JULIE STEENHUYSEN and KATE KELLAND VACCINE makers Moderna Inc and Pfizer Inc, with their German partner BioNTech, have been vocal in their view that the world will soon need COVID-19 booster shots to maintain high levels of immunity. What is less clear, however, is who should make that decision - and based on what evidence. THE MODEL FOR FLU VACCINES Public health officials have a well-coordinated mechanism, first established in 1952, for determining when seasonal flu vaccines should be updated to cope with fast-mutating strains of influenza. The World Health Organization's Global Influenza Surveillance and Response System gathers data collected…