A unit dose of a dengue vaccine composition comprising: a tetravalent virus composition including four live attenuated dengue virus serotypes wherein the unit dose is lyophilized and upon reconstitution with 0.5 mL of a pharmaceutically acceptable diluent comprises: (i) a chimeric dengue serotype 2/1 strain, in a concentration of at least 3.3 log10 pfu/0.5 ml, (ii) a dengue serotype 2 strain, in a concentration of at least 2.7 log10 pfu/0.5 ml, (iii) a chimeric dengue serotype 2/3 strain, in a concentration of at least 4.0 log10 pfu/0.5 ml, and (iv) a chimeric dengue serotype 2/4 strain, in a concentration of at least 4.5 log10 pfu/0.5 ml, wherein upon said reconstitution with a pharmaceutically acceptable diluent (i), (ii), (iii), and (iv) provide a total concentration of pfu/0.5 ml and based on said total concentration of pfu/0.5 ml the concentration of (i) in pfu/0.5 ml is at least 1%, the concentration of (ii) in pfu/0.5 ml is less than 10%, the concentration of (iii) in pfu/0.5 ml is at least 10%, and the concentration of (iv) in pfu/0.5 ml is at least 50%, wherein the dengue serotype 2 strain is derived from the wild type virus strain DEN-2 16681 and differs in at least three nucleotides from the wild type as follows: a) 5'-noncoding region -57 (nt-57 C-to-T) b) NS1-53 Gly-to-Asp (nt-2579 G-to-A) c) NS3-250 Glu-to-Val (nt-5270 A-to-T); and wherein the three chimeric dengue strains are derived from the serotype 2 strain by replacing the structural proteins prM and E from serotype 2 strain with the corresponding structural proteins from the other dengue serotypes, resulting in the following chimeric dengue strains: - a DENV-2/1 chimera, - a DENV-2/3 chimera and - a DENV-2/4 chimera. a tetravalent virus composition including four live attenuated dengue virus serotypes wherein the unit dose is lyophilized and upon reconstitution with 0.5 mL of a pharmaceutically acceptable diluent comprises: (i) a chimeric dengue serotype 2/1 strain, in a concentration of at least 3.3 log10 pfu/0.5 ml, (ii) a dengue serotype 2 strain, in a concentration of at least 2.7 log10 pfu/0.5 ml, (iii) a chimeric dengue serotype 2/3 strain, in a concentration of at least 4.0 log10 pfu/0.5 ml, and (iv) a chimeric dengue serotype 2/4 strain, in a concentration of at least 4.5 log10 pfu/0.5 ml, wherein (i) a chimeric dengue serotype 2/1 strain, in a concentration of at least 3.3 log10 pfu/0.5 ml, (ii) a dengue serotype 2 strain, in a concentration of at least 2.7 log10 pfu/0.5 ml, (iii) a chimeric dengue serotype 2/3 strain, in a concentration of at least 4.0 log10 pfu/0.5 ml, and (iv) a chimeric dengue serotype 2/4 strain, in a concentration of at least 4.5 log10 pfu/0.5 ml, wherein upon said reconstitution with a pharmaceutically acceptable diluent (i), (ii), (iii), and (iv) provide a total concentration of pfu/0.5 ml and based on said total concentration of pfu/0.5 ml the concentration of (i) in pfu/0.5 ml is at least 1%, the concentration of (ii) in pfu/0.5 ml is less than 10%, the concentration of (iii) in pfu/0.5 ml is at least 10%, and the concentration of (iv) in pfu/0.5 ml is at least 50%, the concentration of (i) in pfu/0.5 ml is at least 1%, the concentration of (ii) in pfu/0.5 ml is less than 10%, the concentration of (iii) in pfu/0.5 ml is at least 10%, and the concentration of (iv) in pfu/0.5 ml is at least 50%, wherein the dengue serotype 2 strain is derived from the wild type virus strain DEN-2 16681 and differs in at least three nucleotides from the wild type as follows: a) 5'-noncoding region -57 (nt-57 C-to-T) b) NS1-53 Gly-to-Asp (nt-2579 G-to-A) c) NS3-250 Glu-to-Val (nt-5270 A-to-T); and a) 5'-noncoding region -57 (nt-57 C-to-T) b) NS1-53 Gly-to-Asp (nt-2579 G-to-A) c) NS3-250 Glu-to-Val (nt-5270 A-to-T); and wherein the three chimeric dengue strains are derived from the serotype 2 strain by replacing the structural proteins prM and E from serotype 2 strain with the corresponding structural proteins from the other dengue serotypes, resulting in the following chimeric dengue strains: - a DENV-2/1 chimera, - a DENV-2/3 chimera and - a DENV-2/4 chimera. - a DENV-2/1 chimera, - a DENV-2/3 chimera and - a DENV-2/4 chimera.