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Lower in-hospital mortality for vaccinated patients hospitalized with COVID-19

Kaiser Permanente research examined role of vaccination in deaths during delta surge


COVID-19 patients in Kaiser Permanente Northern California hospitals during the delta surge of 2021 were less likely to die in the hospital of COVID-19 if they were vaccinated, new research finds. The study was published in the Journal of Internal Medicine.

Laura Myers, MD, MPH. Research Scientist, Division of Research.

“We wanted to learn more about what happened to fully vaccinated patients with breakthrough infections and how their outcomes compared to those of unvaccinated patients,” said lead author Laura Myers, MD, MPH, a research scientist with the Kaiser Permanente Division of Research and practicing intensivist with The Permanente Medical Group. “These findings suggest that COVID-19 vaccines were protective against in-hospital mortality during the delta wave.”

The study examined 7,305 hospitalized patients with confirmed COVID-19 who required supplemental oxygen between April 1 and November 30, 2021. Of these, 78% were unvaccinated, 20% were fully vaccinated, 2% were partially vaccinated. Fully vaccinated patients were older and had a higher severity of illness at admission. Fully vaccinated patients also had greater comorbidity burden (49% had diabetes, for example).

To address these differences between the groups, the researchers matched 1,463 fully vaccinated patients with 1,463 unvaccinated patients on age, sex, comorbidity burden, diabetes, and hypertension to generate comparable groups and understand the effect of vaccination.

They were able to create 2 similar groups, though the fully vaccinated group of matched patients had a higher comorbidity burden. Nevertheless, the fully vaccinated patients were less likely to die as an inpatient (9%) compared to the unvaccinated group (16.3%) and less likely to need intensive care (16.1% vs. 21.4%). Fully vaccinated patients also had a shorter length of stay in the hospital (5 days vs. 6 days). The groups were equally likely to receive hospice referral (4.4% vs. 4.5%).

Nicola Klein, MD, PhD, director, Vaccine Study Center.

The results are encouraging, the authors said, because vaccination provided protection against death even when comparing vaccinated patients with more comorbid factors to unvaccinated patients with less of a comorbidity burden.

“It is important to study specific outcomes, such as mortality, to best evaluate effectiveness of vaccines and their role in combating a pandemic,” said co-author Nicola Klein, MD, PhD, director of the Kaiser Permanente Vaccine Study Center. “These findings are consistent with other research – including evaluations of mRNA vaccine effectiveness the Vaccine Study Center participated in – showing protection against real-world outcomes. This is particularly important for patients whose other health conditions may put them at greater risk.”

The study focused on a period of time when the delta variant was predominant, and the omicron variant behaves differently. Further research is needed to study the effect of vaccination on risk of death as new viral variants arise, Myers said.

Other co-authors were Vincent Liu, MD, MS, Patricia Kipnis, PhD, John Greene, MA, Brian Lawson, PhD, Gabriel J. Escobar, MD, Bruce H. Fireman, MA, all of the Division of Research.

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About the Kaiser Permanente Division of Research

The Kaiser Permanente Division of Research conducts, publishes and disseminates epidemiologic and health services research to improve the health and medical care of Kaiser Permanente members and society at large. It seeks to understand the determinants of illness and well-being, and to improve the quality and cost-effectiveness of health care. Currently, DOR’s 600-plus staff is working on more than 450 epidemiological and health services research projects. For more information, visit or follow us @KPDOR.

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