Key Messages​​​​​

• Increases in norovirus activity have been noted throughout California and the United States since mid-January 2023, via laboratory testing and wastewater surveillance.
• As norovirus is highly contagious and can survive for weeks on surfaces and objects, thorough disinf​ection while using personal protective equipment (PPE) should follow any patient seen for confirmed or suspected infection.
• Healthcare providers should report outbreaks of acute gastroenteritis, including suspected outbreaks of norovirus, to the local health department​.​
  

​Background

The California Department of Public Health (CDPH) is alerting healthcare providers about elevated norovirus activity throughout California and nationwide.

Norovirus activity has been elevated throughout the United States, with sustained increases observed in wastewater concentrations at many sites in California, since mid-January 2023. The U.S. Centers for Disease Control and Prevention (CDC) reports steadily increasing norovirus test percent positivity and numerous outbreaks have been reported throughout the U.S.

Wastewater monitoring in California by the Stanford Sewer Coronavirus Alert Network (SCAN) and WastewaterSCAN programs have shown similar rising trends of norovirus concentrations detected in wastewater monitoring sites throughout the state, with large increases since mid-January. To date, the trends at many California sites have not yet decreased to previous low levels, suggesting continued elevated norovirus disease activity in many areas.

Since February 1, 2023, at least 25 outbreaks of norovirus in California have been confirmed by laboratory testing, likely reflecting hundreds of illnesses in the state. 

This included several outbreaks in long-term care facilities, and at least one elementary school outbreak, which required closure for control. However, gastrointestinal outbreaks are often not confirmed by laboratory testing, so the true number of norovirus outbreaks likely well exceeds those confirmed by laboratory testing.

CDPH reminds healthcare providers to report all outbreaks of acute gastroenteritis, including suspected outbreaks of norovirus, to the local health department (LHD).

Recommendations

Clinical Presentation

After an incubation period of 12–72 hours, norovirus infection presents with an abrupt onset of nausea, vomiting, watery diarrhea, and abdominal pain. Most illnesses are self-limited and typically last 48 to 72 hours. Dehydration can occur and illness may be severe in older adults, infants, and immunocompromised persons.

Norovirus infections are most communicable during the acute stage of the disease, but infected persons can continue to shed the virus for two to three weeks, even after clinical recovery. Transmission requires only a very small viral inoculum.

Norovirus is highly transmissible and can readily spread from person to person, through contaminated food or drink, contaminated surfaces, or through direct contact. Norovirus survives readily in the environment and is relatively resistant to common disinfectants. Therefore, norovirus can spread easily, particularly in closed and crowded settings, and can be challenging to control.

Reporting and Surveillance

Confirmed and suspected outbreaks of norovirus are reportable to public health authorities per the California Code of Regulations (CCR) Title 17 § 2500 (PDF). However, because individual cases of norovirus are not reportable, current disease surveillance methods rely on laboratory testing of clinical samples from suspected viral gastroenteritis outbreaks and monitoring of wastewater data.

Laboratory testing confirms the diagnosis of norovirus infection, as symptoms of norovirus alone cannot distinguish it from other gastroenteric viruses (such as rotavirus, sapovirus, astrovirus, and adenovirus) or bacterial causes (such as Salmonella). 

Laboratory confirmation is especially useful in the setting of a suspected outbreak as genotyping can help determine if other illnesses or outbreaks are linked. Testing could also be useful in immunocompromised patients, or those with severe or persistent symptoms.

In suspected viral gastroenteritis outbreak settings, local health departments (LHDs) attempt to collect stool specimens from three or more ill persons to confirm the viral etiology. The CDPH Viral and Rickettsial Diseases Laboratory (VRDL) and select California public health laboratories (PHLs) can perform genetic sequencing on positive norovirus specimens to determine genotype and relatedness to other circulating strains.

The information provided by these tests can help CDPH and LHDs identify sources of outbreaks and implement infection control measures to prevent the spread of illnesses.

In addition to tracking outbreaks, norovirus activity in the community is reflected by wastewater surveillance. Many California cities participate in WastewaterSCAN, a national network of sites that monitor levels of several viruses in wastewater, including norovirus. Monitoring of norovirus levels in wastewater is also conducted by the Stanford Sewer Coronavirus Alert Network.

Prevention and Control

Norovirus is very contagious and the virus can survive for weeks on surfaces and objects. The infection can spread quickly in settings such as schools and childcare centers, congregate living settings, healthcare facilities, and cruise ships.

Additional Resources

The CDPH norovirus webpage includes links to numerous tools and resources for responding to norovirus in common outbreak settings:

• CDPH Norovirus Toolkit for School and Childcare Center Outbreaks (PDF)
• CDPH Norovirus/Viral Gastroenteritis Control Recommendations for Evacuation Centers/Shelters (PDF)
• CDC Norovirus Outbreak Control Resource Toolkit for Healthcare Settings
• CDC Norovirus Webpage ​
  
• CDC Norovirus Multimedia​

• ​​CDC Norovirus Guidelines for Healthcare Settings

For questions regarding norovirus, contact your LH​​D.​