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The durability of the antibody response following severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections has not been fully elucidated. We have performed a cross-sectional study in one of the largest slums of Buenos Aires, Barrio Padre Mugica in June 2020, detecting a seroprevalence of 53.4%. To evaluate the persistence of the humoral response against SARS-CoV-2 in this area, we designed a second study assessing only the people that were IgG positive in the first survey. The IgG levels against the full spike (S) protein in 175 individuals that were seropositive, at least 6 months before, were evaluated in a second survey. The positivity rate was 92.0%, 161 from 175 individuals remained IgG positive. We observed a contraction in the overall IgG levels measured by ELISA. The median IgG dropped 62% from June to December 2020. Most of the individuals tested (87%) reported to be asymptomatic or oligo-symptomatic. No difference was found between men and women, but people aged less than 50 showed a lower IgG level in each period compared to older individuals. Our data indicate sustained humoral immunity for at least 6 months in a specific socio-economical setting in a population that was mainly asymptomatic for COVID-19.