Aetna, the health insurer, announced this week that it is quitting 11 of the 15 states where it has been participating in Obamacare exchanges.

This means customers will have fewer options in 536 of the 778 counties where the insurer had been selling its services. In at least one county in Arizona, there may be no insurance plans available at all. Reduced competition will add upward pressure to premiums, which have risen continuously since Obamacare began.