BACKGROUND: Viral load is an important factor determining the likelihood of transmission of COVID-19. Assuming that
Cycle threshold (Ct) values of the diagnostic RT-PCR assay are a surrogate marker of viral load, we wanted to determine
whether Ct values can be used to estimate the infectivity in terms of secondary attack rate among COVID-19 positive
individuals so as to inform public health surveillance.
METHODS:We conducted a cross sectional study to compare cycle threshold values by secondary attack rates, age, gender,
symptom status and comorbidity of COVID-19 individuals. We extracted data of Ct values from the sample referral form
of all 485 COVID-19 positive individuals from three districts, tested in the state public health laboratory in three days.
We extracted patient details like symptom status, number of contacts who tested positive for COVID-19 from the district
surveillance records. We report median with interquartile range of Ct values and tested between groups using MannWhitney test.
RESULTS: There was no statistically significant difference in median Ct values across sub groups of secondary attack rate.
COVID-19 positive individuals with symptoms had lower Ct value in all three genes with statistically significant difference
in E gene (21.8; IQR 18.5-24.6) vs 23; IQR 19.7-26.1, p<0.02) and RDRP gene (21.4; IQR 18.5-25) vs 22.7; IQR 19.3-26,
p<0.03). There was no significant difference in median Ct values between individuals with and without comorbidity.
CONCLUSION: We could not identify a relationship between Ct values and secondary attack rate. Symptomatic COVID-19
positive individuals have a significant lower Ct values corroborating with a higher viral load among them. Our findings
indicate that Ct values of RT-PCR assay cannot be used in public health surveillance to predict transmission of COVID-19.