As scientists raced to develop Covid-19 vaccines, public health specialists were hoping that more than one group would succeed. Having multiple companies producing vaccines would make it easier to inoculate a lot of people fast. Be careful what you wish for. A range of vaccines with different efficacy results now has given rise to worries that some people may refuse the shot on offer in hopes of getting a “better” one later. In reality, comparing efficacy numbers isn’t necessarily the best way to measure a vaccine’s value. And as suppliers struggle to meet global demand, experts say the best vaccine for you is probably whichever one you can get now.
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.