The rapid development of vaccines to prevent infection by SARS-CoV-2 virus causing COVID-19

makes necessary to compare the capacity of the different vaccines in terms of development of

a protective humoral response. Here, we have used a highly sensitive and reliable flow

cytometry method to measure the titers of antibodies of the IgG1 isotype in blood of volunteers

after receiving one or two doses of the vaccines being administered in Spain. We took advantage

of the multiplexed capacity of the method to measure simultaneously the reactivity of

antibodies with the S protein of the original strain Wuhan-1 and the variant B.1.1.7 (Alpha). We

found significant differences in the titer of anti-S antibodies produced after a first dose of the

vaccines ChAdOx1 nCov-19/AstraZeneca, mRNA-1273/Moderna, BNT162b2/Pfizer-BioNTech

and Ad26.COV.S/Janssen. Most important, we found a relative reduction in the reactivity of the

sera with the B.1.1.7 versus the Wuhan-1 variant after the second boosting immunization. These

data allow to make a comparison of different vaccines in terms of anti-S antibody generation

and cast doubts about the convenience of repeatedly immunizing with the same S protein