Tertiary prevention is doing something to keep a problem from getting worse. An example may be: My first day in Jamaica I stayed in the sun only 20 minutes as tertiary prevention of a bad sunburn. Or: The doctor gives you Tetanus shots as when you puncture your skin from a rusty nail as tertiary prevention against infection. Tertiary means a third level or an exacerbated level.

it is a secondary intervention

Upstream deals with primary prevention while downstrem deals with secondary and tertiary prevention.

Tertiary.

Primary refers to the first level or basic stage, secondary refers to the second level or intermediate stage, and tertiary refers to the third level or advanced stage in a process or classification system. This terminology is often used to describe different sectors of the economy, education levels, or healthcare services.