The study investigates the virological characteristics of the KP.2 (JN.1.11.1.2) variant of SARS-CoV-2, which emerged from the JN.1 (BA.2.86.1.1) variant and has been rapidly spreading in multiple regions as of April 2024. KP.2 carries three substitutions in the spike (S) protein (S:R346T and S:F456L) and one additional substitution in a non-S protein. Using genome surveillance data from the USA, United Kingdom, and Canada, the relative effective reproduction number (Rₑ) of KP.2 was estimated to be 1.22-, 1.32-, and 1.26-times higher than that of JN.1, respectively. This suggests that KP.2 has higher viral fitness and may become the predominant lineage worldwide. However, the infectivity of KP.2 is significantly lower (10.5-fold) than that of JN.1. Neutralization assays using monovalent XBB.1.5 vaccine sera and breakthrough infection sera showed that the 50% neutralization titer (NT₅₀) against KP.2 was significantly lower than that against JN.1, particularly in vaccinees without infection (3.1-fold) and those with infection (1.8-fold). These findings indicate that KP.2's increased immune resistance contributes to its higher Rₑ compared to previous variants.The study investigates the virological characteristics of the KP.2 (JN.1.11.1.2) variant of SARS-CoV-2, which emerged from the JN.1 (BA.2.86.1.1) variant and has been rapidly spreading in multiple regions as of April 2024. KP.2 carries three substitutions in the spike (S) protein (S:R346T and S:F456L) and one additional substitution in a non-S protein. Using genome surveillance data from the USA, United Kingdom, and Canada, the relative effective reproduction number (Rₑ) of KP.2 was estimated to be 1.22-, 1.32-, and 1.26-times higher than that of JN.1, respectively. This suggests that KP.2 has higher viral fitness and may become the predominant lineage worldwide. However, the infectivity of KP.2 is significantly lower (10.5-fold) than that of JN.1. Neutralization assays using monovalent XBB.1.5 vaccine sera and breakthrough infection sera showed that the 50% neutralization titer (NT₅₀) against KP.2 was significantly lower than that against JN.1, particularly in vaccinees without infection (3.1-fold) and those with infection (1.8-fold). These findings indicate that KP.2's increased immune resistance contributes to its higher Rₑ compared to previous variants.