Not All Viral Culture Approaches Are Equal

TO THE EDITOR—Rhee and colleagues contemplate the issue of infectivity post-onset of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and provide justification for isolation recommendations [1]. They propose that additional data are required, and some have since become available (Table 1). The data provide further evidence that viable virus can be obtained from clinical samples after a 10-day period albeit for a minority of patients. Nevertheless, the presence of such outliers for viral excretion can nevertheless complicate infection control both in the community and healthcare settings when the burden of infection is considerable. One reference in particular has documented viable virus for up to 21 days during the course of a mild infection [2]. Citations for Viable SARS-CoV-2 Isolation After 10 days of Disease Onset Abbreviations: CPE, cytopathic effect; RT-PCR, reverse transcriptase-polymerase chain reaction. SARS-CoV-2, severe acute respiratory syndrome coronavirus 2 Citations for Viable SARS-CoV-2 Isolation After 10 days of Disease Onset Abbreviations: CPE, cytopathic effect; RT-PCR, reverse transcriptase-polymerase chain reaction. SARS-CoV-2, severe acute respiratory syndrome coronavirus 2 The argument for how a viral culture may imply infectivity is live, but it is possible that this may in part be ultimately answered by susceptible animal inoculation studies [2]. Regardless, it must be acceded that not all approaches to viral culture from clinical samples are the same, and such variation in part may justly explain the variable frequency with which the virus may be cultured in late clinical samples. The site and manner of sample collection can vary; so too may the method of transport and storage. A simple freeze-thaw may reduce titers of virus [5]. The sample quantity applied, the cell line used, incubation circumstances and time, and potential passage all can affect viral recovery. Presence of the virus may be variably assessed by cytopathic effect, reverse transcription polymerase chain reaction (RT-PCR), or immunofluorescence. Viral debris from the sample may interfere with RT-PCR detection of viral RNA in cell culture [6]. There is no uniform culture methodology accepted worldwide for comparable studies. Although viral culture potential correlates with viral load, there can be little difference in culture-positive frequency among samples from patients that are either asymptomatic or symptomatic [8]. It is not surprising that there is a wide range for confidence intervals ascribed to the ability to culture virus in late samples. For example, one study found a range of 6–24% tail end 95% confidence interval for samples acquired at 11–15 days after onset of symptomatic disease [8]. Fundamentally, variation that is inherent in viral culture is no less an issue than the variation that is inherent in RT-PCR or serological diagnostic methods for SARS-CoV-2 infections among laboratories. In this light, it would be preferable for studies of presumed infectivity, which are abstracted on the basis of viable virus in tissue culture, to maximize the finding of virus by whatever methods that would require. The data thus far imply that infectivity can well extend past a 10-day period, and due consideration for this finding should be borne for medical decision making in deisolation and infection control. The bias in this decision making should lean toward an abundance of caution rather than the contrary. It is conceivable that adjustments in the quarantine may prevent more infection and save more lives than any vaccine in the interim. Disclosure. Funding was not sought for this publication. There is no third party support including that from the pharmaceutical industry. Potential conflicts of interest section. The author: No reported conflicts of interest. The author has submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

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