I'm obviously missing something here. How can it be 82% effective after the first dose, but then only 52% effective between the first and second doses? And if it really is 82% after the first, why wouldn't Pfizer be playing that up instead of telling the NYT that the first dose is only 52% effective?

I puzzled over that all yesterday, then finally broke down and read the paper (actually, just this part of it) Here's my understanding of the numbers:

**From Day 0 (First dose given) to Day 105 **Vaccine had

**82%** efficacy in decreasing infections: 50 cases in vaccinated group, 275 in placebo

But that calculation includes

*all* the time after the first dose was injected. In the first week after injection, it looks like efficacy is close to 0 - there are about 20 cases in each group, making it look like the vaccine has no effect. But we know that this is because it takes a few days to synthesize enough antibodies to prevent infection, and it's only around day 12 that we start to see a change. In the third week (days 14-21), 18 new cases occurred in placebo group, but only 2 in vaccinated group. So they did another calculation to summarize this early 3-week period:

**From Day 0 (first dose given) to Day 21 (before second dose)**Vaccine had

**52%** efficacy: 39 cases in vaccinated group, 82 in placebo

Good to show that %, because it emphasizes that the vaccine starts to have an effect quickly, but it looks misleadingly low because it includes that first week when the immune response was developing.

After the second injection, they did two calculations:

From Day 21 (second dose given) to Day 28 (to allow one week for immune response to develop)

Vaccine had 90% efficacy: 2 cases in vaccinated group, 21 in placebo

**From Day 28 (one week after second dose) to Day 105**Vaccine had

**95%** efficacy: 9 cases in vaccinated group, 172 in placebo

They can't really say what the efficacy would be after just one dose, because they only looked for three weeks, and after that everyone got a second dose. It's possible that immunity would wear off more quickly after a single dose. So why didn't they have a third group, that had just one dose, and they could follow that group for longer? It's been shown for some other vaccines that two doses are needed for maximal effectiveness, and it would have taken that much more time to recruit an additional 22,000 people for that third group. Perhaps this was one of the corners they cut to work at warp speed?