Cell substrates managed in controlled culture environments have become, over the past few decades, the subject of intensive technological developments for the biomanufacturing of viral vaccines. The driving force of such work is an expanding demand for safety, high production capacities, cost savings, and flexibility. Egg, tissue, and primary-cell–based manufacturing methods of limited capacity are now considered to be outdated technologies. In the influenza vaccine field, for example, time delays in vaccine delivery (especially during pandemic responses) have increased concerns…