Inhibition of receptor-binding domain-ACE2 interaction after two doses of Sinovac's CoronaVac or AstraZeneca/Oxford's AZD1222 SARS-CoV-2 vaccines.

Practical laboratory proxies that correlate to vaccine efficacy may facilitate trials, identify nonresponders, and inform about boosting strategies. Among clinical and laboratory markers, assays that evaluate antibodies that inhibit receptor-binding domain (RBD) ligation to angiotensin-converting enzyme-2 receptor (receptor-binding inhibition [RBI]) may provide a surrogate for viral neutralization assays. We evaluated RBI before and after a median of 34 days (interquartile range [IQR]: 33-40) of the second dose of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Sinovac's CoronaVac (CN) or AstraZeneca/Oxford's AZD1222 (AZ) vaccines in 166 individuals. Both vaccines elicited high inhibitory titers in most subjects, 95% (158/166), with signal inhibition above 30% and 89% (127/143) with more than fourfold increase from prevaccination titers, but titers tend to decrease over time. Both postvaccination inhibitory titers (95%, IQR 85%-97% for AZ vs. 79%, IQR 60%-96% for CN, p = 0.004) and pre/post-titer increase (AZ 76%, IQR 51%-86% for AZ vs. 47%, IQR 24%-67% for CN, p < 0.0001) were higher among AZ vaccinees. Previous serological reactivity due to natural infection was associated with high prevaccination signal inhibition titers. The study documents a robust antibody response capable of interfering with RBD-angiotensin-converting enzyme binding. Evaluation of SARS-CoV-2 infection incidence in these populations is necessary to assess its association to protection and its duration.

Humoral response to spike S1 and S2 and nucleocapsid proteins on microarray after SARS-CoV-2 infection.

Many aspects of the humoral immune response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), such as its role in protection after natural infection, are still unclear. We evaluated IgA and IgG response to spike subunits 1 and 2 (S1 and S2) and Nucleocapsid proteins of SARS-COV-2 in serum samples of 109 volunteers with viral RNA detected or seroconversion with different clinical evolution (asymptomatic, mild, moderate, and severe coronavirus disease 2019), using the ViraChip® Test Kit. We observed that the quantification of antibodies to all antigens had a positive correlation to disease severity, which was strongly associated with the presence of comorbidities. Seroreversion was not uncommon even during the short (median of 77 days) observation, occurring in 15% of mild-asymptomatic cases at a median of 55 days for IgG and 46 days for IgA. The time to reach the maximal antibody response did not differ significantly among recovered and deceased volunteers. Our study illustrated the dynamic of anti-S1, anti-N, and anti-S2 IgA and IgG antibodies, and suggests that high production of IgG and IgA does not guarantee protection to disease severity and that functional responses that have been studied by other groups, such as antibody avidity, need further attention.