Immigrants are putting far more into Medicare than they're taking out — to the tune of about $115.2 billion over seven years — according to one recently released study.

In 2009 alone, immigrants contributed $13.8 billion more to Medicare's Hospital Insurance Trust Fund than they took out, Harvard researchers have found. That's in contrast to a $30.9 billion deficit created by U.S. born citizens.

Analyzing data from both the U.S. Census Bureau and the Bureau of Labor Statistics, the study's authors found that the Medicare surplus has stayed relatively consistent between 2002 and 2009, hovering between $11.1 billion and $17.2 billion.

Reasons for the excess are numerous, the study speculates: Some immigrants retire to their country of origin; elderly immigrants may have poor access to care; other elderly immigrants may not have worked the required 40 quarter-years in the U.S. to be eligible to use the program, etc. Whatever the cause, researchers expect the trend to continue, pointing to census estimates that net immigration will continue to increase for the next 18 years.

Government guidelines that restrict their arrival, meanwhile, may further erode Medicare, the study states. The most recent trust fund annual report from the Medicare Board of Trustees predicts that the fund will be depleted by 2024.

"Policies that reduce immigration would almost certainly weaken Medicare's financial health, while an increasing flow of immigrants might bolster its sustainability," the authors write.

Here are a couple of other tidbits from the piece:

  • In 2009, immigrants accounted for 4.8 percent of hospitalization expenditures, 14.2 percent of home-health expenditures, and 11.8 percent of trust fund expenditures on Medicare Advantage premiums.
  • The average expense for immigrant Medicare enrollees ($3,923) was $1,465 lower than the average expense per U.S. born enrollee ($5,388).
  • Noncitizen immigrants accounted for the majority of the $13.8 billion surplus in 2009, at about $10.1 billion. They typically contribute to the fund through the use of social security numbers tied to invented names, or self-employment taxes.