The 2011-2012 flu season was more mild and peaked later than in previous years, the Centers for Disease Control & Prevention reports.

Flu activity generally starts around the year’s end, but this season it remained low through February and only rose above the baseline for a week in March. That’s the shortest and lowest peak since the agency started keeping track in 1997, the CDC said.

While it’s impossible to pinpoint one reason, researchers said mitigating factors were the mild winter, effectiveness of the flu vaccine and the majority of the influenza viruses circulating were similar to those of the past two seasons.

A mandatory flu vaccination proposal for healthcare employees, including those in long-term care, remains contentious. However, the Joint Commission debuted a report Wednesday that said that although it will not require mandatory vaccinations, its accredited healthcare organizations must “set incremental goals for meeting a 90 percent coverage rate by 2020. Organizations also will be required to measure and improve vaccination rates for staff.” The Joint Commission primarily accredits hospitals but also reviews skilled nursing and other facilities.