RESPWatch

Public Health prepares this report to summarize current respiratory illness surveillance data in LA County*. Weekly surveillance data are preliminary and subject to change. More information regarding methods can be found on the surveillance system specific pages of this report.

*For information on trend indicator arrows, see Technical Notes.

**Public Health's Syndromic Surveillance Project monitors initial self-reported symptoms as well as diagnosis codes from patients presenting to participating emergency departments (EDs) throughout LA County. The syndrome of influenza-like illness (ILI) is defined as mention of influenza; or fever (subjective or measured greater than 100° F) plus cough or sore throat, and can include someone with flu, COVID-19, RSV, or some other respiratory infection. ED visits can also be classified by specific ICD-10 diagnosis codes, which indicate a positive laboratory test for COVID-19, influenza, or RSV. The ILI classified and ICD-10 coded ED visits for COVID-19, influenza, and RSV are analyzed weekly and year-round and shown for all ages and by age group. The ILI and ICD-10 coded categories for COVID-19, influenza, and RSV are not mutually exclusive.

Viral surveillance data is provided by clinical laboratories serving hospitals and healthcare networks across Los Angeles County. Participating laboratories provide the number of positive tests and total number of specimens tested for SARS-CoV-2, influenza, and respiratory syncytial virus. Some laboratories also share testing data on other common respiratory viruses with Public Health.

*Baseline is defined as mean percentage of specimens testing positive during non-epidemic weeks for the previous three seasons plus two standard deviations. Non-epidemic weeks are periods of ≥2 consecutive weeks during which each week accounted for < 2% of the season’s total number of specimens that tested positive. The COVID-19 pandemic affected the circulation of other respiratory viruses. Pandemic weeks are excluded from the baseline calculation for influenza and RSV data.

**Data for the 2020-2021 season for influenza and RSV was excluded because non-COVID respiratory viruses were not circulating at this time.'

Wastewater data includes data from the Warren, Hyperion, and Lancaster sewersheds. These three sewersheds cover 90% of the population of LA County. All data courtesy of WastewaterSCAN. Data are normalized average wastewater SARS-CoV-2 concentrations. A weighted average is calculated by using the SARS-CoV-2, influenza, and RSV viral concentrations across the three sewer systems, with the weights representing the population size served by each sewer system.

*Threshold values representing 30% and 60% of the previous season’s winter peak are shown in the light blue and yellow horizontal lines on the COVID-19 graph. These thresholds can aid in interpreting the current season; for example, values at or below the 30% threshold indicate lower concentrations of SARS-CoV-2 (the virus that causes COVID-19) in the wastewater, whereas values at or above the 60% threshold indicate higher concentrations.

Public Health's Syndromic Surveillance Project monitors initial self-reported symptoms as well as diagnosis codes from patients presenting to participating emergency departments (EDs) throughout LA County. The syndrome of influenza-like illness (ILI) is defined as mention of influenza; or fever (subjective or measured greater than 100° F) plus cough or sore throat, and can include someone with flu, COVID-19, RSV, or some other respiratory infection. ED visits can also be classified by specific ICD-10 diagnosis codes, which indicate a positive laboratory test for COVID-19, influenza, or RSV. The ILI classified and ICD-10 coded ED visits for COVID-19, influenza, and RSV are analyzed weekly and year-round and shown for all ages and by age group. The ILI and ICD-10 coded categories for COVID-19, influenza, and RSV are not mutually exclusive.

*The grey bar represents a two week period when syndrome classifications (e.g. for COVID, ILI, etc.) are in flux due to delayed diagnosis information -- these percentages are preliminary and subject to increase.

COVID-19, influenza, and RSV hospitalizations are reported weekly by hospitals through the CDC’s National Healthcare Safety Network (NHSN). The data shows the number of total influenza hospitalizations in Los Angeles County by ICU admission status, and the total number of new patient hospitalizations by MMWR week.

*The number of current hospitalizations and new patient hospitalizations may not align on a weekly basis because of different reporting methods. Current hospitalizations are reported as a snapshot of the number of people hospitalized on the Wednesday of a given week, whereas new patient hospitalizations are reported as a weekly total.

