Association Between Caseload Surge and COVID-19 Survival in 558 U.S. Hospitals, March to August 2020

Abstract

Background: Several U.S. hospitals had surges in COVID-19 caseload, but their effect on COVID-19 survival rates remains unclear, especially independent of temporal changes in survival.

Objective: To determine the association between hospitals' severity-weighted COVID-19 caseload and COVID-19 mortality risk and identify effect modifiers of this relationship.

Design: Retrospective cohort study. (ClinicalTrials.gov: NCT04688372).

Setting: 558 U.S. hospitals in the Premier Healthcare Database.

Participants: Adult COVID-19-coded inpatients admitted from March to August 2020 with discharge dispositions by October 2020.

Measurements: Each hospital-month was stratified by percentile rank on a surge index (a severity-weighted measure of COVID-19 caseload relative to pre-COVID-19 bed capacity). The effect of surge index on risk-adjusted odds ratio (aOR) of in-hospital mortality or discharge to hospice was calculated using hierarchical modeling; interaction by surge attributes was assessed.

Results: Of 144 116 inpatients with COVID-19 at 558 U.S. hospitals, 78 144 (54.2%) were admitted to hospitals in the top surge index decile. Overall, 25 344 (17.6%) died; crude COVID-19 mortality decreased over time across all surge index strata. However, compared with nonsurging (<50th surge index percentile) hospital-months, aORs in the 50th to 75th, 75th to 90th, 90th to 95th, 95th to 99th, and greater than 99th percentiles were 1.11 (95% CI, 1.01 to 1.23), 1.24 (CI, 1.12 to 1.38), 1.42 (CI, 1.27 to 1.60), 1.59 (CI, 1.41 to 1.80), and 2.00 (CI, 1.69 to 2.38), respectively. The surge index was associated with mortality across ward, intensive care unit, and intubated patients. The surge-mortality relationship was stronger in June to August than in March to May (slope difference, 0.10 [CI, 0.033 to 0.16]) despite greater corticosteroid use and more judicious intubation during later and higher-surging months. Nearly 1 in 4 COVID-19 deaths (5868 [CI, 3584 to 8171]; 23.2%) was potentially attributable to hospitals strained by surging caseload.

Limitation: Residual confounding.

Conclusion: Despite improvements in COVID-19 survival between March and August 2020, surges in hospital COVID-19 caseload remained detrimental to survival and potentially eroded benefits gained from emerging treatments. Bolstering preventive measures and supporting surging hospitals will save many lives.

Primary funding source: Intramural Research Program of the National Institutes of Health Clinical Center, the National Institute of Allergy and Infectious Diseases, and the National Cancer Institute.

Visual Abstract.. Association Between Caseload Surge and COVID-19 Survival in U.S. Hospitals.

The effect of surges in COVID-19 caseload on COVID-19 survival rates is unclear, especially independent of temporal changes in survival. This retrospective cohort study used data from a large U.S. hospital database to study the association between caseload surges and risk-adjusted mortality in patients with COVID-19.

Appendix Figure.. Flowchart depicting cohort selection, 558 U.S. hospitals, March to August 2020.

ICU = intensive care unit.

Figure 1.. Distribution of U.S. hospital-months' surge indices by admission month and hospital census region, 558 U.S. hospitals, March to August 2020.

These violin plots show the distribution of patients within each hospital-month, stratified by admission month and log surge index (top) and surge index (bottom), with colors indicating the hospital census region. The overlaid box plots indicate the median, interquartile range, and 95% CI for each month's distribution. The size of each dot represents the total number of encounters in each hospital-month. Peak surges can be observed in the Northeast in April and in the South and West in July.

Figure 2.. Crude mortality rate for categorical parameterizations of the surge index, 558 U.S. hospitals, March to August 2020.

Crude mortality rates across admission month, stratified by shrinking surge index categories, enable visualization of secular patterns beyond the relationship between surge index and crude mortality.

Figure 3.. Adjusted odds of mortality for categorical parameterizations of the surge index, 558 U.S. hospitals, March to August 2020.

Risk-adjusted odds ratios of mortality were calculated using surge index deciles above the median (A) and shrinking percentile categories (B) for the primary study cohort (admissions in March to August 2020). In panel B, the shrinkage distribution is applied to evince the prognostic effect in categories of extremely high surge index. Panels C and D illustrate effect modification of the relationship between surge index and mortality by period of admission. The slopes in the relationship between log surge index and log odds of mortality (see Supplement Figure 5) for June through August versus March through May intersect (slope difference, 0.10 [95% CI, 0.033 to 0.16]), indicating a significant quantitative interaction by period of admission.