In COVID-19 convalescents, a single intramuscular dose of FINLAY-FR-1A vaccine against coronavirus was an effective booster of the pre-existing natural immunity, with a very good safety profile.

An open-label phase 1 clinical trial aimed to investigate the safety and immune responses following a single intramuscular injection of the FINLAY-FR-1A vaccine in people (aged 22-57 years) having documented pre-existing coronavirus natural immunity. 

This monocentric study recruited 30 SARS-CoV-2 convalescents and segregated them into 3 arms: Convalescents of mild coronavirus disease (N=11), asymptomatic convalescents (N=10), both with polymerase chain reaction (PCR)-positive at the time of diagnosis; and people having subclinical infection identified by viral-specific IgG (N=9). A single dose of FINLAY-FR-1A vaccine (50 µg of  recombinant dimeric receptor-binding domain [RBD]) was given.

Reactogenicity and safety, evaluated over twenty-eight days after vaccination were the major endpoints. Vaccine immunogenicity was the secondary endpoint. Using enzyme-linked immunosorbent assay and live-virus neutralization test, the humoral response was determined at baseline and post-vaccination. The effector T cellular response was also examined. 

No severe noxious events were noted. Minor side effects were noted, the most common, redness: 2 (6·7%) and local pain: 3 (10%). The FINLAY-FR-1A vaccine induced a >21 fold rise in IgG anti-RBD antibodies twenty-eight days following vaccination. Compared to the COVID-19 convalescent panel, the median of inhibitory antibody titres (94%) was reported to be 3 times higher. In 24 (80%) of people, the virus neutralization titres greater than 1:160 was noted. A rise in RBD-specific T cells producing interferon-gamma and tumor necrosis factor-alpha was also witnessed.

Immunization with the FINLAY-FR-1A vaccine elicited high humoral and cellular responses, thus implying a protective immunity against SARS-CoV-2. This must be confirmed in larger phase II trials.