Help: Easy language?

Question

How can I reword this:

He was a pioneer in vaccine therapy.

Answer 1

I rewrote your sentence using simple words:

1. "He was first in vaccine therapy"

2. "He was the first in healing with vaccines"

3. "He was among the first in healing with vaccines"

Answer 2

In vaccine therapy, he was a pioneer.  

Answer 3

Well, I guess you could say this:

He was a leader in the development of vaccine therapy.