SARS-CoV-2 subunit vaccine adjuvants and their signaling pathways.

Mekonnen, Daniel; Mengist, Hylemariam Mihiretie; Jin, Tengchuan

INTRODUCTION Vaccines are the agreed upon weapon against the COVID-19 pandemic. Adjuvanted subunit vaccines are well controlled, safe and effective. This review discusses about COVID-19 subunit vaccines and their signaling pathways which could provide a glimpse into the selection of appropriate adjuvants for prospective vaccine development studies. AREAS COVERED In the introduction, a brief background about SARS-CoV-2 pandemic and the current state of vaccine development race were highlighted. Then the general classes of vaccine adjuvants were briefly introduced. In the body of the review, the included articles were identified at PubMed and SCOPUS on 19th may 2021 using keywords such as "SARS-CoV-2" or "COVID-19" and "Immunologic adjuvant" or "subunit vaccine". Then, the antigen, trial stage and types of adjuvants were extracted. Finally, the pattern recognition receptors (PRR), their classes, cognate adjuvants and potential signaling pathways were comprehended. EXPERT OPINION Adjuvants are unsung heroes of subunit vaccines. The in silico studies are very vital in avoiding several costly trial errors and save much work times. Majority of (pre) clinical studies are promising. It is encouraging that, most of the selected adjuvants are novel meaning that they have known PRR to study their effector mechanisms. Much emphasis must be paid to the optimal paring of antigen-adjuvant-PRR for obtaining the desired vaccine effect. A good subunit vaccine/adjuvant is one that has high efficacy, safety, dose sparing, rapid seroconversion rate and induce a broad spectrum of immune response. In the years to come, COVID-19 adjuvanted subunit vaccines are expected to have superior utility than any other vaccines including the breakthrough mRNA vaccines for various reasons.