To the Editor In reading the article on transmission of coronavirus 2019 (COVID-19) by Cheng et al,1 I identified 5 statistical issues:
The first issue is truncated exposure windows. Some of the 70 imported cases had developed symptoms before arrival and were identified in the airport screening.2 The exposure windows of such cases for secondary cases were left truncated at the time of departure for Taiwan. Among the 30 locally infected cases, the sources of infection were all identified. Once a suspected case of COVID-19 was laboratory confirmed in Taiwan, that person would be admitted to an isolation ward of a hospital immediately, and thus the exposure window of this index case for secondary cases was right truncated at diagnosis or hospitalization except for the medical care personnel. Yet exposures before the symptom onset of an index case less likely occurred in the health care setting, and thus the exposure window of such an index case for secondary cases might have been left truncated at diagnosis or hospitalization for the medical care personnel. Hence, the authors should consider a complicated form of a length-biased sampling problem3 due to the truncations of the exposure windows in the comparison of the secondary clinical attack rate across different exposure window periods and exposure settings.
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Hu F. COVID-19 Transmission Conclusions Justified by the Analysis Results? JAMA Intern Med. 2020;180(9):1262. doi:10.1001/jamainternmed.2020.4097
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