The two advisory firms also recommend shareholders approve Centene's $6.3 billion proposed acquisition of Health Net, saying that together the two Medicaid plan providers will be able to better compete in an environment of health care consolidation.
Centene's shares have fallen nearly 17 percent since the deal was announced in early July—a steeper decline than the overall health insurance sector, which has fallen about 5 percent over the same period.
Still, ISS analysts say they do not believe Centene's decline represents negative shareholder support for the merger, or an indication of the Medicaid insurer's financial performance.
"It is possible that (Centene's) relatively greater decline (versus the segment) since announcement reflects the fact that pre-announcement prices were inflated by takeover speculation, which has been leaking out of the market price since then," ISS wrote.
If approved by shareholders, Centene and Health Net expect that the deal will close in early 2016. The firms received early antitrust clearance in August from the justice department under the Hart-Scott-Rodino act.
In addition, the merger still faces approval from state insurance regulators.
For Aetna and Humana the regulatory hurdles will be more complex, because of the size of the deal. Last month, Department of Justice officials made a second request for more information from the firms as part of an antitrust review, which observers had expected.
Analysts say regulators will likely look at the Aetna-Humana deal in conjunction with Anthem's $54 billion proposed merger with Cigna. Federal and state regulators are expected to consider carefully how consolidation among the four major health insurers impacts not just consumers but providers across health care markets. Meanwhile, both mergers may require divestitures in order win approval.
Anthem and Cigna have filed preliminary proxy information and are urging their shareholders to support the deal, but the firms have not yet scheduled a vote. Neither firm has provided any regulatory updates yet on the status of their Justice Department submission under the Hart-Scott-Rodino antitrust act.
CORRECTION: A headline in an earlier version misspelled Centene.