness on average. But the variant-specific data come with a caveat for older adults. In the Japanese study, patients aged 80 and older showed no significant difference in severe disease risk between the original virus and Omicron.⁸ Similarly, the Spanish data showed the gap between Omicron and Delta hospitalization rates narrowed in people 65 and over.⁹ A matched cohort study found that among patients with zero or one vaccine dose, mortality from Omicron was not significantly different from Delta.¹⁰ The milder reputation of current variants applies most reliably to younger, healthier people. For an elderly unvaccinated person, the variant matters less than the lack of immune preparation.

Long COVID Without Vaccination

Surviving the acute infection is not the end of the story. A large systematic review and meta-analysis found that vaccinated individuals had about 23% lower odds of developing long COVID compared to unvaccinated individuals, with booster doses offering further protection beyond the primary series.¹¹ A 12-month follow-up study in a lower-income country put finer details on this: roughly 38% of unvaccinated patients developed new, worsened, or persistent symptoms consistent with long COVID, compared to 20% of vaccinated patients. Severe fatigue, shortness of breath, cough, and difficulty walking due to breathlessness were all more common in the unvaccinated group, and lack of vaccination was an independent predictor of long COVID even after adjusting for age, sex, other health conditions, and disease severity.¹²

Neurological effects deserve particular attention. A large study using U.S. Department of Veterans Affairs data found that in the year following a COVID infection, the risk of a wide array of neurological problems was elevated, including stroke, cognitive and memory disorders, peripheral nerve damage, migraines, seizures, and movement disorders. The overall hazard ratio for any neurological problem after infection was about 1.42, and these risks were elevated even in people who were never hospitalized during their acute illness.¹³ Case reports have specifically documented COVID-related encephalopathy, a condition involving brain dysfunction and confusion, developing in unvaccinated patients with delayed onset after infection.¹⁴

Spreading the Virus to Household Members

Being unvaccinated doesn’t just affect the infected person. It affects everyone around them. A meta-analysis of household transmission studies found that the average secondary attack rate from an unvaccinated index case was about 27%, compared to about 14% from a fully vaccinated index case. The difference was statistically significant across the paired studies.¹⁵ In plainer terms, if you are unvaccinated and bring COVID home, roughly one in four household members catches it from you, compared to about one in seven if you are vaccinated.

Variants complicate this picture further. The same meta-analysis found that household secondary attack rates rose across successive waves: about 15% for studies conducted in early 2020, climbing to roughly 37% for studies in 2021 and 2022, with Omicron reaching about 43%.¹⁵ A Japanese study of unvaccinated household contacts during the Delta wave found a striking 48.5% household secondary attack rate, with spousal contacts reaching above 63%.¹⁶ The relationship between the infected person and their contacts mattered: spouses were far more likely to be infected than other household members, presumably because of shared sleeping quarters and closer physical contact.

One nuance worth noting is that peak viral loads, measured by RNA in nose and throat swabs, were similar between vaccinated and unvaccinated people infected with the Delta variant in at least one longitudinal study.¹⁷ This might seem contradictory, but the transmission difference likely comes from vaccinated people clearing the virus faster and experiencing shorter symptomatic windows, even if the peak amount of virus is similar. The household transmission data, which capture real-world spread rather than lab snapshots, consistently show the gap.

Pregnancy and Children

Pregnant people who are unvaccinated and infected with SARS-CoV-2 face a distinctive set of dangers. Among pregnant patients hospitalized in the ICU with COVID, one study reported a maternal mortality rate of about 156 per 100,000, a stillbirth rate near 12%, preterm births in about 58% of deliveries, and cesarean sections in roughly 79% of deliveries.¹⁸ These are startling numbers. The high cesarean rate often reflects emergency delivery decisions made to protect the mother’s oxygen supply or the baby’s viability when the mother is critically ill.

