Protective titre of the WHO international standard for anti-chikungunya virus antibodies in a mouse model.

Chikungunya virus (CHIKV) is an arthritogenic mosquito-borne virus of public health concern, causing incapacitating febrile illness and, in some cases, chronic long-lasting recurring arthritis. The epidemic potential and unpredictable nature of CHIKV outbreaks provided a stimulus for establishing the WHO International Standard (1502/19) for anti-CHIKV antibodies in 2022. To enhance the value of this serological material, the in vivo immune protection of 1502/19 was assessed to determine the protective titre. Dilutions of 1502/19 were administered to naïve mice and circulating antibody levels measured prior to challenge 24 h later with CHIKV isolate LR2006-OPY1. Mice administered with undiluted 1502/19 survived an otherwise lethal infection to the end of the study (14 days post-challenge), indicating protection was afforded by 1502/19 and establishing a protective titre can be attained. Our results confirm that standalone humoral immunity can provide protection and aid in future vaccine evaluation by elucidating a standardised protective threshold.