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From the published article in JAMA

"asymptomatic/presymptomatic secondary attack rate is not statistically different from zero"




"Four researchers from the University of Florida Department of Biostatistics co-authored a study published online by the Journal of the American Medical Association"

"The secondary attack rate for symptomatic index cases was 18.0% (95% CI 14.2%-22.1%), and the rate of asymptomatic and presymptomatic index cases was 0.7% (95% CI 0%-4.9%), “although there were few studies in the latter group.” The asymptomatic/presymptomatic secondary attack rate is not statistically different from zero, and the confidence interval is technically 0.7 ± 4.2, resulting in a range of -3.5%-4.9%, but attack rates cannot be negative, so it is truncated at 0."