To strengthen the serological profiling of the donor plasma samples, Attana QCM was used to study the interaction kinetics between the ACE2 receptor and Wuhan and other variants (Alpha, Beta, Gamma, Delta, Epsilon and Kappa and Omicron). The affinity varied by more than an order of magnitude and clear differences in the interaction kinetics were observed. Overall, neutralization in Wuhan was among the strongest among the variants tested, which was expected as the vaccine was developed against the Wuhan variant. Significantly inferior neutralizing efficacy was observed against the Beta and Gamma strains compared to Wuhan at all time points tested up to 6 months post-vaccination. The average neutralization against all variants was just over 40%. The results also showed the following significant differences:

• Women showed better protection against all variants than men.

• 60 years and older showed a significantly lower protection compared to those under 40 years.

• Vaccinees of Indian ethnicity showed a small, but significantly stronger, peak response to the Gamma variant compared to those of Chinese ethnicity. In addition, differences were detected against the alpha and beta variants between these two cohorts. Although this needs to be validated in larger cohorts, this raises the possibility that significant ethnic differences in vaccine response may exist.

• Former, but not active, smokers showed higher protection compared to non-smokers.

• BMI and comorbidities such as high blood pressure and high cholesterol had no effect on the observed neutralizing titers.