Longitudinal evaluation of two different COVID-19 vaccination strategies in individuals with and without previous SARS-CoV-2 infection.

Background: We tested the antibody response to SARS-CoV-2 vaccination in individuals with and without previous infection that received different vaccination strategies. Methods: We recruited 203 volunteers. Individuals who have had SARS-CoV-2 infection during the six months preceding vaccination received one dose (group 1), the others received two (group 2). After 3 months, 98 subjects received a booster dose. Anti-SARS-CoV-2 Spike RBD IgG were tested in all subjects before vaccination (T0), and at 15 (T15), 90 (T90), 180 (T180) and 360 (T360) days after second or single dose; additionally, in group 2, IgG were tested 10 days after the vaccination (T10). Results: The difference of IgG concentration between the groups was statistically significant (p<0.05) at T0, T15 and T90, but not at T180 (p=0.713) and T360 (p=0.069). At T0 and T90 the antibody titre was higher in group 1, but it dropped in all volunteers 90 days after vaccination. Most of infections after vaccination occurred between T90 and T180. Conclusions: Antibody titre is significantly associated with a previous SARS-CoV-2 infection. Probability of contracting the infection increases after three months from primary vaccination, even among who had a previous infection, confirming the efficacy of vaccination as a preventive measure against SARS-CoV-2 infections and the need of booster administrations.