**RSV hospitalization data is only shown for the 24-25 season, as RSV hospitalizations became reportable on November 1, 2024

Angelenos in Action is a voluntary text-based public health survey that collects information to monitor COVID-19 symptoms across LA County in real time. Volunteers across LA County provide information on their health as part of a weekly SMS survey. Respondents are assigned to a random day, and then texted once per week about their symptoms. Rates are then calculated weekly.

*Angelenos in Action was not active for a portion of the weeks of August 4th and August 11th, 2024 due to an issue with the survey platform. Fewer responses were collected in this time, which may impact the rate of illness.

Each week, Public Health reports the total number of death certificates received and the number of those for which pneumonia, influenza, or COVID-19 (PIC) was listed as the underlying or contributing cause of death by age group. Technical note: the number of deaths reported in recent weeks does not represent all deaths that occurred in the reporting period. Data may be incomplete due to a lag between when the death occurred and when the death was registered. Previous weeks' counts or percentages may change as vital records are updated with lagged death certificates. This analysis includes all certificates of death (excluding fetal deaths) occurring in the County of Los Angeles (excluding Long Beach and Pasadena) regardless of the residence of the deceased.

*The seasonal baseline of pneumonia, influenza, and COVID-19 (PIC) deaths is calculated using a periodic regression model that incorporates a robust regression procedure applied to data from the previous 5 years. An increase of 1.645 standard deviations above the seasonal baseline of PIC deaths is considered the “epidemic threshold,” i.e., the point at which the observed proportion of deaths attributed to pneumonia, influenza or COVID-19 was significantly higher than would be expected at that time of the year in the absence of substantial virus-related mortality.

**Influenza and COVID-19 deaths reported here are from death certificate data and based on ICD-10 codes in which influenza or COVID-19 was listed as a primary or contributing cause of death. ICD-10 coded death certificates are not necessarily laboratory confirmed. Influenza deaths are likely an undercount (more information can be found at this CDC resource: Facts About Estimated Flu Burden). COVID-19 deaths reported here are not expected to match COVID-19 deaths reported elsewhere due to differing definitions and data sources.

Multisystem inflammatory syndrome in children (MIS-C) is a rare inflammatory condition in children following infection with SARS-CoV-2 (the virus that causes COVID-19). Public Health tracks cases of MIS-C in LA County. MIS-C cases are reported to Public Health by providers. Cases are confirmed by Public Health following medical record review using the relevant Council of State and Territorial Epidemiologists (CSTE) case definition. Data presented is for confirmed MIS-C cases; additional cases may be under investigation that are not presented here. Public Health continues to receive reports from earlier in the pandemic; historic case counts may be revised as new reports are confirmed.

Trend Indicators:
Increasing:
Decreasing:
Stable:
Clinical Labs: The up and down arrows indicate a change of at least 0.5 percentage points in the percentage of specimens positive for influenza, COVID-19, or RSV compared to the previous week reported by LA County sentinel surveillance laboratories.
Emergency Department: The up and down arrows indicate a change of at least 0.1 percentage points in the percentage of COVID-19 and ILI classified emergency department visits in LA County compared to the previous week.

Note about weeks: We present Morbidity and Mortality Weekly Report (MMWR) weeks. The MMWR week corresponds with the week of the epidemiologic year for which the National Notifiable Diseases Surveillance System (NNDSS) disease report is assigned. The MMWR week is used by local or state health departments for the purposes of MMWR disease incidence reporting and publishing. Each MMWR week begins Sunday and ends the following Saturday.

The respiratory virus surveillance period starts with MMWR week 40 and runs through week 39 of the following year. The 2024-25 season started on September 29, 2024.

Public Health surveillance data excludes the cities of Long Beach and Pasadena.

• LA County Respiratory Virus Homepage
• CDC Influenza Surveillance
• CDPH Influenza Surveillance
• WastewaterSCAN
• CDPH Wastewater Surveillance Dashboard
• CDC COVID-19 Variant Surveillance