Single-Dose Vaccine Candidate Boosts Neutralizing Antibody Response

The COVID-19 pandemic continues to spread rapidly across the globe. New research attempts are being made to develop vaccines. A new study released on a pre-print server provides results of the single-dose FINLAY FR 1A recombinant dimeric receptor-binding domain (RBD) base vaccine during a phase I clinical trial with 30 COVID-19 convalescents to test its capacity for boosting natural immunity.

The study included 0 people in the convalescent stage after SARS-CoV-2 infection. All had a positive polymerase chain reaction (PCR) test for the virus that had become negative two or more months from the start of the study. They consisted of three groups. The first group had mild COVID-19, the second group had the asymptomatic disease, and the third were seropositive but never had a positive PCR. Of the 30 participants, anti-RBD immunoglobulin G (IgG) antibodies were markedly increased on day 7 post-vaccination, as reported earlier in healthcare workers.

The peak was reached on day 28, at 722 AU/mL, which is 21-fold the level before vaccination, and ten times higher than the average pre-vaccination Cuban Convalescent Serum Panel (CCSP). This reflects the results obtained with a single dose of the Pfizer/BioNTech mRNA vaccine BNT162b2. Anti-RBD IgG was found to be increased in all three groups, with four non-responders among them, corresponding to 13% of the whole group. Before vaccination, the RBD-ACE2 binding was inhibited by less than 60%; after one dose, the inhibition ratio increased in 26/30 subjects, over time, to 94% by day 14.

The majority achieved this level of antibody-mediated inhibition even earlier, by day 7. This is in contrast to the 47% inhibition achieved with the CCSP. The researchers also observed a 50% molecular virus neutralization titer (mVNT50) – that is, the titer at which serum dilutes 50% of RBD-ACE2 interactions.

The mVNT50 geometrical mean titer (GMT), of ~2240, on post-vaccination day 28, was almost 120-fold higher than the pre-vaccination titer of ~22 and 27-fold higher than the CCSP value of ~83. These results indicate that a single dose of this vaccine candidate is both safe and immunogenic in convalescent COVID-19 patients. The safety profile is excellent, while the antibody response is boosted 20-fold at one-week post-vaccination.

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