Determinants of Humoral and Cellular Immune Responses to COVID-19 Vaccines in Healthcare Workers at a Tertiary Care Hospital

Aims: This study aims to assess the immunological responses elicited by COVID-19 vaccines in healthcare workers (HCWs), to age, vaccination regimen, and prior COVID-19 infection status. Methods: This prospective study population consisted of HCWs who were administered mRNAbased, adenovirus-based, and inactivated COVID-19 vaccines, in doses ranging from one to three. Blood samples were collected between 3 to 18 months post-final vaccination. Humoral responses were measured using the Elecsys® anti-SARS-CoV-2 S assay, while cellular responses were assessed with the QuantiFERON® SARS-CoV-2 assay. Results: Analysis of 44 vaccinated HCWs revealed that 98% had detectable anti-S antibodies, and 75% exhibited detectable cellular responses post-vaccination. Age group, type of vaccine, and the time interval between final vaccination and blood collection did not influence humoral and cellular responses. The number of vaccine doses administered did not correlate with the humoral response. However, a statistically significant positive correlation was observed between levels of CD4 (p=0.029) and CD8 (p=0.015) responses and the number of vaccine doses received. HCWs with two or more previous COVID-19 episodes demonstrated a correlation between the number of vaccine doses given and both humoral (p=0.007) and cellular (p<0.05) responses. Conclusion: Cellular immunity is correlated with the number of vaccine doses administered. HCWs who had encountered two or more COVID-19 episodes exhibited a positive correlation between their immune response and the number of vaccine doses administered.