Pandemic H1N1 and seasonal H3N2 influenza infection in the human population show different distributions of viral loads, which substantially affect the performance of rapid influenza tests.

A wide range of sensitivity has been reported for rapid influenza antigen tests (RIAT). In this study, we analyzed the viral loads of 778 pandemic H1N1- and 227 seasonal H3N2-virus positive clinical specimens collected during the same period and found that viral loads in pandemic H1N1 viruses was characterized by lower copy numbers than seasonal H3N2 viruses. Among various factors including the timing of specimen collection, patient age, patient gender and subtype of influenza, we found that the subtype of influenza was the most important determinant of viral load. To investigate whether these different patterns of viral load distribution affect the clinical performance of RIAT, the RIAT reagent itself and the various virus subtypes were considered and analyzed further. Based on three strategies, including cut-off values, performance on a subset of clinical specimens and evaluated performance curve of the Espline influenza A&B-N RIAT, the clinical sensitivities were 48.7-55.9% for pandemic H1N1 and 64.0-70.5% for seasonal H3N2 viruses in this study. These results indicate that the distributions of viral loads of different influenza A subtypes substantially influence the sensitivity of RIAT for clinical specimens. The lower sensitivity of RIAT for pandemic H1N1 than seasonal H3N2 virus is mainly due to differences in viral load in clinical samples rather than a diminished capacity of RIAT itself to detect these two subtypes of influenza A viruses.