Health-care stocks slumped after three companies led by Internet giant Amazon.com Inc. said they plan to collaborate on developing ways to cut the cost of employee health care.

Pharmacy-benefit manager Express Scripts Holding Co. fell as much as 11 per cent, the most intraday since April, at the open of U.S. trading Tuesday, while rival CVS Health Corp. dropped as much as 6.4 per cent. Health insurers also fell, with Anthem Inc. losing as much as 6.5 per cent and Aetna, which is being bought by CVS, sliding as much as 4.3 per cent.

Amazon, Berkshire Hathaway Inc. and JPMorgan Chase & Co. are planning to set up a new independent company  “that is free from profit-making incentives and constraints,” and will focus on developing technology to make health care simpler and cheaper to access, according to a short statement on Tuesday.

While this new company would be for the companies’ U.S. staff only, this is the first concrete move toward greater involvement in health care by the Internet giant. The mere possibility of the retailer entering the business had already started to cause far-reaching reverberations for a range of companies in the sector, roiling the shares of drugstore chains, drug distributors and pharmacy-benefit managers.