Viral Respiratory Surveillance for LA County

Last updated: March 24, 2023
Information on COVID-19, influenza, and other illnesses in L.A. County for 2023 Week 11, ending on March 18, 2023.

Los Angeles (LA) County Department of Public Health (DPH) collects information on indicators of COVID-19 and influenza activity year-round. These indicators track the intensity, spread, and temporal trends of viral respiratory illness in LA County. COVID Watch provides a weekly summary of these indicators for healthcare providers, infection control practitioners, administrators, and policymakers.

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The weekly surveillance data presented here are preliminary and subject to change.

Key Points for Week 11 ( March 12 -March 18, 2023)
  • Virology: In Week 11, 5.1% of specimens tested at LA area viral surveillance laboratories were positive for SARS-CoV-2 (COVID-19), which is less than  in the previous week.

  • Severity: In Week 9, COVID-19 was listed as a cause for 3% of deaths registered in LA County. Pneumonia, influenza, and COVID-19 accounted for 11% of all deaths registered in LAC, which is about the same in the previous week.

  • Activity: In Week 11, around 3.7% of emergency-department (ED) visits were for COVID-19, which is less than in the previous week. The rate of ED visits for influenza-like illness* was 4.4%, which is about the same as in 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.

Virologic Surveillance

Virologic surveillance data is provided by clinical laboratories serving hospitals and healthcare networks in Los Angeles County. These laboratories provide DPH with weekly data on the total number of respiratory specimens tested and the number of specimens that were positive for SARS-CoV-2, influenza and RSV; a subset of laboratories also report data for other viral respiratory pathogens. Having the same group of laboratories reporting to DPH allows for uniform monitoring trends over time in a defined population.

COVID-19 Testing

The percent of specimens that were positive for SARS-CoV-2 is one of the indicators used to monitor trends in COVID-19 activity. Data reported from viral surveillance laboratories will differ from the overall County testing data because of differences in the population tested, types of tests used, and changes in the number of laboratories conducting testing over time.

Note about weeks: These are Morbidity and Mortality Weekly Report (MMWR) weeks. Useful for comparing diseases across years, these week numbers are set by the CDC's National Notifiable Diseases Surveillance System. Each MMWR week lasts from Sunday through Saturday.

Other Respiratory Viruses

Surveillance data for influenza, COVID-19 and other respiratory viruses is provided by clinical laboratories serving hospitals and healthcare networks across Los Angeles County. The proportion of specimens testing positive for these other viruses provides context for interpreting the rates of influenza or COVID-like illness in the community.
Seasonal and COVID-19-related changes in testing practices should be kept in mind when comparing viral surveillance data between winter and summer, or while comparing data from the current season to data from previous seasons.

Each week, participating laboratories report the total number of specimens tested and the number of specimens that were positive for Influenza, RSV, and several other respiratory viruses. Specimens that are positive for influenza are further broken down by influenza type (A or B). Using this information, LAC DPH calculates the percent positive rate for each of these viruses. Clinical laboratories primarily test respiratory specimens for diagnostic purposes and data from these laboratories provide useful information on the timing and intensity of influenza and other virus activity.

Note about week numbering

Pneumonia, Influenza and COVID-19 (PIC) Mortality

Public Health uses information provided on death certificates to calculate the percentage of deaths caused by pneumonia, influenza, or COVID-19 (PIC) occurring in LA county each week. A pathogenic etiology is not identified for the majority of persons who die with pneumonia. However, the number of persons with pneumonia listed as a cause of death increases when influenza and COVID activity increases. Monitoring PIC trends can provide a more comprehensive assessment of the burden of influenza and COVID-19 mortality.

Cause of Death

The number of deaths is based on the week in which deaths occurred. These are based on certificates received by the Office of Vital Records as of the last day of the last reported MMWR week. There is a lag between the time of death and when a death certificate is registered. Therefore, the number of deaths reported in recent weeks should be considered preliminary; the number of deaths are continually revised and may increase or decrease as new and updated death certificate data are received.

Number of deaths by illness. PIC total (%) gives the percentage of total deaths attributed to Pneumonia, Influenza, and COVID-19 combined.

  • If a death certificate has more than one PIC cause of death, that death is placed into the most specific category. For example, if a death certificate lists both COVID-19 and pneumonia as a cause of death, it will be counted as a COVID-19 death and not as a pneumonia death.
  • 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.
  • The count includes all certificates of deaths (excludes fetal deaths) occurring in the County of Los Angeles (excluding Long Beach and Pasadena), regardless of the residence of the deceased.
  • Note about week numbering

Seasonal Mortality

Although PIC deaths can occur any time of year, PIC mortality generally tends to be highest during the winter. To account for this seasonal trend, DPH estimates the proportion of weekly PIC deaths that would typically been expected to occur in the absence of influenza and COVID-19 circulation (the baseline). The difference between the baseline and the observed PIC deaths represents an indicator of severity. The epidemic threshold is the point at which the observed proportion of deaths attributed to pneumonia or influenza or COVID-19 was significantly higher than would be expected at that time of the year in the absence of substantial COVID-19- or influenza-related mortality. More technical details about the method used can be found in the Details tab below.

Percentage of deaths attributed to Pneumonia, Influenza, and COVID-19 (PIC). Note that observed values of 0.00% mean that data is not yet available.

Note about baseline and threshold:

  • 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.
Note about week numbering

Emergency Department Visits & Reported Symptoms

Public Health conducts syndromic surveillance through participating LA County emergency departments (EDs). These EDs provide DPH with information on symptoms reported by patients on initial ED presentation. These symptoms are categorized into different clinical syndromes according to specific code words:

  • COVID-19: Visits are included in the COVID-19 category if the chief complaint field includes any mention of "COVID", "Coronavirus" or similar key words.
  • Influenza-Like Illness (ILI): ILI is defined as mention of influenza; or fever (subjective or measured greater than 100 ̊F) plus cough or sore throat.
  • The COVID-19 and ILI categories are not mutually exclusive.

Emergency Department (ED) Visits due to COVID-19 and Influenza-like Illness (ILI)

ED Visits due to COVID-19 by Age Group

ED Visits due to ILI by Age Group

ED Visits due to ILI by Year

Angelenos in Action Survey Responses

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.

Anyone who is over the age of 18 years, lives in LA County (including Pasadena and Long Beach) and has a cell phone with texting capabilities is eligible to participate. Participants enroll in the voluntary survey either online or by texting @PROTECT to 833-228-3056. After giving consent, respondents are asked for their age group, sex, race/ethnicity, zip code, and, if enrolling online, phone number. Upon enrollment, participants were randomized to one day of the week. Once per week, respondents are sent a question asking how they are feeling. If the respondent feels sick, two further yes/no questions were asked regarding additional symptoms of COVID-19: whether or not they were experiencing cough/shortness of breath (1 text), whether or not they were experiencing at least two of the following: headache, body ache, fever, chills, sore throat, or loss of sense of smell or taste (1 text). These respondents were reminded to call their doctor or 211, but no further follow-up occurred.
The SMS surveys were generated using Qualtrics [Provo, UT].

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