The present invention relates to an auto injector (1) comprising a prefilled liquid medicament container, a hollow auto injector body (2) housing the medicament container. The auto injector body (2) has an elongated part, a first end side (21) and a second end side (22) and at least one through hole (3) arranged between the end sides (21, 22). The through holes (3) allow for flow of gas through the auto injector body (2) when the auto injector body (2) houses the medicament container. The invention further relates to a method comprising inserting the auto injector (1) into the package via an opening in a non-inert environment, and, in a non-inert environment and under atmospheric pressure: replacing the air in the package with inert atmosphere by one or several cycles of removing air and inserting inert gas through the opening, wherein the air and inert gas in part flows through the at least one through hole (3) of the auto injector (1). The method further comprises sealing the opening under vacuum.