Declining Effect in the Immunogenicity of Covishield Vaccine among Healthcare Workers Postvaccination- A Prospective Cohort Study

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Introduction: The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) pandemic had a severe impact on global healthcare systems and the economy. The introduction of Coronavirus Disease 2019 (COVID-19) vaccines has provided a solution for the disease. However, with mutations in the strains of COVID-19, there is a need to understand the immunogenicity and duration of protection provided by the vaccines. Aim: To assess the immunogenicity of the covishield vaccine among Healthcare Workers (HCWs) postvaccination following the administration of the second dose of the covishield vaccine. Materials and Methods: This was a single-centre prospective cohort study in the Department of Microbiology, Sapthagiri Institute of Medical Sciences and Research Centre, Bengaluru, Karnataka, India, from January 2022 to June 2022. Study included HCWs who had received the first and second doses of the Covishield vaccine and were willing to participate in the study. Only 30 HCWs who had received two doses of the COVID- 19 vaccine (Covishield) at least two weeks before registration were enrolled in the study. The serum was tested to determine the qualitative detection of COVID-19 Immunoglobulin G (IgG) antibody in HCWs using the Enzyme-linked Immunosorbent Assay (ELISA) method using COVID Kawach IgG Microlisa kit. The P/N ratio is the Optical Density (OD) value of the sample tested/average OD value of the negative control. The P/N ratio was calculated and the participants were divided into different groups based on the months after receiving the second dose of the vaccine. The non parametric Kruskal-Wallis test was applied to test the difference between the P/N ratio based on the months of vaccination since the second dose. Results: In the present study, the majority of HCWs, 20 (66.6%), were uninfected by the SARS-CoV-2 virus, while 10 (33.3%) became infected after receiving two doses of the covishield vaccination. Out of 30 samples, 29 (96.6%) samples had crossed the cut-off OD value of 0.3970, and the P/N ratio was greater than 1.5, suggestive of the presence of antibodies against SARS-CoV-2. Only 1 (3.3%) sample had an absorbance value lower than the cut-off, and the P/N ratio was <1.5, suggestive of the absence of antibodies against SARS-CoV-2. The P/N values of the participants were compared between different groups, which were divided based on the months since the second dose of the covishield vaccine. Participants who had completed five months since the last vaccine dose showed the highest P/N value (median 13.9), followed by median values of 7.1, 4.8 and 3.3 on completion of 6, 7 and 8 months and beyond, respectively. A steady decline in the median P/N ratio was observed from 5 to 8 months. Conclusion: The immunogenicity of the covishield vaccine among HCWs Postvaccination after the second dose of the covishield vaccine showed a steady decline in the subsequent months.