NEW YORK--(BUSINESS WIRE)--A combination of any of the five largest US health insurers could accelerate further merger and acquisition activity in the managed care sector, says Fitch Ratings. Just one mega M&A deal could lead to similar responses by competing firms seeking to shore up competitive disadvantages in scale and product lines. Fitch sees the M&A potential in the health insurance sector as a direct response to anticipated market conditions in a post-Affordable Care Act (ACA) w