By Aidin Vaziri, San Francisco Chronicle The Tribune Content Agency

After the holiday season and an uptick in travel, California faces a rise in respiratory illnesses, prompting health experts to talk of a "quad-demic." The term refers to the simultaneous spread of four major viruses: COVID-19, influenza, respiratory syncytial virus and norovirus. While distinct in their effects, their combined impact could strain health care systems and disrupt daily life.

As of Friday, the U.S. Centers for Disease Control and Prevention reports that acute respiratory illnesses remain at high levels nationwide. Flu activity, in particular, is elevated, with COVID-19 cases low but rising in many areas and RSV peaking in several regions. However, the sharp increase in norovirus - a highly contagious stomach bug - has become one of the most worrisome trends.

In mid-January, nearly 28% of weekly tests for norovirus came back positive - levels not seen since 2019. According to the CDC, "the number of reported norovirus outbreaks this year has surpassed those seen in recent years, including pre-pandemic levels."

Flu and COVID-19 impact in California

So far this season, the flu has caused approximately 9.1 million illnesses, 110,000 hospitalizations and 4,700 deaths, including 16 pediatric fatalities. Six states, including California, are experiencing particularly high flu activity.

In California, wastewater surveillance shows "very high" levels of the flu virus, with emergency rooms seeing an increase in visits and hospitalizations. The current flu positivity rate in California is about 20%, while RSV positivity has slightly decreased to 7.5%. The COVID-19 positivity rate stands at just 2.6%, much lower than the surges seen during earlier stages of the pandemic.

While COVID-19 is no longer the widespread threat it once was, hospitalizations - especially among older adults and children - are rising as we head into the new year.

The rise of norovirus

The most alarming development this year is the surge in norovirus cases. In early January, the CDC reported 91 outbreaks of the intestinal virus, which causes vomiting, diarrhea and stomach cramps. This marks the highest number of norovirus outbreaks since before the pandemic.

Wastewater monitoring in the Bay Area has revealed a significant increase in norovirus levels, with one San Francisco facility showing a staggering 458% rise in viral traces compared to last year.

Unlike the flu, COVID-19 or RSV, there is no vaccine for norovirus, making it particularly difficult to control. Hand sanitizers are largely ineffective against it, so to prevent spread the CDC recommends frequent hand washing with soap and water for at least 20 seconds - especially after using the bathroom and before eating.

Health care strain and hospitalizations

Nationally, flu activity remains elevated, with both hospitalizations and emergency department visits continuing to climb. The CDC has reported that influenza-related hospital admissions are higher than usual for this time of year.

Although the CDC expects the peak number of combined hospitalizations from COVID-19, flu and RSV this season to be similar to or lower than last year, overall hospitalizations from all respiratory viruses are anticipated to exceed pre-pandemic levels.

Vaccination gaps remain

To mitigate the impact of these respiratory illnesses, the CDC continues to urge vaccination, particularly for the flu and COVID-19. However, vaccination rates remain troublingly low. Only 43% of U.S. adults have received their flu shot, and just 23% have received the updated COVID-19 vaccine. RSV vaccine coverage is similarly low, especially among adults.

Reach Aidin Vaziri: [email protected]