| In this week's edition of the Covid Q&A, we look at what vaccine efficacy really means. In hopes of making this very confusing time just a little less so, each week Bloomberg Prognosis is picking one question sent in by readers and putting it to an expert in the field. This week's question comes to us from Lois in Atlanta. Lois wonders what goes into determining a vaccine's efficacy rate. Lois asks:
When the news reports that such-and-such vaccine is 85% effective, what does that actually mean? With the Johnson & Johnson shot now rolling out across the U.S., we're guessing that this is on the minds of many readers. We talked about it a little last week. The double-dose Pfizer and Moderna vaccines were shown to have an efficacy rate of about 95% in clinical trials. The single-shot J&J vaccine was shown to have an estimated efficacy of 72%.
But what do those numbers mean anyway?
"Efficacy is a measure of relative reduction in risk," says Natalie Dean, a biostatistician specializing in infectious-disease epidemiology at the University of Florida. Let's say for example that a vaccine has an efficacy rate of 95%, Dean says. "If you could clone yourself and you had one version of you that was vaccinated and then one version of you was unvaccinated then the vaccinated one is 95 percent less likely to get sick," she says.  A vial of Johnson & Johnson's Covid vaccine Photographer: Michael Ciaglo/Getty Images North America OK, so that's what the numbers mean, but how do vaccine makers come up with them?
"We can't clone people, so what we do is we use randomization," says Dean. Clinical trials of Covid-19 vaccines, for example, take large groups of people and randomly give some of them a Covid vaccine and others a placebo. Then they wait to see how many people in each group winds up getting ill.
"So we see, you know, 95% or 70% fewer cases of people getting sick in the vaccine arm than in placebo arm," says Dean.
Voila — there's your efficacy rate!
But once vaccines enter the world outside of clinical trials, it gets a tad more complicated, says Piero Olliaro, a professor of infectious diseases of poverty at the University of Oxford in England. Those rates are specific to the clinical trials in which the vaccine was tested. "It is not an absolute risk; it is a risk relative to the risk one would have without vaccine in the conditions of that trial, in that population, at that time," he says. For instance, if the virus was surging during trials on one vaccine, it might result in a lower efficacy.
"Therefore, one should refrain from making comparisons between the efficacy percentages of these vaccines – that's comparing apples and oranges," he says.
In other words, don't go thinking the J&J vaccine is necessarily a lesser option than those from Pfizer or Moderna just yet. As more people around the world get vaccinated, we'll get a clearer picture of just how well all of the shots work. Thanks to all of you for writing in this week! Next Sunday, we'll be answering the best question we receive again. So if you have any, we want to hear from you. Write to us at CovidQs@bloomberg.net — Kristen V. Brown | 
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