Pandemic Navigator, which simulates day-by-day immunity growth. “That’s good news for the domestic market, but in the context of international travel, we do have to realize that it’s not just about one country — it’s a country at the other end as well.”

Factoring in the spotty vaccine rollout across the pond, Mr. Usman said it’s reasonable to assume that Europe’s herd immunity will lag several months behind the United States. Over the next several months, he added, European countries will follow in Iceland’s footsteps and open individually, complete with their own regulations about vaccinations, testing and quarantines. To spur travel across the continent this summer, the European Union is considering adopting a vaccine certificate for its own residents and their families.

“It’s not going to be a binary open-or-shut,” Mr. Usman said. “Countries are going to start getting more selective about who they’re going to start letting in.”

Italy’s numbers — plus new lockdowns and growing Covid variants — seem to be stifling optimism; Hopper flight searches from the United States to Italy have remained relatively flat.

For now, Ms. Lieberman, of Skylark, has adopted a “beyond the boot” mind-set: “Our theory is that if you’re willing to go beyond the boot — meaning, Italy — there will be fabulous, desirable summer destinations for you to take advantage of.”

Portugal surged in January but has recently eased lockdown measures as infection rates have slowed. The country is now aiming for a 70 percent vaccination rate this summer.

American interest in Portugal is spiking in response. In the first week of March, following an announcement that Portugal could welcome tourists from Britain as soon as mid-May, Hopper searches on flights from the United States to Lisbon rose 63 percent. (That’s not far behind Athens, for which travel searches shot up 75 percent in the same time period.)

will next month start nonstop service between Boston and Reykjavik — and resume its Iceland service from New York City and Minneapolis.

“Unless demand spikes rapidly enough to outpace the increase in supply, flash sales can be found as airlines attempt to entice travelers to return amid piecemeal easings of travel restrictions,” said Mr. Damodaran. Icelandair, for example, is running sales on flights and packages through April 13.

And with prices for summer flights to Europe still relatively low in general — down by more than 10 percent from 2019, according to Hopper — experts see little downside in penciling in a trip.

“If you’re willing to take some risk, plan early and lock in your preferred accommodations and ideal itineraries,” Ms. Lieberman said. “But of course we caution you to be prepared to have to move deposits and dates if it comes to that.”

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