Aetna's announcement that it will spend about $35 billion to purchase its smaller rival Humana was greeted by concern from many quarters.

"The largest unknown is antitrust approval,” Liz Hoffman and Anna Wilde Mathews wrote in the Wall Street Journal. “Aetna and Humana are the third- and fourth-biggest U.S. health insurers by revenue, and together would have about a million more Medicare subscribers than their next-closest competitor."

Aetna had already prepared for possible divestitures to address overlaps with Humana's business, Aetna CEO Mark Bertolini said, according to Reuters, adding that he is confident any antitrust review would allow the deal to close in the second half of 2016.

Consumer concerns have also arisen since the announcement.

"The question most people care about outside Wall Street – what effect these mergers will have on what we pay for health care is a source of some disagreement among specialists," Carolyn Johnson reported in the Washington Post. "One of the most surprising – and weird – things about the insurer merger mania is that ultimately some analysts think it might have benefits for consumers. But others say there is some evidence that a marketplace with fewer insurers will be bad for consumers."

Despite Bertolini's assurances that such worries would not stop the deal, however, Humana shares closed Monday 15.4 percent below the value of Aetna’s cash-and-stock offer and continued to slide Tuesday. Aetna's stock also fell Monday and Tuesday.