SAO PAULO, Feb 18 (Reuters) - Shareholders of Brazilian

healthcare operator Hapvida approved a change to the

company's bylaws that will force any shareholder or group of

shareholders with a stake larger than 20% to launch a tender

offer for all remaining shares.

The mechanism, known as a "poison pill," was approved by a

majority of Hapvida shareholders in a general meeting on

Tuesday, according to the meeting minutes.