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. 2023 Mar 28. Online ahead of print. doi: 10.1016/S1473-3099(23)00129-9

Figure 3.

Figure 3

Index patient Ct values and the probability of tested contacts being RT-PCR positive or LFD positive (A) and estimated index cases detectable by LFDs among case–contact pairs with probable transmission (B)

Contacts of asymptomatic index patients were 0·76 times as likely to test positive as contacts of symptomatic index patients at a Ct value of 10; 0·70 times as likely at a Ct value of 20; and 0·65 times as likely at a Ct value of 30. The model in part A was adjusted for index patient age (predictions plotted for age 40 years), index patient sex (female), index patient vaccination status (boosted, ≥3 vaccine doses), contact event type (household or accommodation), contact age (40 years), contact sex (female), and index test date (July 1, 2021). We found no evidence that fitting an interaction between index patient symptom status and Ct values improved model fit. In part B, error bars indicate 95% CIs calculated by non-parametric bootstrapping (1000 iterations). Ct=cycle threshold. LFD=lateral flow device.