WHYY – PBS: “Several listeners also had questions about vaccine effectiveness and clinical trial reports. What do companies mean when they say their vaccines are 95% effective? First of all: Effectiveness and efficacy aren’t exactly the same thing. A vaccine’s efficacy tells us how well it worked to prevent disease in people in clinical trials, while its effectiveness tells us how well it works when given to people in real-world conditions. For example, the Pfizer vaccine has 95% efficacy. That doesn’t mean that only 95 out of every 100 people vaccinated are protected and an unlucky five are not. Instead, efficacy measures relative risk. It means that 95% of the people who got COVID-19 during the observed trial were those who hadn’t received the vaccine. That indicates that the people who were vaccinated had a 95% lower risk of getting COVID than people in the control group who didn’t get the immunization. Perhaps more importantly, the small group who were vaccinated and got COVID mostly had mild cases; they weren’t hospitalized, and none of them died. So the vaccine, which aims to prevent severe sickness and death, is ultimately successful. The same is true for vaccines that haven’t hit the market in the United States yet, which have lower efficacy rates: 66% for Johnson & Johnson. While these efficacy rates look much lower on paper than Pfizer and Moderna, no one in the vaccine groups for these trials became severely ill or died; all three vaccines are highly effective. Finally, neither measure — efficacy or effectiveness — is an accurate or useful way to tell how likely you, personally, are to get sick. That’s because your personal risk is based on a variety of factors: the amount of exposure you have to the virus, the strength of your immune system, your age or underlying health conditions, and more. That’s also why most experts are saying you shouldn’t focus too much on efficacy percentages; instead you should just try to get whatever vaccine is authorized and available…” [h/t Pete Weiss]