An announcement could come as soon as Monday, said the person, who asked not to be identified because the talks are private. CVS is likely to agree to pay at least $200 a share for Aetna, with more than 30 percent paid for with cash, according to the person. Talks could still be delayed or fall apart.

Timing of the potential deal was first reported by The Wall Street Journal on Thursday. Aetna and CVS declined to comment.

A deal between Aetna and CVS could upend the health insurance, pharmacy and drug benefit sectors. Many of the biggest players in those industries are also looking over their shoulders at Amazon.com Inc., which has been reported to be exploring various health-care businesses.

It would also come less than a year after two major health insurance deals – between Anthem Inc. and Cigna Corp., and Aetna and Humana Inc., fell apart amid antitrust challenges.

Unproven Rationale

Aetna’s shares are still trading well below the reported deal price, a sign that investors believe the deal may not close. Investors may be skittish about the amount of CVS stock included in the deal, since cost savings or revenue increases from the deal rely on melding two large, complex companies, said Ana Gupte, an analyst at Leerink Partners.

“It’s a vertical deal, it’s unproven, it’s defensive, it’s got more limited cost synergies, and the revenue synergies are unproven at best,” she said. “There may be some medical-cost savings or drug-cost savings, but they’d have to be likely passed on to customers” to keep them from defecting to rival companies.

Aetna was up 1 percent to $181.40 at 12:30 p.m. in New York. CVS was up 3.1 percent to $75.66.

A deal between the two companies could heighten speculation about the next moves of other players, including independent pharmacy chains and standalone pharmacy benefit management companies.

On Thursday, Express Scripts Holding Co.’s chief executive said he’s open to a deal with a health insurer or partnering with Amazon.com Inc. CEO Tim Wentworth’s pharmacy benefits company has been battered by departing clients and the vague specter of Amazon’s entry into the drug business.

If a major insurer was interested in a deal, “I would be open to it,” Wentworth said on the sidelines of a conference in New York sponsored by Forbes. “We don’t need to sell to be very successful in the future, but we are always open to others who may all of sudden conclude they want what we have.”

Wentworth said the company isn’t actively seeking a large deal or sale. Express Scripts shares were up 1.9 percent to $64.08.