There has been efficacy for people with dyslexia. Fonts like comic sans are closer to their own writing and therefore are easier to read.

You can also look at the Geronimo Stilton book series, a lot of words appear in different colors / fonts to emphasise words. These books are often easier for children and those with dyslexia to read.

Note: I still feel like calling it a typeface that makes reading easier is inappropriate. No study has specifically been conducted on this typeface, and drawing conclusions from (limited, and arguably unrelated) studies and and anecdotes is dubious at best.

No - many of the claimed benefits of these various speciality fonts do not actually hold up in real research. For example: https://pmc.ncbi.nlm.nih.gov/articles/PMC5934461/

It was claimed that OpenDyslexic could mitigate some of the common reading errors caused by dyslexia.

https://en.m.wikipedia.org/wiki/OpenDyslexic

That heavily depends on your definition of "positive impact". In design/typesetting theory there are different "kinds of reading" and some fonts have positive effects, as in "works well with that kind of reading", while others are not very well suited for a specific task.

For example letters with very distinct shapes and different heights between lower and uppercase letters, like often found in serif fonts, are generally said to be easier to process for your eyes and brain.

Your brain learns to "read without reading" by scanning for known shapes and groups of shapes and just recognizing letters and words by that. You start to skip words, letters, whatever, once your brain has internalized that font.

That effect helps with reading faster and with less "stress" which is ideal for longer texts like in a book. Combine that with a good mixture of line length, font size and line height and you can create long texts that can be read very well.

Now take the same font, set it really tiny because you're working on an Encyclopaedia and don't want it to have 300 pages more and those font features that helped you before, actually make it more difficult to read.

Fine shapes might break away in the printing process or run up and your text will be harder to read. A sans-serif font might be better suited here. Straight crisp lines, that can be reproduced very well might actually make a better job here.

So... Fonts can have a positive impact on reading, depending on your definition of impact. ;-)

There's certainly a large amount of anecdotal evidence that a decent percentage of dyslexic people benefit from using Comic Sans. I don't know if there has ever been a formal study though.

There's also a view that all dyslexia doesn't have a single cause. If that is true, then there may be different things that are helpful depending on the exact cause.

I remember reading somewhere that reading a text with an unfamiliar font face you spend more time reading it, so you're using more cognitive load and are more likely to understand the text. Which might suggest it is just the novelty impacting the reading and not the font face itself.