RR:C19 Evidence Scale rating by reviewer:
Reliable. The main study claims are generally justified by its methods and data. The results and conclusions are likely to be similar to the hypothetical ideal study. There are some minor caveats or limitations, but they would/do not change the major claims of the study. The study provides sufficient strength of evidence on its own that its main claims should be considered actionable, with some room for future revision.
The study addressed whether T-cell numbers are associated with protection from COVID-19 infection. Many studies have demonstrated T-cell immunity in unexposed donors and in SARS-CoV-2-infected individuals. Similarly, antibodies against many SARS-CoV-2 antigens were reported in unexposed and infected individuals. Two questions have remained unanswered – 1. How are individuals unexposed to SARS-CoV or SARS-CoV-2 mounting B and T-cell responses against SARS-CoV-2 antigen 2. Are unexposed individuals with B and T-cell immunity protected from SARS-CoV-2 infection? This study has provided evidence that unexposed individuals carrying B-cell and/or T-cell immunity are indeed protected from new SARS-CoV-2 infection.
The methodologies used by the authors are standard assays that the field is using. The statistical analyses are in line with other reported studies. There are a few key parameters missing from their study:
The Methods Section does not indicate the incubation time of the T-cell activation assay. The assay time should be short (16 – 48 h) to make the point that the individual was carrying antigen-experienced T-cells.
The assay method – T-spot assay fails to indicate whether activation is coming primarily from CD4 or CD8 T-cells. Many papers have shown a stronger CD4 T-cell activation over CD8 using peptide pools. This question should be addressed at least with a subset of samples.
The magnitude of T-cell activation is a critical parameter for protection. Similar analysis as shown in Figure-5 may be considered in which the variable is # of T-spots observed in an individual and their susceptibility to infection.
Other questions that need to be addressed:
Why do authors see a lack of correlation between T-spot numbers and self-reported symptoms in seronegative individuals? In other studies, disease symptoms were reported in seronegative individuals, and these individuals were shown to exhibit T-cell immunity.
Is there any correlation between the severity of disease and T-spot numbers?
Why do older infected individuals show higher T-cell activity than younger individuals? The expectation is that younger individuals are likely to have a broader naïve T-cell pool compared to older individuals and would mount a stronger response.
The overall finding of the paper is in line with what has been published so far. The authors add new information that pre-existing immunity may provide protection to SARS-CoV-2 infection. The authors have pointed out several limitations in their study many of which will be addressed as more and more infected patients are analyzed. However, the study has addressed an important gap in our understanding of pre-existing T-cell immunity and protection using a well-designed clinical experiment.
The paper can be improved by focusing the correlations on a few key conclusions. The results section is dominated by correlational analysis, where most Figures and Tables are in the supplements masking the important conclusions that the paper is trying to convey.