The surge in patients consisted of US citizens who live in Mexicali, capital of the Mexican state of Baja California, and were turned away from hospitals overrun with coronavirus cases there, said Dr. Adolphe Edward, chief executive officer of the El Centro Regional Medical Center.
Edward said his 161-bed hospital in El Centro, the main city in Imperial County about 100 miles (160 km) east of San Diego, ended up with 65 COVID-19-positive patients from Monday night's influx, while the 106-bed Pilgrims Memorial Hospital in nearby Brawley admitted 28.
"Our numbers just skyrocketed last night," Edward said on his hospital's Facebook page.
The emergency rooms of both hospitals have imposed a "divert" order requiring any additional COVID-19 cases be redirected to other medical facilities in the region, he said.
The two ERs will remain open to non-coronavirus cases, and most COVID-19 patients already admitted would remain, he said.
A spokeswoman for Pilgrims Memorial, Karina Lopez, confirmed Edward's account, adding no one at the Brawley hospital had been turned away.
The two hospitals serve all of Imperial County, consisting of about 175,000 residents and a local economy based largely on irrigated agriculture.
About 80% of residents in the larger Imperial Valley, straddling both sides of the border, are Hispanic, with many considered bi-national. An estimated 265,000 U.S. citizens and Mexicans with American "green cards" conferring permanent-residency status, live in Baja. Many are retirees.
Imperial County has registered fewer than 800 known coronavirus infections and just 15 deaths to date. Baja California, by comparison, has reported 3,458 confirmed COVID-19 cases and 134 deaths.
Two main medical centers in Baja's state capital - Mexicali Hospital General and IMSS Regional Hospital 30 - are both "saturated" by the outbreak, Mario Cervantes, head of relief services for the Red Cross of Mexicali, told Reuters.
Some arriving ambulances have had to wait hours to deliver new patients, while others were turned away altogether, he said.
Baja health department officials said neither Mexicali hospital had exhausted its bed space, but Dr. Rafael Abril, president of the Mexicali College of Surgeons, told local news in April that half the IMSS hospital's doctors were infected with COVID-19, which could lead to staffing shortages.
US officials recently voiced concern a coronavirus outbreak in Mexico could send a wave of dual citizens over the border into the United States, putting extra strain on American hospitals.