Safety, immunogenicity, and breakthrough infections during 1-year follow-up after COVISHIELD™ vaccination among healthcare workers in a teaching institute in North India: A prospective longitudinal study.

Background: A prospective longitudinal study was planned to assess immunogenicity, safety, and breakthrough infection rates among health care workers (HCWs) after COVISHIELD™ vaccination in a teaching institute in North India.

Methods: A total of 518 HCWs were enrolled at baseline for receiving first dose of COVISHIELD™, 429 continued participation for second dose, 415 followed up at 28 days after second dose, 405 at 6 months, and 403 at 1 year from February 2021 to November 2022. At each visit prior, 2 ml of blood sample was collected and plasma separated. Anti-SARS-Cov-2 IgG antibodies against spike protein were detected by Chemiluminescent Immuno-Assay on the VITROS 3600 platform (Ortho-clinical Diagnostics, NJ, USA). Data were analysed for 403 HCWs (completed all follow-ups), using Statistical Package for Social Sciences version 21.0.

Results: The mean age of participants was 35.3 years (SD ± 11.11), and males were 50.2%. At baseline (N = 518), proportion of reactive HCWs was 25% (95% CI, 21-29%), which increased significantly to 92% (95% CI, 89-95%) after receiving first dose (N = 403), 95% (95% CI, 92-96%) after second dose (N = 403), and declined to 85% (95% CI, 83-93%) at six months (N = 403). At 1 year, reactivity increased to 89.3% (95% CI, 86-90%; value < 0.0001). Fever was reported by 31.2% of HCWs, followed by pain at injection site (27.6%), malaise (16.4%), headache (3.7%), and dizziness (3.7%).

Conclusions: Immunogenicity post-COVISHIELD™ vaccination increased after first and second doses but declined after 6 months and inclined after 1 year of the first dose. The safety profile was within acceptable limits.