(CNN) — As more people get vaccinated and coronavirus infection rates drop in many parts of the world, people are traveling again in larger numbers.
The European Union has announced that it is lifting restrictions for visitors from more than a dozen countries, including the United States. Cruises from the United States will soon be sailing again, too.
What should people consider when deciding whether and where to travel this summer? What if they are traveling with children who can't yet get vaccinated -- what additional considerations should there be?
We asked CNN Medical Analyst Dr. Leana Wen to weigh in. Wen is an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health. She's author the forthcoming book "Lifelines: A Doctor's Journey in the Fight for Public Health."
Here's her advice:
CNN: Is it safe to travel abroad this summer, or is it safer to stay closer to home?
Dr. Leana Wen: That depends. There are two major factors to consider. The first is your vaccination status. If you are fully vaccinated -- unless you are severely immunocompromised -- you are well-protected against Covid-19, and I hope that you will resume activities you enjoy again, including international travel.
On the other hand, I'd advise those who are unvaccinated to refrain from nonessential travel, either domestically or abroad. If you are in large crowds of people indoors -- for example, at airports or train stations -- there will likely be others who are unvaccinated there too. They will pose a risk to you, and you will pose a risk to them.
Greece is starting to welcome international visitors.
Byron Smith/Getty Images
Second, consider your destination. Many countries are allowing travelers from the United States again, but they may still have rules that may make you rethink your trip. For example, there may be a required quarantine period during which you can't visit any tourist destinations. If the quarantine period is essentially the duration of your trip, it may not be worth it to go.
Also look up the coronavirus transmission numbers. You wouldn't want to go to a country in the middle of a major surge of cases. Not only would you be at risk for contracting the virus, if you were to become ill, it might be a challenge to find medical care. Countries with high Covid-19 infections might also be imposing lockdowns and other restrictions that would hamper your trip. Depending on the conditions at the time, you may even have difficulty returning to the United States.
The US Centers for Disease and Control Prevention keeps an up-to-date list of countries by risk level. Make sure you consult this list, and realize that it is constantly changing. Once you decide on a destination, know the rules before you go -- whether testing is required, for example.
If you aren't an American citizen or green card holder, make sure to look up the rules about returning to the US. Some foreign nationals who live in America have been restricted from re-entering the US during the pandemic, and you wouldn't want to accidentally get stuck somewhere else.
From a logistics standpoint, it is definitely easier to travel domestically. On the other hand, there are plenty of countries with low enough rates of Covid-19 and favorable regulations that will be very welcoming of international visitors.
The number of air travelers has been ticking up in the US.
Alex Wong/Getty Images
CNN: Is air travel safe?
Wen: When passengers are masked on planes, the rate of Covid-19 transmission is low. I have little concern about vaccinated people flying, though I'd caution that they still keep their mask on during the flight and limit their time in crowded, indoor settings before and after their trip.
I do have concern for unvaccinated people going on flights. Though masks are required on flights, many people have let down their guard and are not being as cautious as before. Ideally, the traveler should get vaccinated before they go on the trip. Unvaccinated people should wear a high-quality mask (i.e. an N95 or KN95) or double-mask during the entire flight.
CNN: Is air travel safe for kids under the age of 2, or other kids who just can't keep their masks on?
Wen: I have a 1-year-old, and I'm not taking her on flights. My almost 4-year-old is pretty good about wearing a mask, and if we had a short trip -- an hour flight, for example -- I'd consider taking him. But we wouldn't go across the country or on an international flight at this time.
Other families will make different decisions. This is an in-between time where people will make different choices based on their perception of risk and their individual circumstances.
CNN: You're going to say no cruises for unvaccinated people, too, right?
Wen: Right. Let's take a step back and look at the unique characteristics of cruise ships. In pre-pandemic times, a cruise experience involved thousands of people in close quarters with one another, with lots of mingling, and for prolonged periods of time.
There are shared meals, shows, drinking and dancing. The cruise ship makes stops along the way, and often, passengers are disembarking and interacting with people in other regions of the country or the world. They might be purchasing souvenirs, visiting attractions, or going to a bar, and then coming back on board the ship to interact with one another again.
Cruises from US ports are set to resume this summer.
Joe Raedle/Getty Images
These are some of the highest risk settings for Covid-19 transmission. We saw this at the beginning of the pandemic, when more than 800 people on board three cruise ships became infected. An outbreak on a single cruise ship, the Diamond Princess, sickened over 700 people; nine died.
Studies show that vaccination dramatically reduces your chance of acquiring Covid-19 and transmitting it to others. Even if someone who's vaccinated gets coronavirus, chances are that they will have a much lower viral load and not be able to infect many others. If you have a cruise of only vaccinated people, the chance of them endangering one another is low. It's not zero -- breakthrough infections among fully vaccinated people could happen -- but the chance of having a superspreader event is much decreased.
However, if those on board are not vaccinated and someone ends up getting Covid-19, that infection could ripple through the unvaccinated. There's also a chance it that it could put the vaccinated at risk, too, in particular those who are elderly and with some degree of immunosuppression.
Bottom line: If you're considering a cruise and you're vaccinated, choose a cruise that requires proof of vaccination. If you're not vaccinated, please don't go on the cruise, for your own safety and for that of the other passengers.
Dr. Leana Wen advises unvaccinated travelers to skip going on a cruise.
Joe Raedle/Getty Images
CNN: What if the cruises allow kids and other unvaccinated people to be on board, as long as they have a recent negative test? