This article reports on the results of a few different studies, including Pfizer’s preliminary data and real world data from Israeli HMOs.
an HMO found no decrease in the odds of developing covid from 5-12 days after the first shot, but found a 33% decrease 14 days after the first shot.
Another study found a 60% decrease in risk after 14 days.
A small study where they actually drew blood and looked at antibody levels found that after a week, only 1% of people had enough antibodies to matter, but after 14 days, 50% did. (Which sort of demonstrates that it may not be that YOU are x% less likely to be infected, but that the odds are y% that you will be z% less at risk…)
The NEJM published a very nice study based on Israeli experience, with a lot of people and carefully matched controls. It didn’t analyze the data prior to 14 days, because they didn’t expect meaningful results. But they did publish some nice graphs.
Here’s the link to the whole article.
https://www.nejm.org/doi/full/10.1056/NEJMoa2101765
They studied the Pfizer vaccine, and the predominant variant of virus during their study was B.1.1.7 (the “UK” variant.) There was also some “original strain” running around, but not much B.1.351 (“south african”) variant.
Other data suggests the Pfizer vaccine is less effective against the B.1.351 variant.