Passive transfer of serum from mice immunized with vaccine plus KIN1148 protects mice against lethal H1N1 challenge. Immune passive transfer sera was generated from mice immunized either once (prime), or twice (prime/boost), 21 days apart, with vaccine plus vehicle, or vaccine plus KIN1148. Groups of 6 C57BL/6N mice received 200 μl of pooled immune sera by intraperitoneal injection on day −2 and day −1 prior to challenge. Mice were then challenged with 10× the LD50 dose of mouse-adapted influenza virus A/California/04/2009 and observed for 14 days after challenge. (A) Passive transfer of prime sera. (B) Passive transfer of prime-boost sera. **P < 0.01, ****P < 0.0001.