Molina Healthcare Inc. had the greatest rise in short interest relative to shares outstanding among SNL-covered insurance companies between April 28 and May 15.

The percentage of the company's shares held short relative to shares outstanding increased 184 basis points to 15.74% as of May 15. The company also topped the list of most shorted insurance companies during the period.

EHealth Inc. took the top spot on the list of insurance companies with the largest declines in short interest in the first half of May.

The percentage of the company's shares held short relative to shares outstanding fell 116 basis points to 7.25% as of May 15.