Determining the Seasonality of Respiratory Syncytial Virus in the United States: The Impact of Increased Molecular Testing.

Background: In the United States, the seasonality of respiratory syncytial virus (RSV) has traditionally been defined on the basis of weeks during which antigen-based tests detect RSV in >10% of specimens (hereafter, the "10% threshold"). Because molecular testing has become more widely used, we explored the extent of polymerase chain reaction (PCR)-based RSV testing and its impact on determining the seasonality of RSV. Methods: We assessed antigen- and PCR-based RSV reports submitted to the National Respiratory and Enteric Virus Surveillance System during July 2005-June 2015. To characterize RSV seasons by using PCR-based reports, we assessed the traditional 10% threshold; subsequently, we developed 3 methods based on either PCR-based detections or the percentage of positive test results. Results: The annual number of PCR-based reports increased 200-fold during 2005-2015, while the annual number of antigen-based reports declined. The weekly percentage of specimens positive for RSV by PCR was less than that for antigen-detection tests; accordingly, the 10% threshold excluded detections by PCR and so was imprecise for characterizing RSV seasons. Among our PCR-specific approaches, the most sensitive and consistent method captured 96%-98% of annual detections within a season, compared with 82%-94% captured using the traditional method. Conclusions: PCR-based reports are increasingly relevant for RSV surveillance and determining the seasonality of RSV. These PCR-specific methods provide a more comprehensive understanding of RSV trends, particularly in settings where testing and reporting are most active. Diagnostic practices will vary by locality and should be understood before choosing which method to apply.