Researchers investigated the incidence of breakthrough COVID-19 infection and the associated risk factors in a vaccinated cohort of patients with autoimmune rheumatic diseases (AIRDs). Further, they ascertained the reliability of using antibodies to the receptor binding domain of spike protein (anti-RBD) for predicting susceptibility to such infections.

• A total of 630 patients of AIRDs (mean age 55.2 (±11.6) years, male to female ratio of 1:5.2) who had completed two doses of SARS-CoV2 vaccines were included.

• AZD1222 vaccine was received by the majority of the patients (495, 78.6%) while the remaining received the BBV152 vaccine.

• Following determination of anti-RBD antibodies 4–6 weeks after the second vaccine dose, stratification of the patients was done into good responders (GR) (> 212 IU), inadequate responders (IR) (0.8–212 IU) and non-responders (NR) (< 0.8 IU).

• The patients had mean antibody titer of 854.1 (± 951.9); 380 patients (60.3%) were GR, 143 (22.7%) IR and 107 (16.9%) NR.

• At a mean follow-up of 147.3 (±53.7) days, 47 patients (7.4%) had occurrence of breakthrough infections, which were proportionately highest in the NR group (19; 17.75%), followed by the IR group (13; 9.09%) and least in the GR group (15; 3.95%).

• There appeared correlation of breakthrough infections with seronegativity following vaccination.

• Overall, these findings support exploring postvaccination antibody titres as a biomarker in patients with AIRD.