Editor’s Note: Kent Sepkowitz is a physician and infection control expert at Memorial Sloan Kettering Cancer Center in New York. The views expressed in this commentary are his own. View more opinion on CNN.

CNN  — 

A sense of normalcy has once again settled onto large regions of the United States as Covid-19 cases have quieted down.

For many, this relative calm, occurring just as the holiday season approaches, means only one thing: an opportunity to leave the United States, to go overseas to that favorite tourist destination – somewhere in Europe where there aren’t so many tourists.

Kent Sepkowitz

Unfortunately for travelers, the US Centers for Disease Control and Prevention has implored Americans to think long and hard before they leap. Europe, it seems, is in the early stages of a new Covid-19 surge. The list of places labeled by the CDC as “avoid travel” due to risk of infection, updated weekly, includes about half the countries in Europe, with Belgium, Slovakia and Russia added just this week.

Note that “Europe,” from a public health perspective, is not the European Union or a travel agency conceit but rather the 53 countries on the World Health Organization’s map. This large area includes Russia and numbers about 750 million people, giving it nearly twice the population of the US.

Which brings us to the two questions for potential holiday travelers: Is it really this bad or is the CDC just being cautious? And is it going to get worse over there and, gulp, over here, too?

As to the first question – the CDC is not being too cautious. Not at all. The newest Eurosurge is quite real in countries as different culturally and politically as Russia and Germany, the Czech Republic and the Netherlands. Cases overall have been increasing for at least four weeks and are highest in those under 50 years of age, though older people are beginning to see an increase as well.

The reasons for the newest increase resemble those of the last big European surge from Marchunder-vaccination, weak enforcement of public health interventions and general refusal to accept the risk as real.

This lack of a clear and singular explanation for this uptick in cases has led to considerable speculation and handwringing. Countries in Eastern and Central Europe, many of them once part of the Iron Curtain, such as Bulgaria, Romania and Slovakia, have some of the lowest vaccination rates in the world, likely explaining the rise.

But in Western Europe, countries including Germany and Belgium have vaccination rates similar to the US – yet this does not seem sufficient to contain spread. To try to gain control once again, these countries are giving booster shots, trying even harder to convince the unvaccinated to take the shot and even considering re-instituting some components of the never popular lockdown. To my mind, a plausible explanation is that the same Delta variant is spreading to unvaccinated and unboosted people as weather cools and everyone returns indoors, where social distancing and fresh air are in short supply.

No matter the causes, the trends in Europe (and the Caribbean and some other areas around the world) are not amenable to a quick fix. The surge is here to stay, at least for a little while. In other words, Europe is likely to get worse rather than better in the weeks ahead.

As to the other question of whether the Eurosurge will presage a new global surge … well, it’s complicated. In the US, cases are rising in a few states, as has been true for months. But a more disturbing trend is being seen across large swaths of the country and in New York City as well: an end to the steady decrease of cases registered over the last few months. To turn a once good-news term to the dark side, the curve is flattening – but this time it indicates a stalemate between the virus and humanity, not the inspiring consequences of a hard-fought battle to curb a runaway pandemic.

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    Given all this uncertainty, international travel in the next weeks seems like a singularly bad idea. Yes, maybe this is all just a cold weather pause, or perhaps some more people need a boost, or maybe the virus is doing something new we have not yet discerned. But from what we know right now, there is a real possibility that whatever is driving the Western European increase will also mess things up in the US.