It’s not unusual for health officials to see a rise in the standard trifecta of co-circulating winter viruses — influenza, RSV and COVID-19 — particularly as people return from extended holiday travel and gatherings.

But this year officials with the Denver Department of Public Health and Environment (DDPHE) said norovirus, a highly contagious stomach bug that causes vomiting and diarrhea, is a fourth component joining the mix.

Health experts closely watching the uptick said that while they are seeing a “higher-than-usual” increase in norovirus activity and the usual “triple-demic” of respiratory illness, the rise is most likely seasonal.

Hospitalizations associated with influenza and RSV appear to be increasing, said Hope Shuler, Colorado Department of Public Health and Environment (CDPHE) Immunization Communications manager.

“This is expected at this time of year as these viruses spread more easily when individuals have been spending more time indoors in close proximity, such as during the holiday season,” Shuler said. “COVID-19 hospitalizations are increasing, but do not appear to be increasing as quickly as influenza and RSV.”

For the week ending Dec. 28, 2024, CDPHE data shows positive cases of COVID-19 were up 4.5% from the previous week, Influenza A up 7.15% and RSV up 7.62%.

Preliminarily, 338 norovirus outbreaks were reported in Colorado in calendar year 2024, compared to 277 in 2023, and an average of 168 per year prior to the pandemic (2015-2019), Schuler said.

Norovirus transmissions during the COVID-19 pandemic were significantly less due to mandated social distancing practices.

“The tricky thing about norovirus is that it can be confused for other illnesses such as food poisoning or a ‘stomach bug,’” said DDPHE spokesperson Courtney Ronner. “While norovirus circulates year-round, outbreaks of norovirus are most common from November to April.”

Highly contagious, norovirus is transmitted by direct contact with someone who is ill from the virus or those caring for them, by touching surfaces contaminated with the virus or eating food or drinking beverages contaminated with the virus or by an ill food handler.

Symptoms include vomiting, diarrhea and stomach cramps.

To prevent the spread of norovirus, health experts suggest the following:

• Wash hands with soap and water as alcohol-based hand sanitizer is not effective against norovirus.
• Stay home and do not prepare food for others if you’ve been sick with diarrhea or vomiting. If possible, wait two full days after your symptoms have resolved to prepare food for others.
• If you, or others in your household, have been sick clean with products effective against norovirus.
• Focus on cleaning high touch surfaces.
• Not all cleaners are effective against norovirus. Check the label or use bleach (One cup bleach mixed with 10 cups water). Detailed cleaning instructions are available on the CDC website.
• Launder soiled items on high heat.

Although there are no approved vaccines available, most people develop only mild norovirus symptoms and recover without medical treatment. But the virus can pose serious health risks to vulnerable populations. Children under 5 and adults 85 years of age and older are more likely to have an urgent care or emergency department visit.

“However, anyone can be infected, and outbreaks of norovirus can happen anytime, anywhere,” Ronner said.

DDPHE investigates all reported outbreaks of norovirus, including those in child care centers, schools, and residential or long-term care facilities.

Local health officials said it’s not too late to get a flu or COVID-19 vaccine, and, if eligible, an RSV vaccine.