In a call with reporters on Monday, Harvard University immunologist and infectious disease expert Dr. Barry Bloom further clarified the discrepancy. "There is clearly an upgrading of the formulation of the Moderna vaccine that no longer requires ultra-cold temperatures to maintain stability, which is the characteristic of the Pfizer-BioNTech vaccine," said Bloom, adding later, "There's a practical advantage at the moment for the distribution of the Moderna vaccine."