T he public health community anticipated widespread co-circulation of infl uenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes coronavirus disease (COVID-19), during the 2020-21 infl uenza season. However, infl uenza activity in California was unusually low (1). The California Department of Public Health (CDPH; Richmond, California, USA) matched positive infl uenza test results with SARS-CoV-2 test results to assess the occurrence of infl uenza and SARS-CoV-2 co-infections in California.
The StudyCalifornia laboratories and medical providers must report all positive and nonpositive (i.e., negative, inconclusive, or invalid) SARS-CoV-2 laboratory results to their local health jurisdictions (LHJs) (2). For infl uenza, only positive results that can be submitted electronically by laboratories are reportable. Most data are reported directly to CDPH's web-based platform, California Reportable Diseases Information Exchange (CalREDIE). CalREDIE assigns electronic laboratory reports a unique identifi er, personID, that can be used to link the same person across different disease reports. CalREDIE is used by 59 of California's 61 LHJs for disease tracking and reporting. Two LHJs, Los Angeles and San Diego, which represent one third of California's population, do not use CalREDIE directly; we excluded data from those LHJs.We matched positive molecular infl uenza test results reported during September 1, 2020-April 30, 2021, with positive and nonpositive molecular SARS-CoV-2 test results to identify co-infections. We matched positive infl uenza results with nonpositive SARS-CoV-2 results to determine whether persons infected with infl uenza were negative for SARS-CoV-2 or were potentially not tested for SARS-CoV-2. We deduplicated all positive infl uenza tests results and excluded antigen test results.We matched laboratory results fi rst using Cal-REDIE personID, then by name and date of birth, and fi nally by manual record review if positive infl uenza results did not match to SARS-CoV-2 results by per-sonID or name and date of birth (Figure 1). If a person had both positive and nonpositive SARS-CoV-2 results within 7 days of a positive infl uenza result, we used the positive SARS-CoV-2 result in the analysis. Persons with both positive infl uenza and SARS-CoV-2 test results with ≤7 days between specimen collection dates met criteria for infl uenza and SARS-CoV-2 co-infection. We analyzed co-infection data by week of illness onset and geographic distribution. We summarized co-infected persons by age, race and ethnicity, sex, hospitalization, and survival status. We completed all analyses using SAS version 9.4 (SAS Institute, http://www.sas.com). This study received a nonresearch determination from the California Committee for the Protection of Human Subjects.CDPH received 258 positive infl uenza test results during September 1, 2020-April 30, 2021, and >21.1 million SARS-CoV-2 total test results. Among positive infl uenza results, 255 (99%) matched with a SARS-CoV-2 test result (po...