Children are generally less likely to develop severe acute COVID than adults, but they face a rare and serious complication called multisystem inflammatory syndrome in children, or MIS-C, an immune overreaction that can damage the heart, lungs, kidneys, brain, and other organs weeks after infection. California data showed that among 12-to-17-year-olds, unvaccinated children developed MIS-C at about 23 times the rate of vaccinated children. Among 5-to-11-year-olds, the rate was about three times higher in the unvaccinated group.¹⁹ National U.S. MIS-C surveillance data confirmed that unvaccinated children had a 23% higher risk of needing ICU-level care compared to vaccinated children, and all 21 deaths in that surveillance cohort occurred in unvaccinated patients.²⁰

Secondary Infections and Autoimmune Complications

When COVID-19 damages the lungs and weakens the immune system, it opens the door to secondary bacterial and fungal infections. An Australian study spanning 2020 to 2023 found that being unvaccinated was associated with about 59% higher odds of developing a secondary infection while hospitalized compared to vaccinated patients.²¹ Older age, chronic conditions, and lower socioeconomic status also raised these odds, but vaccination status remained an independent factor. These secondary infections can include hospital-acquired pneumonia, urinary tract infections, and bloodstream infections, any of which can extend a hospital stay or prove fatal in someone whose body is already fighting a severe viral illness.

There is also emerging evidence that severe COVID can trigger autoimmune responses, where the immune system, revved up to fight the virus, starts attacking the body’s own tissues. Case reports have documented new-onset autoimmune hepatitis in unvaccinated patients following COVID infection, with some cases requiring aggressive immunosuppressive treatment.²² This is harder to quantify than mortality or hospitalization because autoimmune conditions are individually rare, but the pattern across case reports and small series suggests that uncontrolled viral replication, the kind more likely in an unvaccinated person, may be the driver.

Natural Immunity and the Case for Hybrid Protection

A common argument against vaccination is that a previous infection provides sufficient immunity. There is real science behind this claim: natural immunity after a COVID infection was associated with a 95% lower risk of reinfection and an 87% lower risk of hospitalization for up to 20 months in a large Swedish population study.²³ That is genuinely strong protection. But the same study found that adding vaccination on top of prior infection, known as hybrid immunity, reduced the risk of reinfection by an additional two-thirds compared to natural immunity alone, and hospitalization was extremely rare in the hybrid group.

Estonian data echoed this finding: during the Delta period, hybrid immunity provided about 39% lower risk of infection compared to natural immunity alone.²⁴ The advantage faded during the Omicron period for infection risk specifically, but hospitalization due to COVID remained extraordinarily rare among those with hybrid immunity. A review of the broader literature concluded that hybrid immunity appears to offer the strongest protection available against both infection and severe outcomes.²⁵ The implication is that even if you have already been infected and recovered, vaccination still meaningfully reduces your future risk.

Antivirals Are Not a Substitute, but They Help

Some unvaccinated people assume that the availability of antiviral treatments like nirmatrelvir/ritonavir (sold under the brand name Paxlovid) makes vaccination unnecessary. The drug does work remarkably well: in its pivotal trial involving unvaccinated high-risk adults, treatment within three days of symptom onset reduced COVID-related hospitalization or death by about 89% compared to placebo, and all 13 deaths in the trial occurred in the placebo group.²⁶ A real-world retrospective analysis of unvaccinated patients treated in emergency departments confirmed these benefits, finding significant reductions in mortality, ICU admissions, inpatient admissions, and dangerously low oxygen levels.²⁷

But relying on antivirals as a substitute for vaccination has practical problems. The drug must be started within five days of symptom onset, ideally within three. That requires recognizing symptoms quickly, obtaining a test, getting a prescription, and filling it, all within a narrow window. People in rural areas, those without a regular doctor, and those who don’t realize they are sick early enough can easily miss this window. The drug also interacts with a long list of medications, making it unsuitable for some patients. And antivirals do nothing about the transmission risk to others, the chance of long COVID, or the systemic inflammatory damage that can begin before treatment starts. Data from combined trial populations showed that nirmatrelvir still provided substantial benefit to vaccinated and previously infected patients as well, reducing hospitalization or death by about 74% in that group.²⁸ Antivirals work best as a second line of defense, not a first.

Hospital Stay and System Strain

Even in outcomes short of death, being unvaccinated adds up. U.S. surveillance data from 13 states spanning January 2021 to April 2022 showed that unvaccinated adults hospitalized with COVID had a longer median hospital stay than vaccinated adults.²⁹ The difference in any single case might look modest, measured in fractions of a day, but multiplied across millions of hospitalizations it translates to enormous pressure on hospital capacity, nursing staff, and the patients with other conditions whose care gets delayed when beds and ventilators are occupied. During surge periods, this cascading burden was one of the most tangible harms of low vaccination rates, extending well beyond the unvaccinated patients themselves to anyone needing hospital care for any reason.