We present a novel vertical flow-based paper immunosensor for the rapid and sensitive electrochemical and colorimetric detection of influenza H1N1 viruses using a different pore size (DP) sample pad. The DP sample pad consisted of two different pore size papers: larger pores (diameter: 11?μm) facing the inlet, and smaller pores (diameter: 0.45?μm) facing the conjugate pad. This sample pad offered moderate and uniform flows, and hence concentrated horseradish peroxidase tagged antibodies (HRP-Abs)-H1N1 complexes from 40?μL of sample volumes on a conjugate pad for 2?min after sample injection, thereby providing fast detection (6?min for both detection methods) with 100?μL of flushing afterwards, high sensitivity, and the simplicity of the sensor. The filtration characteristics of the DP sample pad were evaluated using fluorescent beads, indicating that only small-sized bio-particles such as viruses can pass through the sample pad. Sandwich immunoreactions of HRP-Ab-H1N1-Ab were performed on the gold paper electrode of the immunoStrip, which was determined by electrochemical impedance spectroscopy (EIS) measurements. Simultaneously, the color signal of free HRP-Ab captured on the colorimetric zone was obtained using a scanner, and the intensity was analyzed using ImageJ. This immunosensor detected H1N1 virus concentration as low as 3.3 plaque forming units (PFU)/mL (phosphate buffer saline; PBS) and 4.7?PFU/mL (saliva) by EIS, and 1.34?PFU/mL (PBS) and 2.27?PFU/mL (saliva) by the colorimetric method. Furthermore, integrating these two detection methods can reduce false results with double assurance, and this device can provide a simple and economical on-site detection platform.