Clinical Outcomes in Vaccinated and Unvaccinated COVID-19 Inpatient Admissions March to September 2021: A Single Center Retrospective Cohort Analysis (Preprint)

BACKGROUND COVID-19 is an international public health emergency. Despite challenging initial months plagued by death and uncertainty, the national scientific community developed a set of effective vaccines against the virus. The United States received early and consistent access relative to less industrialized countries. Despite this, vaccination rates in the United States have not been as robust. As SARS-CoV-2 variants developed, the prospect of vaccine-mediated immunity in the face of emerging variants remains guarded. What’s more, surges in COVID-19 cases in vaccinated patients gave pause to consider the underlying cause. OBJECTIVE To identify the effectiveness of Covid vaccination strategies in adult inpatient admissions. METHODS Retrospective cohort analysis of Emanate Health hospital system from March - September 2021. Demographic and clinical factors for adult inpatient COVID-19 positive admissions were analyzed using Meditech software. These included age, sex, comorbidities, date of admission, and immunization status. Immunization status was confirmed via the California Immunization Registry. Virus subtype and immunization status was not an exclusion criterion. RESULTS A total of 475 COVID positive adult inpatient admissions were analyzed. Of the total population sample, 28% of patients were vaccinated and 72% were unvaccinated. Inpatient admissions steadily decreased from March to June 2021, followed by steep increases from June to August 2021 driven predominately by unvaccinated patients. Length of stay was statistically significantly higher for patients 60-79 years old (P = .003) and in patients with four and six comorbidities (P = .002). Mortality was 1.5 times more likely in patients aged 60-79 years (P< .001) and 1.3 times more likely in those with 4 and 6 comorbid conditions (P = .002). Partially vaccinated individuals were admitted earlier to the hospital compared to fully vaccinated individuals (P< .001) and were more likely to require ICU admission and ventilation (P = .018). ICU admission was significantly higher in males compared to females (P = .027). Patients requiring an intensive care unit (ICU) admission and mechanical ventilation had a significantly higher mortality rate compared to those not requiring ICU admission or mechanical ventilation (P< .001). CONCLUSIONS Our institutional data demonstrates strong trends previously described in national literature, including poor outcomes in unvaccinated, older patients, and those with multiple comorbid conditions. Fully vaccinated individuals had a shorter length of stay, irrespective of vaccine type. Mortality increased with age and was highest in patients 60-79 years old. In vaccinated patients, the top three comorbidities with the highest admission and mortality rates were hypertension, diabetes mellitus, and obesity, raising further questions regarding risk profiles in COVID-19 hospitalizations. Furthermore, COVID infection rates in vaccinated patients were lowest at 2 months and highest at <14 days and 5 months post-vaccination providing insight into the development and waning of immunity.