On a basic level, vaccine efficacy of 50%, for example, roughly means that an immunized person has a 50% reduced risk of becoming ill compared with an otherwise similar non-immunized person. However, the measurement can be applied to different questions about a vaccine’s effect. For example, almost all Covid-19 vaccines appear to successfully — 100% — avert hospitalization and death. But since relatively few people infected with SARS-CoV-2 become critically ill, it’s hard to measure such a rare outcome reliably in clinical trials involving only tens of thousands of participants — a comparatively small pool. Instead, the primary aim of most late-stage trials has been to measure broader efficacy against lab-confirmed Covid cases with any symptoms, including mild ones.
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