Slightly different laboratory procedures were used in the 2 studies. For the IRIS, influenza A and influenza B matrix gene-specific reverse-transcription polymerase chain reactions (RT-PCRs) were performed as described elsewhere [2] . Dilutions of an electron microscopic-counted influenza virus A/PR/8/34 stock (Advanced Biosciences) and B/Lee/40 (Advanced Biotechnologies) were run in parallel for conversion of RT-PCR threshold cycle (Ct) values into a quantitative measurement of viral particles per milliliter (vp/mL) [2] .