SACRAMENTO, the United States -- For the first time since the COVID-19 pandemic began, influenza has surpassed COVID-19 as the deadliest respiratory illness in California, prompting hospitals to struggle with overwhelming patient numbers amid a surge in flu cases.

This spike in flu cases comes amidst historically low vaccination rates, with only 44 percent of adults and 46 percent of children receiving flu shots this season, according to the US Centers for Disease Control and Prevention (CDC).

"Our hospital is full to the brim," Peter Chin-Hong, an infectious disease expert at the University of California, San Francisco, told local news outlet San Francisco Chronicle. "Influenza seems to be everywhere," he added.

At local clinics in the Bay Area, more than 70 percent of respiratory virus tests are now positive for influenza, surpassing cases of respiratory syncytial virus (RSV), COVID-19 and the common cold. As of Feb 1, flu test positivity rates hit 27.8 percent, while RSV cases dropped to 5 percent and COVID-19 remained at 2.4 percent, according to the California Department of Public Health.

Since July 1, California has reported at least 561 flu-related deaths, the majority of which are among individuals aged over 65. Additionally, there have been 10 pediatric flu deaths this season, compared to just three pediatric deaths linked to COVID-19 in the same period.

Nationwide, the CDC estimates that there were at least 29 million flu cases, 370,000 hospitalizations, and 16,000 deaths during the 2024-25 flu season as of Feb 8.

John Swartzberg, an infectious disease expert at the University of California, Berkeley, told Newsweek that "this may become the worst influenza season we've had so far in the 21st century."

Medical professionals are particularly concerned about the simultaneous circulation of two influenza A strains -- H1N1 and H3N2 -- in roughly equal amounts, an unusual pattern that increases the risk of sequential infections.

"Immunity to one of them does not give you immunity to the other in any sort of durable way," said Buddy Creech, a pediatric infectious disease expert at Vanderbilt University Medical Center, as reported by CNN Health.

The surge in severe flu cases has led to complications that are rarely seen in such numbers, including acute necrotizing encephalopathy (ANE), a severe brain condition, particularly among children. ANE has a fatality rate of around 50 percent.

California's healthcare workers describe scenes reminiscent of the peak of the COVID-19 pandemic. Critical care nurses report that ICUs are filled with flu patients suffering from pneumonia and respiratory failure.

John Lynch, an infectious disease specialist at the University of Washington Medicine, told CNN that there is a concerning trend of MRSA pneumonia following flu infections, which can cause permanent lung damage. MRSA is a strain of bacteria resistant to many antibiotics.

Public health officials stress that it's not too late to get vaccinated, although it takes about two weeks for the vaccine to provide full protection. While the vaccine may not prevent all infections, it significantly lowers the risk of severe illness and hospitalization.

"These recent flu deaths among our youth are tragic and concerning as we head into what historically is the peak of flu season," Ankita Kadakia, San Diego County's interim public health officer, told the San Francisco Chronicle.

Experts predict high flu activity for at least another month to six weeks, with the possibility of an additional wave of influenza B cases in the spring, highlighting the ongoing need for preventive measures and vaccination.