Health insurance companies including UnitedHealthcare, CVS Health’s Aetna and Blue Cross and Blue Shield plans are seeing lower costs. But there’s trouble ahead.

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CVS Health reported first quarter net income of more than $2.9 billion as costs slowed for subscribers of the Aetna health plans the company sells.

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Oscar Health swung to a $679 million first quarter profit – the highest in company history – as its health plan membership jumped more than 50% and medical costs eased.