Fact check: What is the difference in hospitalization rates between vaccinated and unvaccinated populations?

1. Why the headline numbers differ — the messy reality behind “hospitalization rates”

Studies report different percentages because they measure different outcomes, times, and populations: some report vaccine effectiveness against hospitalization (a relative reduction) while individual-hospital cohorts report raw hospitalization or mortality rates among admitted vaccinated and unvaccinated patients ^{[1] [6]}. The IVY Network multicenter surveillance measured VE for the 2024–2025 vaccine against hospitalization and severe in-hospital outcomes, producing VE estimates of ~40% for hospitalization and 79% for invasive ventilation/death, whereas single-center clinical series can find no mortality difference among already-hospitalized cohorts because those analyses do not measure prevention of admission, only in-hospital outcomes ^{[1] [6]}. These methodological differences drive apparent contradictions.

2. Recent multi-site surveillance: measurable but partial protection in 2024–2025

Large, multi-hospital surveillance studies from the 2024–2025 season found moderate effectiveness against hospitalization—around 29–40% in several analyses—and stronger protection against death or mechanical ventilation (up to 79%) ^{[1] [2]}. These findings are recent (September 2025 and related analyses covering Sept 2024–Apr 2025) and reflect circulating variants and updated vaccines; they indicate vaccines continue to prevent many, but not all, hospital admissions. The fluctuating VE numbers reflect variant immune escape, waning immunity, and differences in booster uptake, which the surveillance networks accounted for in adjusted models ^{[1] [2]}.

3. Clinical cohorts: why hospitalized vaccinated patients may still have high mortality

Hospital-based clinical series sometimes report similar or even higher in-hospital mortality among vaccinated versus unvaccinated patients because hospitalized vaccinated patients are often older, more medically complex, and residents of long-term care facilities, and the studies analyze outcomes conditional on admission rather than prevention of admission ^{[6] [4]}. A Brazilian cohort reported mortality of 45% (vaccinated) vs 37% (unvaccinated) among hospitalized patients, coupled with trends toward greater respiratory support needs and longer stays among the unvaccinated, illustrating how case mix and severity at presentation can produce counterintuitive in-hospital comparisons ^[6].

4. Imaging, biomarkers, and severity signals: mixed but generally favorable for vaccinated people

Smaller clinical investigations using imaging and laboratory markers showed lower CT severity scores and lower need for oxygen or ventilator support in vaccinated patients, while sometimes finding elevated inflammatory markers—indicating complex host responses but overall fewer critical respiratory events ^[7]. These studies, often from single centers and earlier periods, provide biologic plausibility that vaccination reduces disease severity even when breakthrough infections occur, supporting the surveillance estimates that vaccines lower severe outcomes even if they do not eliminate all hospitalizations ^[7].

5. Who remains hospitalized — age, comorbidities, and long-term care driving the statistics

Surveillance across the U.S. found that adults aged 65 and older accounted for ~70% of COVID-19 hospitalizations during Oct 2023–Apr 2024, and most hospitalized adults had multiple underlying conditions and a significant share were long-term care residents ^[4]. This demographic concentration means that population-level hospitalization rates among vaccinated vs. unvaccinated groups depend heavily on vaccination coverage in older and frail populations, and that hospitalizations may increasingly cluster among those with weaker vaccine responses or incomplete coverage ^{[4] [5]}.

6. How public-health messaging and study framing create different impressions

Population surveillance emphasizes preventing admissions, producing VE estimates that highlight benefits of vaccination at the community level; clinical cohort studies among admitted patients emphasize in-hospital outcomes, which can understate population-level benefits if they lack a denominator of infections prevented ^{[1] [6]}. Researchers, policymakers, and media may highlight different metrics—relative VE, absolute risk reduction, or in-hospital mortality—leading to divergent public impressions. Recognizing these framing choices is essential: both views are valid but answer different questions ^{[1] [6]}.

7. Bottom line for policy and individuals — targeted protection matters

The combined evidence shows vaccines remain an important tool: they substantially reduce the risk of hospitalization and greatly reduce the worst in-hospital outcomes, though protection against any hospitalization is partial and varies by vaccine update, timing, and patient factors ^{[1] [3]}. For policymakers, the implication is to prioritize boosters for older adults and residents of long-term care facilities and to communicate that vaccination reduces, but does not eliminate, risk—especially for medically vulnerable groups who still account for most hospitalizations ^{[4] [5]}.