1.7.1. Comparison of viral load Ct by vaccination status In the NHS Test and Trace (NHSTT) case data, the mean and median lowest Ct values for all cases with Delta, where Ct data are available, since the 14 June 2021 are similar, with a median of 17.8 for unvaccinated and 18.0 for those with 2 vaccine doses (Figure 12). This means that whilst vaccination may reduce an individual’s overall risk of becoming infected, once they are infected there is limited difference in viral load (and Ct values) between those who are vaccinated and unvaccinated. Given they have similar Ct values, this suggests limited difference in infectiousness. To note, this analysis is undertaken on case data and are not age-stratified. Findings can be influenced by test-seeking behaviour, as well as true changes in the data, for example the age distribution of cases, which can also influence Ct values.