There's also the unknown of 'how does a foreign pathogen affect a human'? Will alien virus affect us? I'm guessing not often. Bacteria maybe, but viruses for different body chemistries maybe not so much. So most of what we'd be exposed to would be things we took with us or successive evolutions of same. (Bioweapons not accounted for)
I can see high-tech medical being available at good starports to quickly scan for common threats and the higher the tech, the more it might recognize variations on a known threat. That might help catch a lot of it. At lower tech places, you have the Spanish wiping out Mezoamericans.... to some degree.
It's an interesting thought experiment. COVID is not necessarily any more harmful than any other coronavirus, most of which cause only minor symptoms. What makes it such a threat is its novelty, and the consequent inability of our immune system to hit it hard, early. (In fact, one of the suspicions about why it is so lethal is that the immune response is what is killing most people, and the immune response is only fully kicking in once the virus has multiplied for a week or so ... versus kicking in the first few hours as with a non-novel coronavirus.)
So, given MagicTech, would we assume that the screening at the starport is trying to identify any variation of a pathogen that is not recognised in the world's medical databases? I think the difficulty with that is that pretty much _every_ inbound traveller is going to have a "novel pathogen" of one kind or another, and most of those will be essentially harmless. (Or, rather, the costs of attempting to routinely exclude those pathogens will be greater than the costs of putting up with a pandemic every decade or so. Imagine New Zealand trying to use border protection to prevent seasonal influenza from being imported each year!)
Given the efficacy of the various responses to COVID, I think a HiTec society would implement widespread social isolation with minimal disruption as a first-line response to any AI-identified statistical abnormality in the types of disease presenting for medical attention. Potentially reusable head-and-neck masks to filter out respiratory pathogens, or full-body suits, might be standard family equipment above a certain TL. Ubiquitous use of personal computing devices would make contact tracing much easier. (One of the advantages of social isolation is that, if you DO acquire COVID, it is now much easier to identify who you might have acquired it from and who you might have given it to, as compared to when you were in contact with hundreds of people on a daily basis.)
Thus, the Traveller milieu is one where it is near impossible (given communication difficulties, and frequent contact between generally-isolated worlds) to prevent the inter-world spread of a pandemic, but relatively easy for HiTec worlds (compared to TL-2020) to shut down a pandemic when it arrives. In fact, given the likely frequency of importing novel pathogens, I think you could make a case that _every_ HiTec world would be living under somewhat socially isolated conditions (by our standards) _all_ the time ... because the worlds that don't will be racked by pandemics on a regular basis.
Cheers,
KenB