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Investors hammered insurers and other health care companies Wednesday amid fears over the potential impact of reforms being discussed in Congress.

The S&P 500 health care sector ended the day down 2.9 percent, its biggest one-day percentage drop since Dec. 14, as investors remain spooked by policy uncertainty.

Among companies hit Wednesday were Clayton-based Centene Corp., which saw shares drop 6.1 percent, and insurer Cigna Corp., parent of Express Scripts, which fell 3.7 percent.

Shares of UnitedHealth, the nation’s biggest health insurer, slid 1.9 percent; Anthem Inc., 3.6 percent; and Humana, 0.9 percent. CVS Health, which owns Aetna, fell 2.5 percent. Hospital chain HCA Healthcare Inc. fell 2.1 percent, and Community Health Systems Inc. lost 1.2 percent.

The slide began in earnest on Tuesday after UnitedHealth — treated by investors as a bellwether for the insurance sector — waded into the debate over “Medicare for All,” which is being pushed by several candidates contending for the Democratic presidential nomination.

For months, health insurers have kept largely out of the fray over the proposal to expand the government program that covers about 60 million Americans, most of them elderly. That changed Tuesday during UnitedHealth’s earnings call with analysts.

The proposal would be a “wholesale disruption of American health care” and would “surely have a severe impact on the economy and jobs — all without fundamentally increasing access to care,” CEO Dave Wichmann said on the call.

As a source of coverage, UnitedHealth is almost as large as Medicare itself. It provides health-insurance services to 49.7 million people, and last year recorded revenue of $226.2 billion. While UnitedHealth had kept a low profile, there are signs it was already becoming a target before Wichmann’s remarks Tuesday.

Sen. Bernie Sanders, I-Vt., a presidential candidate and a leading advocate for a government-run health-care system, called out UnitedHealth last week, saying in an April 12 tweet that “your greed is going to end.”

Sanders specifically referenced Steve Nelson, head of the company’s insurance division. Sanders cited a Washington Post story about remarks Nelson made at a company meeting.

According to the Post, Nelson answered a question from an employee about the insurer’s role in the political discussion, saying, “the last thing you want to do is become the poster child during the presidential campaign.”

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