Epitope-based Multi-variant SARS-Cov-2 Vaccine Design: Shared Epitopes Among the Natural SARS-Cov-2 Spike Glycoprotein and 5 of its Variants (D614G, α, β, γ, δ) with High in Silico Binding Affinity to Human Leukocyte Antigen (HLA) Class II Molecules