Why more vaccinated people are getting Covid

Or: why denominators matter

Slowly but surely, we’re getting vaccinated. (In high-income countries, at least, but this isn’t a post about vaccine nationalism).

Covid case counts in the US have steadily decreased since mid-April, reaching numbers we haven’t seen since March of last year. It’s pretty phenomenal.

New Covid cases per day in the US (Source: New York Times)

So, this is a good time to talk about why we’re about to see reports of increasing infections among vaccinated people.

And it’s true! By the time 90% of the population is vaccinated, nearly a third of Covid infections will be in people who’ve been vaccinated!

In fact, here’s what it’s going to look like:

Oh my god, the vaccines are going to kill us

The heck? What kind of vaccine madness is this? Does it mean the vaccines don’t work?

(Spoiler: No, it doesn’t.)

Making ice cream

Let’s take a detour.

Pretend, for a second, that you’re an ice cream maker, and you pride yourself on two things: the sheer number of flavors you serve and the absolute decadence of your chocolate ice creams.

In fact, your chocolate ice creams are so mind-blowing because you’ve dedicated years perfecting the way you turn a humble gallon of chocolate milk into ice cream. It’s pretty amazing, if you do say so yourself.

Your recipe’s got one downside, though: a gallon of chocolate milk simply doesn’t make as much ice cream as a gallon of regular milk.

(This is important.)

On any given day, your store sells 50 different flavors (none of that Baskin Robbins 31-flavor madness!). Because of your patented chocolate process, only 2 of these flavors are chocolate, but those two flavors slap.