What does "mean" refer to here? As a verb? As an adjective? As a noun?

Mean is a verb.
In this case it can be read as... When I say that I really mean this.
When Africa has a drought it means starvation.
When the US fears something, they will work hard to find a cure to the problem. Because SARS could be used as a bio weapon, the US has continued to study it, therefore much of the work on MERS has already been done. US paranoia means better vaccines.

Thanks.
If being a verb, it should be "means" (singular form) in the context.

US fears mean research has continued.
When in doubt Ori, try paring down the content. That usually helps clarify things.

Yeah, I've guessed that at the begining.
But "US fears" appears more to be Subject + verb.

The progressive isn't much used to describe general states, things that always are, Ori.

If it's a verb,

Good stuff, Ceili!!

No; it's a noun phrase - US fears = fears of the US [government]

'Fears' can be a noun. One may have fears [apprehensions, worries] about something, or fears that something may be the case.

I have fears that my wife is lying. He has fears that his son is associating with criminals. John's fears that his daughter has become a prostitute to pay for her drug habit are, mercifully, unfounded.

[the]US [government's] fears that SARS could be used as a bioweapon mean [are the reason that] research on the virus has continued

Thank you all.