Final answer:
Phase 3 clinical trials for the Moderna vaccine assess its efficacy on a large scale to ensure safety and effectiveness before seeking regulatory approval.
Step-by-step explanation:
Phase 3 clinical trials are distinguished from earlier phases by their large-scale testing, involving up to several thousands of patients. This is crucial to establish the efficacy and safety profile of a compound like the Moderna vaccine. The efficacy is measured by looking at how well the vaccine prevents the disease compared to a placebo and is determined by the number of symptomatic, laboratory-confirmed COVID-19 cases in the vaccinated group versus the control group. Large-scale testing in these trials is significant because it provides a comprehensive database that reflects a diverse population and how the vaccine performs across varied demographics and health conditions. A vaccine like Moderna's is considered ready for regulatory approval after Phase 3 if it demonstrates clear benefits that outweigh any risks and can be considered effective and safe for public use.