Review 1: "SARS-CoV-2 infection results in lasting and systemic perturbations post recovery"

RR:C19 Evidence Scale rating by reviewer:

• Potentially informative. The main claims made are not strongly justified by the methods and data, but may yield some insight. The results and conclusions of the study may resemble those from the hypothetical ideal study, but there is substantial room for doubt. Decision-makers should consider this evidence only with a thorough understanding of its weaknesses, alongside other evidence and theory. Decision-makers should not consider this actionable, unless the weaknesses are clearly understood and there is other theory and evidence to further support it.

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Review:

This investigation by Frere and colleagues examined the short- and long-term systemic responses of SARS-CoV-2 and compared its pathology with the influenza A virus (IAV) infection in a golden hamster. Besides finding the greater injury to the lung and kidney by SARS-CoV-2 than IAV, an insightful finding they made is the lasting impact of the SARS-CoV-2 infection in the olfactory bulb (OB) and olfactory epithelium (OE), which showed prolonged inflammation beyond a month of the infection. Of note, they observed microglial and T cell activation, proinflammatory cytokine production, and interferon in the OB/OE despite the lack of any detectable viral copies. They further implicated the impact of lasting OB/OE pathology on the behavioral changes. Readouts of SARS-CoV-2 pathology from a golden hamster model were taken further to evaluate with sequencing data of individuals who recovered from COVID-19 that corroborated the sustained inflammation in OB/OE tissue a month after disease resolution. Taken together, a present investigation by Frere and colleagues is interesting and timely as it helps to understand the lasting impact of SARS-CoV-2 pathology and highlights infection severity to the olfactory bulb (OB) and olfactory epithelium (OE) and thereby reflects frequent issues of sensory, emotional, and cognitive processes-related perturbations in the COVID-19 patients. It’s a term now widely defined as long COVID.

An insightful finding of this report is the analysis of the lasting impact of SARS-CoV-2 infection on the olfactory bulb (OB) and olfactory epithelium (OE) as validated by transcriptomic and histochemical assays. To understand the lasting impact of SARS-CoV-2 infection, authors made extensive attempts by including various systems/tissues viz. lung, heart, kidney, and brain/neural system, and underpinned key anomalies. The inflammatory profile of OB/OE long after SARS-CoV-2 infection and with no detectable virus trace led the authors to suggest that either an undetectable level of replication is persisting in a nearby region or left-over debris is responsible for the continued inflammatory profile, which is an interesting point and could be investigated further.

Besides the above key findings, this work suffers several inadequacies as well. Even though the author has carried out an extensive investigation, stating that SARS-CoV-2 infection results in lasting and systemic perturbations post-recovery are misleading. It needs a comprehensive and thorough investigation to rightly state such anomalies as systemic perturbations. Also, the statement- ‘These data highlight a molecular mechanism for persistent COVID-19 symptomology and characterize a small animal model to develop future therapeutics’ is not factually accurate, where transcriptomic/histochemical analysis is performed that provides details about expression profiles not mechanism. While several other reports earlier described/characterized the utility of golden hamsters for COVID therapeutics. 

Another major issue is the interpretation of SARS-CoV-2 pathology finding of golden hamster to human pathology while taking a considerably very small sample size. In the data (Figure 7), for both olfactory bulb (OB) and olfactory epithelium (OE) -related observations, the human sample size is sparse which could potentially limit the quantitative outcome of their finding on human physiology.

Additionally, histochemical validation at the protein level in Figure 4 could strengthen the claim of the findings in this section.

There are a few minor issues evident related to the mislabeling and missed information. For instance, lines 591- Figure 6G-I, here Figures 6H and 6I are missing. Legend Fig.2 D-F- labels for kidney, heart, and lungs are not in accord the figure 2.