There’s been great news on the Covid-19 vaccination front the past couple of weeks, with excellent preliminary results for vaccines from Pfizer and Moderna. Both report higher-than-expected effectiveness (Pfizer 90%, Moderna 94.5%).
But what does it mean for a vaccination to be “90% effective”? (I’ll use the Pfizer number because it makes the math easier). Technically, what’s being reported is the “efficacy” of the vaccine, which in clinical trials measures the difference in outcomes between the treatment group (those who got the vaccine) and the control group (those who got a placebo). In the case of the Pfizer study, 90% efficacy means that you are 90% less likely to get Covid-19 if you have the vaccination than if you didn’t. Another way to say “90% less likely” is that you are 10 times less likely. (For every 10 people in the control group who got Covid, only one person in the treatment group did).
The next question is, “if I get this vaccine, what are my chances of getting Covid?”. Well, they are 10x less than they were before you got the vaccine. Which then raises the next question, “where were my chances of getting Covid before?” Which, of course, varies widely from person to person, depending on your behavior and exposure to other people. Getting the vaccine lowers your risk in all cases, but we really don’t know what your original risk was.
I think what people really want to know is, “if I get the vaccine, can I get back to my normal life?” And, like many interesting questions, the answer is “it depends”. In fact, the answer really is, “we don’t know.” Here are the variables:
- X – The risk of getting Covid in your current situation.
- Y – The risk of getting Covid if you return to your normal behavior (unvaccinated)
- Y / 10 – The risk of getting Covid if you return to your normal behavior (vaccinated)
If Y/10 is less than X, then getting the vaccine allows you to return to your normal life, and the risk is lower than the risk you currently face. That sounds pretty good to me. But in reality you don’t really know X and Y, so you can’t make this call.
But this really isn’t about your individual situation. You won’t get getting the vaccine by yourself; the rest of the country will be too, over several months. Experts estimate that once 60-70% of the population is inoculated, community protection (popularly called herd immunity) kicks in and the disease peters out. It isn’t completely eliminated, but it can’t spread and flare-ups die out quickly. Looks like we’ll reach that point next year some time (fingers crossed).
