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.
The authors report a cross-sectional assessment of SARS-CoV-2 PCR cycle threshold values among positive cases over two months of routine testing at two sites in California, stratifying by site, vaccination status, and presence of symptoms. Healthy Yolo Together is a free testing program for asymptomatic people over age 2 in the City of Davis and Yolo County and utilized a saliva-based assay. Unidos en Salud provides free community-based testing in San Francisco for asymptomatic and symptomatic individuals via anterior-nasal swabs. The HYT saliva testing assessed N1 and N2, and UeS detected N and E genes. They found no significant different in CT values between vaccinated and unvaccinated or symptomatic and asymptomatic groups infected with the delta variant.
This is a well-reported study and consistent with other analyses using similarly collected data of CT values among people infected with delta. While at first glance these findings might suggest transmission potential is the same for vaccinated and unvaccinated individuals with delta infection (e.g., this is how the Provincetown MMWR has been interpreted by many), there are several very important limitations to keep in mind that preclude any strong conclusions about infectiousness or transmission potential. Importantly, studies with stronger designs support the opposite conclusion. I suggest including the following issues to the manuscript as additional context for an otherwise well-done analysis.
1. Sampling strategy – CT values are highly variable based on the timing of collection, both in terms of the individual infected (peaking over a period of about 24 hours) and the overall epidemic (routinely collected specimens will have lower CT values during rising epidemics, higher CT values during falling epidemics; e.g. https://doi.org/10.1093/infdis/jiab367). This makes studies without a systematic sampling strategy (i.e., patients self-presenting for testing) challenging to interpret and vulnerable to biases based on the timing of the sample relative to both infection onset and the overall epidemic. We are also not told the timing relative to symptom onset for the symptomatic positives, which would be useful if available. The authors cite the REACT-1 study in the UK, which uses a more robust systematic random sampling strategy and found that vaccinated individuals with infection have higher CT values than those who have not had the vaccine. By nature of the study design, this is a more reliable assessment of the difference in CT values between these groups.
2. Cross-sectional design – In a somewhat related issue, the cross-sectional study design offers only a snapshot of the CT value, which as above is highly dynamic over time. The Chia et al study cited by the authors offers a helpful counterexample from Singapore, where CT values were assessed over a 4-week period and, while CT values were initially similar between vaccinated and unvaccinated individuals with infection (no systematic sampling strategy for the initial testing), there was a much more rapid rise in the CT values (drop in viral loads) among those who had been vaccinated.
3. Cycle thresholds are not equivalent to infectious virus or transmissibility, especially with vaccination – The first issue to highlight here is that multiple studies have now shown that when comparing equivalent CT values between vaccinated and unvaccinated people, there is a lower likelihood of being able to culture virus from vaccinated people. This makes Click here to access/download;Review;c19rr cycle threshold 10.18.21.docx immunological sense. Second, contact tracing transmission studies in the delta era have found reduced transmission risk with delta despite similar CT values at diagnosis (e.g., https://doi.org/10.1101/2021.08.12.21261991 and https://doi.org/10.1101/2021.09.28.21264260).