The Massachusetts Nurses Association sued Brigham and Women's Hospital this week, seeking to block a policy not yet in effect that would require nurses to get flu shots if they want to keep working there.

The action in Suffolk Superior Court comes as state public health officials are leaning on hospitals to improve the influenza vaccination rate among health care workers, which varied in hospitals from 62 percent to 99 percent during the most recent flu season.

Health care workers can transmit the flu to patients, whose illnesses make them especially vulnerable to complications.