If I had to guess I would estimate a prayer to be as effective as a placebo, but to proclaim that as fact seems a bit unscientific.
No affects have been found.
> In 1872, the Victorian scientist Francis Galton made the first statistical analysis of third-party prayer. He hypothesized, partly as satire, that if prayer were effective, members of the British Royal Family would live longer than average, given that thousands prayed for their well-being every Sunday, and he prayed over randomized plots of land to see whether the plants would grow any faster, and found no correlation in either case.
https://en.wikipedia.org/wiki/Studies_on_intercessory_prayer
there's been a number of studies showing lots of effects. Most studies seem to show slight positive effects from prayer, some show none and some show negative effects.
> Meta-studies of the literature in the field have been performed showing evidence only for no effect or a potentially small effect.
I'm an atheist so I take the "no effect" over the "small effect".
I take what I consider to be the simplest explanation in the absence of a credible alternative, that prayer does nothing when the recipient is unaware of it.
I take that view because I've never heard an alternative explaination that didn't appear to break the second law of thermodynamics.
A magical, invisible, omnipresent, omnipotent "all known laws of nature defying"-being that answers prayers, requires significantly more evidence than a small effect in some meta study.
A more properly scientific line of reasoning would be to instead say that you'll assume the small-to-nonexistent effect is nonexistent until somebody produces a model capable of predicting the proposed small effect, or a myriad other arguments along the same general lines.