I understand what Dr. Frieden is trying to say here but this makes it sound like vaccines will drive the emergence of new variants and compel the virus to mutate in new and scary ways. That’s just not true. I’ll try to clarify. https://twitter.com/drtomfrieden/status/1361362197780303872

Immune status of a population can be an evolutionary selection pressure, but that assumes there’s enough replication going on for mutations to be under positive selection.

This means that we might see more/better immune escape variants emerge if these scenarios occur:
1. Widespread community transmission with slow vaccine uptake
AND/OR
2. Vaccines don’t provide any protection against infection and transmission continues unchecked after vaccination.

#1 is our current scenario. We need to get transmission under control as we ramp up immunization. #2 is an unlikely scenario, as several lines of evidence suggest vaccines protect at least partially against infection and thus transmission.

Mutation is a relatively constant process. It happens randomly whenever the virus replicates. Selection pressures don’t increase mutation or make the virus mutate differently.

What is “positive selection”? That means that a randomly acquired mutation changes the virus in a way that gives it an advantage based on the prevailing circumstances. In this case, evading immunity would be an advantage.

But acquiring mutations to be selected is a numbers game, because it happens randomly. It’s like buying lottery tickets. Buy one? Chances of winning are very low. Buy millions? Your chances go up depending on how many millions or billions more.

Because transmission is so high, it’s the virus is buying unlimited Powerball tickets. Thus growing population immunity exerting selective pressure will result in some jackpots for the virus.

“Hey I’m sick of stupid SARS-CoV-2 winning evolutionary Powerball all the time.” Me too. So let’s stop selling it tickets.

By doubling down on efforts to reduce transmission while simultaneously ramping up vaccination, we will cut it off from its supply of chances to win.

I think that’s what Dr. Frieden was getting at based on his next tweet but I wanted to make it clear that immunization is a solution to the pandemic and won’t automatically select for vaccine-resistant variants, especially if we do everything we can to knock down transmission now