The forecast illustrates the administration’s confidence in enrolling more people and keeping those who are covered from dropping out in a challenging year. But the Obamacare exchanges are still not attracting enough young, healthy and higher-income individuals who could help spread the health-care costs of the sickest over a bigger group.

“What we are still missing is the young and invincible,” said Deep Banerjee, an analyst at S&P Global Ratings. “The exchange market has to grow a lot more to become stable.”

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