Vaccines against viruses can reduce antibiotic prescribing by reducing the incidence of viral infections that are inappropriately treated with antibiotics, as well as by reducing the incidence of secondary bacterial infections caused by viral infections. We will estimate the magnitude of this effect for influenza vaccination and the potential magnitude for Respiratory Syncitial Virus (RSV) vaccination and enhanced influenza vaccination. First, we willestimate the antimicrobial prescribing attributable to RSV and influenza in the Kaiser Permanente population in Northern California, USA, in total and by drug class and age group. Next, we will estimate the antimicrobial prescribing currently averted by influenza vaccination by comparing antimicrobial prescribing in (1) persons who have not received influenza vaccine vs. (2) persons who have, controlling for age and location within northern California, in aggregate and stratified by age and antimicrobial class. Using these results, we will estimate the number and proportion of antimicrobial prescriptions that may potentially be averted by improved influenza vaccines and (separately) by RSV vaccines that are now investigational, assuming various possible levels of coverage and effectiveness. Notable features of our analysis will be extensive measures to avoid confounding and an examination of waning of influenza prescribing effects with time since vaccination.