“Complete live viruses are necessary for transmission, not the fragments identified by PCR. Prospective routine testing of reference and culture specimens and their relationship to symptoms, signs and patient co-factors should be used to define the reliability of PCR for assessing infectious potential. Those with high cycle threshold are unlikely to have infectious potential.”
Jefferson, T, Spencer, E A, et al. Viral cultures for COVID-19 infectious potential assessment – a systematic review. Clinical Infectious Diseases. 03 December 2020. https://doi.org/10.1093/cid/ciaa1764.