Background: Vaccination for COVID-19 is an essential tool to fight the pandemic. Evidence suggests that patients with immune mediated inflammatory diseases (IMIDs) have less response. The application of a booster shot is a strategy that has been implemented in this population, however there is scarce information about its efficacy.

Objectives: To assess the humoral and cellular immune response after a third dose of SARS-CoV-2 vaccine in patients with rheumatoid arthritis (RA) with undetectable antibodies titles after primary regimen of two doses.

Methods: Observational study. Patients with RA (ACR/EULAR 2010 criteria) from two rheumatology centers, ≥18 years old, with no seroconversion after two doses of SARS-CoV-2 vaccine, who received a third dose of either mRNA or vector-based vaccines (BNT162b2 or ChAdOx1 nCoV-19) were included. Anti-SARS-CoV-2 IgG antibodies, neutralising activity and T cell responses were assessed between 21 and 40 days after the third dose. Sociodemographic data, comorbidities, treatment, vaccine applied and the presence of adverse events (AE) were recorded. Statistical analysis: descriptive analysis. Chi ² or Fischer test and T test.

Results: A total of 21 non-responder patients were included, all of them females with a mean age of 63.7 years (SD 11,6) and mean disease duration of 15.8 years (SD 8). Most of them (81%) reported comorbidities, being the most frequent arterial hypertension, obesity and dyslipidemia. At vaccination time, 6 (28.6%) were receiving glucocorticoids, 3 of them ≥10 mg/day, 17 c-DMARDs (methotrexate 57.1%) and 18 (85.1%) b-DMARDs, 6 abatacept (ABT) and 4 rituximab (RTX).

Regarding the primary vaccination regimen, 13 (61.9%) received two doses of BBIBP-CorV, 3 (14.3%) Gam-COVID-Vac, 3 (14.3%) ChAdOx1 nCoV-19 and 2 (9.5%) a mix regimen of Gam-COVID-Vac/mRNA-1273. The majority (95.2%) received BNT162b2 vaccine and only one of them ChAdOx1 nCoV-19, with a mean time between the second and third dose of 151,4 days (SD 46,4). After the third dose, 90.5% of the patients presented detectable anti-SARS-CoV-2 IgG and 76.2% presented neutralizing activity. The median of neutralizing antibodies titers was 1/12 (IQR 1/7-1/48). Both patients who did not present detectable antibodies were obese, recieved BBIBP-CorV during the primary regimen and BNT162b2 as the third dose, one of them was taking methotrexate and ABT and the other one RTX. Compared to other treatments, ABT and RTX was associated with no neutralizing activity in 4 (80%) patients and lower titers of neutralizing antibodies [median 1/3 (IQR 0-1/20) vs median 1/8 (IQR 1/4-1/128), p=0.197].

A T-cell response was present in 41.2% of all patients after the second dose, increasing to 75% after the third dose. The use of ABT was associated with a lower frequency of T-cell response (80% vs 20%, p=0.014).

Sixteen (76.1%) patients reported at least one AE, 66.7% injection site reaction and 25% flu-like syndrome.

Conclusion: In this RA cohort who failed to seroconvert after two doses of SARS-CoV-2 vaccine, 90.5% presented detectable anti-SARS-CoV-2 IgG and 75% T-cell responce after a third dose. The use of ABT was associated with a lower frequency of T-cell response. This data highlights the importance of a third vaccine in this group of patients.

Disclosure of Interests: None declared