Mean ln (RBD) and 95% confidence limits in HCWs (continuous line) and SOT recipients (broken line) between 49 and 153 days after vaccination, adjusted for sex and age. The mean ln (AbR) in the two populations for the periods between 49 and 153 days after vaccination in individuals with positive AbR. The ratio of ln (AbR) between HCWs and SOT recipients ranged between 1.2 and 1.7, i.e., between 3.3 and 5.5 on arithmetic scale; SOT recipients showed a significant increase up to 76 days (p < 0.001), then a non-significant decrease in ln (AbR) after 118 days (p = 0.1); conversely, HCWs experienced a strong decrease in ln (AbR) up to 76 days (p = 0.02), and a less pronounced decrease between 76 and 118 days (p = 0.04).