In patients with severe COVID-19 requiring mechanical ventilation, a large difference in the percentage of positive results between viral culture and PCR testing was revealed. Viral culture testing revealed that viral load decreased over time, while decreases were not observed via PCR testing. Previous reports have also shown that PCR test results remain positive for an extended period, up to 40 days or more in some cases11,12. The CDC does not recommend ending isolation based on PCR testing5. The results of this study also showed that it is difficult to evaluate whether a patient is infectious based on PCR testing, even if the viral load is known.
In terms of infectivity titers, we confirmed that both the viral load and percentage of positive results decreased over time. One patient required 24 days for negative viral culture conversion to occur. In this patient, factors associated with immunosuppression, including treatment with prednisolone and abatacept, and dialysis for renal failure likely influenced the results. The likelihood that these factors caused severe COVID-19 has already been identified. Previous studies revealed that a high proportion of patients receiving prednisolone for rheumatism developed severe COVID-1913, and the mortality rate of COVID-19 is reportedly higher in patients who are on dialysis than in their counterparts14. Nevertheless, the possibility that these factors may lead to viral culture test positivity for prolonged periods has not been fully examined. Though this affected only one patient in this study, the finding suggests that these factors may prolong the negative viral culture conversion time.
For all the other patients in this study, there was no difference in age, BMI, or medical history between the culture-positive and culture-negative groups, and negative viral culture conversion occurred between days 6 and 16 after symptom onset. Furthermore, in this study, approximately one third (30·4%) of all patients with severe COVID-19 were believed to have been shedding the infectious virus 10 days after symptom onset.
This study has a few limitations. This was a single-center study of a group with a small sample size, and only seven patients had positive viral culture test results. Patients were transferred to our hospital when they required mechanical ventilation and the timing of the first specimen collection was different for each patient. A few specimens were collected within seven days after symptom onset, and we could not investigate the percentage of positive viral culture test results obtained prior to initiating therapy. SARS-CoV-2 infectivity was assessed with a viral culture test that used Vero cells expressing TMPRSS2. Vero cells are well suited for SARS-CoV-2 culture15, and Vero cells expressing TMPRSS2 are even more susceptible to SARS-CoV-2 infection16. However, additional knowledge is required to investigate what factors inhibit viral culture and the detection limits of viral culture.
This study showed that 30% of the critically-ill patients remained culture positive for ≥ 10 days after symptom onset. Further, one immunosuppressed patient required 24 days of testing before negative viral culture conversion occurred, suggesting that a longer period of isolation is required for patients with impaired cellular immunity. Further research investigating patients with COVID-19 who exhibit prolonged infectious virus shedding is required.