Influenza is terrible at infecting neurons. You need to use something like rabies instead. Unfortunately, influenza and rabies, while both RNA viruses, have rather small genomes (12-14 kb), which makes your task daunting.
But, fine, you have aliens with interstellar transport capability. They can probably do daunting molecular biology: take a highly infectious airborne virus like influenza or rhinovirus as a backbone, re-engineer the viral capsid proteins to admit a larger payload, add genes that will produce a rabies-like virus (or at least the protein that binds NCAM to give it entry to the nervous system), and then add a reverse transcriptase and such to be able to modify the DNA (good if you want long-lasting effects), using a serotonin-neuron-specific promoter (or a glial cell promoter that drives production of antibodies to serotonin receptors with export sequences on them).
And then go through yet another layer of optimization to get it to work passably well in birds or something for the cross-species infection.
But good grief that's a lot of work. It's very inefficient. 335
Why infect birds when you can just visit a bunch of cities leaving around virus-saturated items where children can pick them up? Why, when you have such powerful control over a person's molecular biology, would you just mess with their serotonin and adrenaline systems? Better to have a virus that has a quick initial infection and then goes latent (Herpesviruses are good at that) with special triggers to activate it in times of high stress. Infect a bunch of people, then cause a scare, and BOOM! You cripple a huge portion of the population. No need to be subtle, is there? Just cause loads of neurological damage.
The biggest plot hole is that people, given that they are competitive with the aliens, will doubtless have extremely advanced molecular biology also. So it will take a matter of days to have sequenced and inferred the function of the entire elaborate construct, and another couple weeks to have ways to generate high affinity binding proteins that provide essentially complete immunity. Any long-lasting futzing with neuromodulators is doomed to failure; you need something that takes out vast numbers of people really fast, or humans will reactively solve the problem before it causes any appreciable harm. 335