Emergence of circulating influenza A H3N2 viruses with genetic drift in the matrix gene: Be alert of false negative test results

In March 2022, we observed samples with a negative fluorescent signal (60.5%, n=43) for the influenza A matrix gene, and a stronger positive signal for subtype A(H3N2). Forty-three samples were positive in InfA (H3N2) (mean Cq 30.9, range 23.9-35.1) and 26 of the 43 samples were negative in InfA matrix (mean Cq 28.0, range 23.2-30.6). Our multiplex test is a laboratory developed four-target, four-color influenza A reverse-transcription PCR assay targeting the matrix gene, subtypes A(H3N2) and A(H1N1)pdm09. Several samples were negative when retested on commercial influenza Point-of-Care assays. As the matrix gene is a stand-alone target in most commercial diagnostic assays, we caution against false negative subtype A test results.