Edit:

Please be aware that following is a speculation and sourced on unofficial Galnet News Digest.

Continuation of https://www.reddit.com/r/EliteDangerous/comments/zadp0c/orthrus_key_to_contain_invasion/.

We have currently 8 systems under Thargoid Alert and 5 days to do something about it. These systems are all around Maelstrom 3:

Hyades Sector BV-O b6-3
29 e Orionis
Ildano
Awara
Trianguli Sector CA-A c15
Ebisu
Hyades Sector FB-N b7-2
Hyades Sector FB-N b7-3

That means that after 5 days, systems that are Human Controlled will become under Incursion, but unpopulated ones become Thargoid Controlled (skipping one stage – without Incursion).

One (only?) way to stop spreading the invasion is to prevent Orthruses from collecting the probes. How to do it is mentioned in the recent Galnet News and linked post.

How?

Go to Threat 4 Non Human Signal Sources with Salvage icon in these systems and prevent Orthruses from collecting probes – destroy/steal probe before Thargoid takes it; or kill the messenger.

Let's get them – we still have 5 days.

o7 Commanders!

Thargoid Alert systems

Source: https://www.reddit.com/r/EliteDangerous/comments/zapmud/prevent_invasion_from_spreading_on_these_8_systems/