COVID-19 as an opportunity to reveal the impact of large hospital expansion on the healthcare delivery system: evidence from Shanghai, China.

Background: The expansion of large hospitals on the medical service market's supply side has always been an intensely debated topic. In this study, we conducted statistical analysis on the natural shock of COVID-19 to investigate whether the large hospitals will draw health demand from the small hospitals when a supply capacity surplus is present, a phenomenon otherwise known as the "siphon effect".

Methods: We collected the monthly hospital income and service data, including outpatient income, inpatient income, number of visits, and discharges, from all public hospitals, from January 2018 to July 2020 in Shanghai. A difference-in-differences (DIDs) method was applied to analyze the existence of the large hospitals' siphon effect by identifying the differences in the healthcare service market share change between large and small hospital groups at the height of pandemic (February and March, 2020) and the postpandemic period (April and May, 2020). Case mix index (CMI) was used to verify whether the reduction in healthcare amount and market share of small hospitals was due to unnecessary care.

Results: In total, 156 public hospitals, including 46 large hospitals and 110 small hospitals, with an average number of beds of 1,079.21 and 345.25, respectively, were involved in this study. At the height of the pandemic, the healthcare service volume and revenue in public hospitals in Shanghai experienced a sharp decline, especially for large hospitals and inpatient services. Compared to small hospitals at the height of the COVID-19 pandemic, large hospitals' market share decreased significantly in outpatient and inpatient services for overall and nonlocal patients (P<0.05). During the postpandemic period, large hospitals' market share increased significantly in outpatient and inpatient services for overall and local patients (P<0.05). This increase was more substantial in inpatient services.

Conclusions: Under conditions of the COVID-19 pandemic of higher care-seeking costs in the large hospitals, some of the healthcare services typically provided by large hospitals were then supplied by small hospitals. Furthermore, the siphon effect of large hospitals could be clearly observed when a supply capacity surplus was present and external constraint on patients' care-seeking behavior was absent.