Fact check: What is the difference in fatality rates between vaccinated and unvaccinated individuals with underlying health conditions?

1. Why aggregated hospital data shows a clear survival edge for vaccinated patients

Large hospital-based cohorts identified a measurable mortality reduction among vaccinated versus unvaccinated hospitalized COVID-19 patients, with one 86,732-patient study reporting 5.1% mortality in vaccinated patients versus 8.3% in unvaccinated patients; reductions were particularly notable in older and obese patients ^[1]. An international cohort of 83,163 hospitalizations across 38 countries confirmed vaccinated patients tended to be older and have more comorbidities, complicating crude comparisons but still indicating varied fatality risk by vaccination status and locale ^[2]. These studies underline that vaccination lowers mortality risk on aggregate after accounting for baseline differences ^{[1] [2]}.

2. Cancer and immunocompromised patients: meaningful protection but limited magnitude

Condition-specific analyses show substantial but incomplete protection for patients with cancer and immunocompromise: a Catalonia study found vaccine effectiveness of 51.8% against hospitalization and 58.4% against death in cancer patients after full vaccination, indicating roughly half the risk compared with unvaccinated cancer patients ^[3]. The CDC’s interim 2023–2024 assessment of updated vaccines estimated modest effectiveness against hospitalization (38% at 7–59 days, 34% at 60–119 days) for adults with immunocompromising conditions, emphasizing waning protection and the need for timely boosting ^[5]. These findings show vaccines reduce but do not eliminate elevated mortality risk in vulnerable clinical groups ^{[3] [5]}.

3. Boosters and bivalent doses: sharper reduction in older adults but time-dependent effects

Surveillance of older adults (≥65 years) found those who received a bivalent booster had lower mortality than unvaccinated peers, with vaccine effectiveness against death starting high and declining modestly over months (reported decline from 94% to 88% in one analysis), and mortality rate ratios falling across variant periods ^[6]. These data indicate boosters restore high short-term protection against fatal outcomes in seniors but that effectiveness wanes, supporting ongoing updates and targeted booster campaigns for high-risk groups ^[6].

4. Heterogeneity by specific high-risk conditions: some diagnoses blunt vaccine benefit

Not all comorbidities respond equally: one England-based analysis reported higher post-booster COVID-19 death hazards for people with learning disabilities, Down syndrome, pulmonary hypertension/fibrosis, and hematologic cancers, with hazard ratios from 2.57 to 5.07, indicating these conditions confer substantial residual risk even after boosting ^[4]. A separate methodological study using a novel COVID Excess Mortality Percentage measure reported relative mortality risks for two-dose vaccinees ranging widely (10.6%–36.2% of unvaccinated risk) and modest further reduction with boosters, highlighting condition- and metric-dependent variability ^[7]. These findings stress that population averages mask critical high-risk subgroups ^{[4] [7]}.

5. Reconciling international variability: age, comorbidity mix and health system factors

International comparisons reveal that country-level differences in age distribution, prevalence of comorbidities and hospitalization thresholds influence observed fatality gaps between vaccinated and unvaccinated patients. The 38-country analysis found vaccinated hospitalized patients were older and sicker, yet vaccination status still influenced outcomes variably across settings, reflecting differences in variant circulation, vaccine uptake timing, and healthcare capacity ^[2]. Therefore, absolute fatality differences reported in one country or study cannot be generalized globally without adjusting for these contextual factors ^[2].

6. Practical implications: targeting boosters and non-vaccine measures for the most vulnerable

The evidence supports prioritizing timely booster doses for older adults and people with immunocompromise or cancer to reduce fatality, while recognizing that some conditions will retain high residual risk despite vaccination. Studies recommend layered protective strategies — vaccination, early therapeutics, and non-pharmaceutical interventions — for those with conditions shown to carry high post-vaccination mortality ^{[3] [5] [4]}. Policymakers should use granular risk stratification rather than population averages when allocating boosters and treatments ^{[3] [4]}.

7. Where the uncertainties lie and what data would help next

Key uncertainties include duration of protection after updated vaccines in diverse comorbidity groups and the absolute benefit of boosters against newer variants in immunosuppressed patients. Existing studies vary by timing, definitions, and outcome measures (mortality vs. hospitalization vs. excess mortality), producing a range of effect estimates; harmonized, longitudinal data linking vaccination timing, variant prevalence, and granular comorbidity details would clarify remaining questions ^{[6] [7]}. Meanwhile, current multi-source evidence consistently shows vaccination reduces but does not eliminate fatality risk among people with underlying conditions ^{[1] [3] [4]}.