Discussion
This study is the first to investigate the relationship between viral
shedding duration in COVID-19 and patient prognosis and disease
progression to critical illness. In this study we determined that
prolonged viral RNA shedding duration in COVID-19 is associated with
improved patient prognosis. Furthermore, patients with long viral
shedding duration as compared to short viral shedding duration had a
lower rate of progression to critical illness. Finally, we determined
the mortality of COVID-19 in patients with long viral RNA shedding
duration (≥26 days) was nearly 50% less as compared to patients with
short viral RNA shedding duration (<26 days). In this study,
we have excluded 5 patients whose viral RNA shedding of SARS-CoV-2 was
also detectable until death. So, the mortality is not a confounder of
viral RNA shedding duration.
This finding is inconsistent with previous findings in different viruses
including avian influenza A (H5N1) and respiratory syncytial virus,
where longer viral shedding duration has been shown to be associated
with increased risk of complications2,4,5,7,8.
Furthermore, longer duration of viral shedding in H1N1 has been
correlated with worse disease severity9. There are
many differences in the pathology of SARS-CoV-2 that could potentially
account for the different observations in the correlation between viral
shedding duration and patient prognosis in COVID-19 as compared to other
viruses (such as influenza). One possibility is that a destabilizing
mutation in SARS-CoV-2 in the nsp3 protein (that is not observed in
other viruses, such as SARS) could explain the unique correlation
between decreased viral shedding duration and worse disease progression
observed with COVID-19, although this remains to be
tested10.
Secondly, differences in virus subtypes of SARS-CoV-2 as compared to
other viruses might explain the contrasting finding that increased viral
shedding duration is associated with better patient prognosis in
COVID-19. A recent study investigated the molecular divergence of
SARS-CoV-2, in which it was determined the virus has two major types,
L-type and S-type, each that has unique
characteristics11. This study determined that L-type
SARS-CoV-2, as compared to S-type, is detected in approximately 70% of
all patients and is more prevalent and may be more transmissible than
the L-type. It has been suggested that the diversity of COVID-19
mortality rates in different regions of China may be explained by the
different virus subtypes, among other
characteristics12,13. Currently, few studies have
focused on defining the clinical features of these two sub-types of
SARS-CoV-2. As we do not know the SARS-CoV-2 subtypes of the patients in
our study, further analysis of the SARS-CoV-2 mRNA sequence of the virus
type could help further stratify the findings in our study.
Previous studies have shown that correlations between viral load and
immune system response are important determining factors in disease
progression. It has also known that SARS-CoV-2 infection can activate
innate and adaptive immune responses14. Therefore, it
is possible an immune system response may influence the differences in
viral load observed in the patients in our study. Consistent with prior
studies that have shown lymphopenia is a common feature in severe
COVID-1914,15, we also frequently observed
lymphocytopenia in the patients in our study. Patients with long viral
RNA shedding duration as compared to short viral RNA shedding duration
had a significantly higher absolute lymphocyte count, as well as higher
levels of inflammatory markers (such as hs-CRP and IL-6). This suggests
that patients with prolonged viral RNA shedding durations may also have
increased innate and adaptive immune responses, although this remains to
be tested.
Previous studies have observed that the median duration of viral
shedding in COVID-19 survivors ranges from 17 days (IQR 13-22 days) to
20 days (IQR 17-24 days)3,16. The longest reported
duration of viral shedding in COVID survivors was 37
days3,16. In our study, the median SARS-CoV-2 shedding
duration was 26 days, and the longest duration was 62 days (reported in
a 72-year-old male patient who survived COVID-19). Approximately 15% of
in our study patients had a viral shedding duration of more than 6
weeks. This duration is significantly longer than has been previously
reported3,16. This difference may be attributed to the
fact that the patients enrolled in our study were from Wuhan, China
where more severe cases of COVID-19 have been reported. Delayed hospital
admission may also be associated with longer viral shedding duration as
patients would not have received any medical care that could have
minimized COVID-19 symptoms. This could also potentially contribute to
the longer viral RNA shedding times detected in our studies.
There are limitations to this retrospective study. First of all,
although we were able to detect viral RNA of SARS-CoV2, viral RNA
shedding may not perfectly correlate with total viral load as quantified
by virus isolation. However, viral RNA can be used as a surrogate for
estimating the total viral load in a patient. Secondly, the viral RNA
specimens were obtained from pharyngeal swabs and it is possible there
may be instances of false negative viral RNA detections. Respiratory
tract specimens such as viral RNA detection from sputum, endotracheal
aspirate, or bronchoalveolar lavage fluid would provide a more accurate
level of detection for viral RNA, but obtaining these specimens from
non-critical patients is not practical as it requires invasive
mechanical ventilation. Finally, there were variations in the antiviral
treatments patients received in this study. Lopinavir/ritonavir and
interferon-α were used for many patients in our study, and because of
the lack of standardized antiviral therapies for the patients in our
study, we do not have statistical power to determine if these antiviral
treatments had an effect on viral RNA shedding.