Induction of humoral and cellular immune responses to COVID-19 mRNA and vector vaccines: A prospective cohort study in Bulgarian healthcare workers

Installing efficient protective immunity by anti-SARS-CoV-2 vaccines is the only current means to overcome coronavirus disease 2019 pandemics. The cellular and humoral immune responses induced with an messenger RNA (mRNA) (BNT162b2) or with a vector (ChAdOx1nCoV-19) vaccine among Bulgarian healthcare workers (n = 123, aged 23-71 years) were studied in the course of 16 weeks after priming. Receptor-binding domain (RBD)-blocking Abs and SARS-CoV-2 RBD immunoglobulin A (IgA) were evaluated in parallel with interferon gamma (IFN gamma)-producing virus-specific T cells. Both vaccines induced RBD-blocking Abs in 100% of the participants after complete immunization while the levels of protection after a single dose largely varied (22%-98%). Advanced age had a negative impact on the level and longevity of virus-neutralizing activity induced by one dose mRNA, but not by the vector vaccine. RBD-binding IgA was detected in 100% of tested donors from the mRNA vaccine cohort, and in 67% of tested from the vector vaccine cohort, at least 1 month after completed immunization. One month after completing mRNA immunization, the number of IFN gamma-producing T cells correlated significantly with the levels of RBD-specific IgA and virus-neutralizing activity induced after priming. Enumeration of circulating virus-specific IFN gamma+ T cells is not recommended for evaluation of protective immunity as their detection may require longer stimulation beyond the firstmonth postimmunization. In conclusion, BNT162B2 and ChAdOx1nCoV-19 induced potent and comparable humoral and cellular anti-SARS-CoV-2 immune responses, peaking between 10 and 30 days after complete immunization. A single dose of any vaccine did not induce adequate protection in a great part of donors, making the shorter interval between mRNA vaccine doses preferable in the settings of increased risk of infection.

Automated summary

Installing efficient protective immunity by anti-SARS-CoV-2 vaccines is the only current means to overcome coronavirus disease 2019 pandemics. The study showed that both mRNA and vector vaccines induced potent and comparable humoral and cellular immune responses against SARS-CoV-2. The number of vaccine doses and age of the participants had an impact on the level and longevity of protection provided by the vaccines.