The Brits created a natural experiment this spring, with a big push to get primer doses to as many people as possible, and then getting the second booster dose as availability permitted.
I remember reading that some knowledgeable people in the UK were speculating that this might be creating a stronger immunity than the 3 week spacing that we used here in the US. At the time, that wasn’t even phrased as a testable hypothesis, but it shouldn’t be hard to at least look at that retrospectively, and there are some good reasons based on previous vaccines that long-term efficacy would benefit from a longer interval between the two doses.
I remember reading that some knowledgeable people in the UK were speculating that this might be creating a stronger immunity than the 3 week spacing that we used here in the US. At the time, that wasn’t even phrased as a testable hypothesis, but it shouldn’t be hard to at least look at that retrospectively, and there are some good reasons based on previous vaccines that long-term efficacy would benefit from a longer interval between the two doses.
Another UK 'natural experiment' just starting - booster shots being offered to all over-70s 6 months after second vaccination. All will be Pfizer, even if initial vaccine was from a different source. I'm double-vaccinated with Astra Zeneca (12-week gap between shots), will get Pfizer boost in a few weeks.