Even just 30 days in space can significantly reduce our immune system’s ability to fight infection, suggests a new analysis of mice that spent a month aboard an orbiting spacecraft.

The research, which was published December 6 in the journal Federation of American Societies for Experimental Biology, is a recent analysis of data from the Bion-M1 mission, which was a collaborative project carried out by NASA and the Russian Institute of Biomedical Problems in 2013.

As part of the study, an international team of researchers compared three distinct groups of mice. The first two groups spent 30 days orbiting the Earth at an altitude of 360 miles (575 kilometers), while the third group, which served as the control, remained on the planet under similar feeding and housing conditions. Of the two space-bound groups of mice, one was examined immediately following its return to Earth, whereas the other was evaluated a week later.

According to the study, which analyzed proteins found within the rodent’s femur bones, the researchers revealed that living in a microgravity environment for even 30 days is enough to dramatically impair the mice’s ability to produce vital immune system cells, and this effect persisted even after a week safely back on Earth.