Table 1.

Candidate COVID-19 vaccines currently in phase 1 or 2 human clinical trials.

Platform	Description	Advantages	Disadvantages
Recombinant Viral vector	Unrelated virus engineered to encode the target gene of the pathogen. Viral vectors can be replicating or non-replicating	Induces high cellular and humoral immune responses	Possible pre-existing immunity against vector Risk of reversion to virulence Limitations in scaling-up production
Candidate vaccines:	Ad5-CoV Adenovirus Type 5 vector; antigen: Spike protein Phase I: CanSino Biologics ChiCTR2000030906 [88]/NCT04313127 [89] Phase II: Institute of Biotechnology; Academy of Military Medical Sciences ChiCTR2000031781 [90]/NCT04341389 [91] Participants: 18–60 years; Phase I n = 108Phase II: n = 508 Phase I: vaccine was safe and tolerable. No serious adverse effects. 63 participants (Low dose n = 18 [50%); moderate dose n = 18 [50%]; high dose n = 27 [75%]) developed four fold rise in neutralising antibody titres by Day 28. Pre-existing immunity to Ad5 neutralising antibody titre was observed in half of the participants before vaccination. Out of these, a low proportion seroconverted[76], [1] AZD1222 (previous known as ChAdOx1 nCoV-19) Simian adenoviral vaccine vector; antigen: Spike protein Phase I/II: University of Oxford NCT04324606 [92] Participants: 18–55 years; n = 1090
Inactivated	Pathogen virus inactivated by chemicals or radiation	Easy to prepare High safety	Variable efficacy
Candidate vaccines:	PiCoVacc Phase I/II: Sinovac Biotech Co NCT04352608 [93] Participants: 18–59 years; Phase I n = 144; Phase II n = 600 Phase I/II: Sinovac Biotech Co NCT04383574 [94] Participants: ≥60 years; Phase I n = 72; Phase II n = 350 Inactivated novel coronavirus vaccine Phase I/II: Beijing Institute of Biological Products/Sinopharm ChiCTR2000032459 [95] Recruitment: Age >/= 3 years; Phase I n = 480; Phase II n = 1168 Inactivated novel coronavirus vaccine Phase I/II: Wuhan Institute of Biological Products/Sinopharm ChiCTR2000031809 [95] Participants: Age >/= 6 years; Phase I n = 288; Phase II n = 1168
Live attenuated	Live virus whose genome(s) is mutated, inducing immune response but not disease	Induces long-term immunity	Expensive to produce
Candidate vaccines:	Nil in clinical trials as yet
Protein subunit	Components of target antigen protein produced in laboratory; some vaccines may use nanoparticle technology	High safety Scalability	High cost Lower immunogenicity and may require adjuvant or repeat doses
Candidate vaccines:	NVX-CoV2373 Phase I: Novovax NCT04368988 [96] Participants: 18–59 years; n = 131
Virus like particle	Non-infectious self- assembling viral structural proteins	Induces strong immune response	Limitations in manufacturing production
Candidate vaccines:	Nil in clinical trial as yet
mRNA	mRNA encoding target antigen (may be complexed with lipid- or polymer-based nanoparticles)	Easier to design Induces strong immune response Rapid manufacture	Requires mRNA to be encapsulated otherwise unstable under physiological conditions
Candidate vaccines:	BNT162 Phase I/II: BioNTech/Pfizer Four candidates, two candidates include a nucleoside modified mRNA, one a uridine containing mRNA and the fourth self- amplifying mRNA. Each combined with a lipid nanoparticle formulation. Germany 2020–001038-36 [97] Participants: 18–55 years; n = 196 USA NCT04368728 [96] Participants: 18–85 years; n = 7600 mRNA-1273 Lipid nano-particle (LNP)-encapsulated mRNA vaccine; Coding antigen: full length S-protein Phase I/II: Moderna/National Institute of Allergy and infectious diseases NCT04283461 [59] Participants: 18–55 years, extended to 99 years n = 155 Interim Phase 1 Data: 15 participants seroconverted by day 15; 8 participants: all developed neutralizing antibodies[77]
DNA	DNA that encodes the target antigen	Easier to design Rapid manufacture	May require a special approach to administer the vaccine (e.g. electroporation device) May requires adjuvant Uncertainty of safety issues
	INO-4800 Phase I: Inovio Pharmaceuticals NCT04336410 [98] Participants : 18–50 years n = 40 Plasmid DNA oral vaccine (bacTRL-IL-Spike-1) Phase 1: Symvivo NCT04334980 [99] Participants: 19 = 55 years; n = 84