Evaluation of the Rubella Surveillance System, California, 2018-2022

Rubella prevalence in the United States is low, and many positive immunoglobulin M (IgM) test results are likely false positive. We evaluated case classification and follow-up time spent on rubella-positive IgM test results from routine surveillance by the California Department of Public Health (CDPH).

We identified and abstracted data from rubella reports submitted to CDPH during January 1, 2018-December 31, 2022. CDPH uses a modified version of the 2013 Council of State and Territorial Epidemiologists (CSTE) rubella case definition to determine cases. The percentage of confirmed cases was the proportion of cases determined via CDPH's modification over reports adhering to the CSTE rubella case definition, calculated by test type and reason. We surveyed local public health staff to estimate person-time spent on report follow-up.

We identified 801 suspected rubella reports. After investigation, CDPH confirmed 4 as cases and 797 as not cases; 467 (58.3%) were erroneously tested on the basis of test reason (immunity screening or ordered in error). Overall, 745 (93.0%) reports had IgM test results, 33 (4.1%) had an unknown test type, and 23 (2.9%) had a polymerase chain reaction test. Most erroneous reports (93.4%, 436/467) included only an IgM-positive test result. Mean time spent to investigate a suspected rubella report was 3.2 hours (range, <1-14.5 h).

Most erroneous rubella reports submitted to CDPH during 2018-2022 included a positive IgM test result, highlighting limitations of using IgM test results to classify rubella cases. The CSTE rubella case definition should be revised to ensure consistent interpretation and classification of confirmed rubella cases.