Consistent with mitigation of clinical CRS, elevated serum levels of cytokines, including interferon-gamma, interleukin-2, tumor necrosis factor-alpha and interleukin-6, were observed in part 1.1 after the priming dose but then substantially attenuated at the RP2D (Extended Data Fig. 1a–d). Here, IL6 is linked to congenital rubella syndrome.