Otherwise, the booster vaccine dose with the rAd5-S vaccine will become redundant so as to make better use of available vaccine doses. triggered antibody response in all study individuals. The second Sputnik V dose had no impact on IgG response for those seropositive for SARS-CoV-2 antigens before vaccination. The 27 seropositive individuals had detectable IgG antibodies against NP or RBD at baseline. The prevalence of individuals in our cohort with specific antibodies against SAR-CoV-2 antigens RBD and NP before vaccination was relatively high because the study was primarily performed with hospital workers, who are highly prone to contracting a SARS-CoV-2 infection. Therefore we assume that all PCI-34051 seropositive individuals in this study can be considered to have been previously infected. For 6 of the seropositive individuals, previous infection was confirmed with a positive PCR. Of the other 21, who were never PCR tested, 16 reported clinical specific COVID-19 symptoms and signs accompanied by a loss of smell and were treated at home as COVD-19 patients. Five individuals in the seropositive cohort reported no signs/symptoms of a SARS-CoV-2 infection. For these 5 we have little proof of infection before vaccination; however, all had antibodies against NP and 4 had RBD antibodies as well. In addition, these 5 displayed a booster or secondary immune response after the first vaccine dose and thus can Mouse monoclonal to mCherry Tag be considered asymptomatic SARS-CoV-2 infections. Elsewhere in Spain, at PCI-34051 least a third of infections determined by serology were asymptomatic (Polln?et?al.?2020). Seven of the seropositive individuals had no PCI-34051 detectable RBD antibodies, likely indicating that the immune response had wained over time; 2 were RT-PCR COVID-19 symptomatic cases diagnosed in August 2020, approximately 7 months before vaccination, and 4 had disease-specific symptoms/signs, including a loss of smell in April and May of 2020. Regarding the sensitivity of a “loss of smell” for COVID-19 diagnosis, see the discussion below. A waning of the antibody response over time has been assessed in several serological studies; however, the findings are not uniform, with some studies claiming rapid waning and others showing that antibody persistence can only be accurately determined at the individual level (Chia?et?al.?2021). Our study suggests longevity of the IgG response against NP over time compared with the RBD antibody response. Our study demonstrates that 42% (25 out of 59) of our seronegative cohort did not develop a detectable IgG response against RBD after the first vaccine dose and seroconverted only after receiving the second dose, highlighting the importance of receiving a second dose of the vaccine. Furthermore, we showed that a single dose did not always protect against a symptomatic SARS-CoV-2 infection. Of the 25 individuals without a detectable IgG response against RBD after the first vaccine dose, 4 developed symptomatic COVID-19 just before or shortly after the second dose. The infection was not confirmed with an RT-PCR; however, the 4 individuals reported fever (4), dry cough (3), headache (2) and tiredness/fatigue (4) as the most common symptoms and 3 out of 4 lost their smell. A loss of smell is a good predictor for COVID-19, with studies reporting high specificity (97%) and sensitivity of 65% (Gerkin et?al., 2021, Haehner et?al., 2020, Said et?al., 2021). A rapid olfactory test as a potential screening tool for COVID-19 has been reported (Said?et?al., 2021). These 4 individuals also developed a strong IgG response against the NP antigen after the second vaccine dose; only a natural infection and not the vaccine induces an antibody response against the NP antigen (Assis?et?al., 2021) Additionally, these 4 people showed a high IgG booster antibody response against.