What research is this? As far as I've been able to tell, CDC hasn't conducted a single randomized survey on any large group, but has only collected convenience samples.

Also keep in mind that due to compounding (keeping these people from infecting others on the plane and at home), the effect extends beyond the people prevented from traveling. You can also speculate that detectable infections are more transmissible during the journey.

Is that 5-9% of total passengers or 5-9% of those that have covid at the time?

Anecdote from a German that visited Spain to see the family: Spains government requires <72h old negative covid test before entering the country, with 6000€ in fines if no test can be presented. Also, a form about symptoms etc had to be filled out.

After arriving at Valencia, Spain, there was a big line where we had to show the form's result QR code only. Not a single passenger was asked for the covid test.

I've heard that the vaccine doesn't make you less likely to transmit COVID - just less acute symptoms. So getting the vaccine doesn't really help other people - just you. From this perspective, this doesn't make much sense (unless the objective is to make $ for the corporations).

You heard wrong. Nobody actually knows if you can transmit after getting vaccinated or not. Thus far anyone who claims otherwise is a liar.

There is some evidence that some of the vaccines prevent transmissions, but we need more study before we can make a statistical determination.

We know why other vaccines have failed to stop transmission. The reasons those vaccines didn't stop transmission do not apply to the covid ones. However there is a lot we don't know about biology so until we have better evidence no expert will state that the vaccine stops transmission even though many believe it will.

It was just published today that in Israel, where around 20% of the population has already been vaccinated with the first dose, they're beginning to see clear evidence that vaccinated people do not transmit it others.