We are so fortunate to have vaccines that are 95% effective against Covid-19. But was does 95% effectiveness mean? 1/

“95% effective” has been misinterpreted widely, with implications for our risk perception. Here are some misinterpretations: 2/

Misinterpretation: “5% of vaccinated people can still develop Covid-19.” Not correct! 100% of vaccinated people are still at risk of getting it (but they are at greatly reduced risk of getting it). 3/

Misinterpretation: “If you were vaccinated, your chance of getting Covid-19 after an exposure is 5%.” Not right. Depends on the type and degree of exposure. Hopefully far less than 5%. 4/

Misinterpretation: “Among people who get Covid-19, 5% have been vaccinated.” Nope! Depends on vaccination rates. Even in a trial with 1:1 randomization, 95% efficacy almost means that, but not quite! See this explanation by my son at @LeffellSchool middle-school: 5/

So what DOES 95% efficacy mean?

Vaccine efficacy is 1 – relative risk. So it means that the proportion of people in the vaccine group who got Covid divided by the proportion of people in the placebo group who got Covid was 0.05.

How about a definition that’s less clunky? 6/

People who were vaccinated had a risk of getting Covid that was 0.05 times the risk among the non-vaccinated group.

THAT is correct (and it’s also amazing!) 7/

Is it important to get this definition right? Or is this just being picky? Is this a case of “Actually, that’s not champagne, just sparkling white wine”? 8/

Yes, it is important! The correct definition (see #7 above) reflects a key fact: even when you’re vaccinated, your risk of developing Covid is still a function of the overall risk of Covid out there.

That, in turn, is a function of virus prevalence and your exposures. 9/

A vaccinated person is far less likely than an unvaccinated person to develop Covid in any given scenario, but the risk of Covid still depends on the scenario.

So even though I’m vaccinated, I’m not heading to a choir practice in a church basement any time soon. 10/

That’s why herd immunity is key—get enough people jabbed and the virus will not have enough places to go.

With background rates falling, your chance of getting Covid if you are vaccinated will plummet to close enough to zero that we can be done with masks and distancing. 11/

So let’s get this right. People who get vaccinated have a risk of getting Covid that is 0.05 times the risk among the non-vaccinated group.

Now to the hard work of getting us vaccinated. end/