First infections are not uniformly distributed. Second, by the nature of travel people get into close contact with more and more importantly different people than usual. 200 people sitting in an aircraft for 2 hours is inherently more risky than 200 people sitting in a theater for 2 hours as aircraft mix people from wider geographic areas. So, regardless of infection rates, this is beneficial for both Europe and the US at the same time.

Multiple cities with hundreds of thousands of people in both the US and Europe are infection free right now. Extending that for even a few weeks is very useful.

This is false. Any reduction in travel is going to have a positive effect. The best situation is for people to reduce their exposure to other people and to stay at home. If they do need to be exposed to others, the best is for them to be exposed to the same set of people all the time, and not to different sets of people.

This seems not fully worked out. Assuming total travel is constrained, selectively limiting travel to particular areas concentrates travel in the remaining areas, which increases the average number of contacts between travelers, which violates your “reducing exposure to other people is best” rule of thumb.

By your model, some reductions in travel ought to have a negative effect. Is that a fair corollary?

There’s no such thing as a fixed amount of travel, and constraining it on one way concentrates it in another. That makes no sense. Constraining travel will reduce travel, which will reduce interactions between people, which will reduce the spread of the virus. We know this empirically from previous outbreaks and countermeasures.

The pressure / desire to travel is also dropping sharply, though.

Limiting all Schengen traffic is an extreme example, but it's not obvious that some level of restriction would necessarily produce a significant bottleneck effect (given that citizens are exempt).

Why would you assume total travel is constrained? Restricting travel from Europe to US will just eliminate that portion of travel, not concentrate it in the US.

There is an effect, because traveling itself creates chances for exposure. Anyone traveling on an airplane or train in close quarters, or passing through an airport or transit facility, can pick up or pass on an infection.

By your last analogy, the red balls would have paint rubbed off them in the exchanges.

Yes, but then we should be canceling all flights in the US (we should be). Only canceling flights to Europe is a tiny fraction of what's actually necessary here.

It’s a matter of timing. Europe has higher concentrations of the virus than the US. The chances that a plane from Europe contains an infected individual are getting quite high.

When the concentrations in the US get higher it will start to make sense to constrain travel more over there. Right now the chances a domestic flight contains an infected individual is quite low, but that won’t always be the case.

Bear in mind the virus cannot be stopped, or contained at this point. All we can do is slow it down while also limiting the harm our countermeasures cause.

It's not clear the US does have fewer proportions of actual cases, just fewer tested ones.

Saying the virus cannot be stopped seems defeatist. China seems to have managed it locally, albeit with some very aggressive measures.

It’s only a matter of time before it starts over there again, even if through back-propagation from other countries. Everyone’s talking as though China solved the problem. Incredible.

They don't need to solve the problem, just keep the infection rate slow enough to be controlled medically. And then keep everything locked down enough until we can get a vaccine produced. Admittedly, a gargantuan task, and I don't think it's going to happen anywhere else.

They don't need to do that either, they could just let more people die.

That's what you're proposing, if you don't realize it. If you get the growth rate below 1, you can wipe out the disease, instead of merely delaying it.

Apart from the other two replies, the corollary of your point is that travel from a lower incidence area to a higher incidence area actually reduces the risk in the latter.

But this is clearly false because population density is another important factor for contact between people.