Immune dynamics at single cell protein level after delta/omicron infection in COVID-19 vaccinated convalescent individuals

Both COVID-19 mRNA or recombinant Adenovirus vector (rAdVV) based vaccines have shown a great efficacy in generating humoral and cellular immune responses. Two doses of the COVID-19 vaccines generate enough antibodies and generate spike-specific T cell responses. However, after 6-8 months there is a decline in antibody production and T cell responses. Due to the rise of new SARS-CoV-2 variants of concern, a third or even fourth dose of vaccine was recommended for the elderly, immune comprised and frontline medical health care workers. However, despite additional booster doses given, those who were infected with either delta or omicron (during December 2021 – March 2022) had symptoms of illness. By what means these COVID-19 vaccines provide immunity against the SARS-CoV-2 virus at the molecular level is not explored extensively yet and, it is an emerging research field as to how the SARS-CoV-2 virus is able to evade the host immunity. Most of the infected people had mild symptoms whilst some were asymptomatic. Many of the people had developed nucleocapsid antibodies against the SARS-CoV-2 delta/omicron variants confirming a humoral immune response against viral infection. Furthermore, cellular analysis shows that post-vaccinated recovered COVID-19 individuals have significantly reduced NK cells and increased T naïve CD4+, TEM CD8+ and B cells. This decrease in cellular immunity corresponds to individuals who recovered from alpha variants infection and had mild symptoms. Our results highlight that booster doses clearly reduce the severity of infection against delta/omicron infection. Furthermore, our cellular and humoral immune system is trained by vaccines and ready to deal with breakthrough infections in the future. ### Competing Interest Statement The authors have declared no competing interest.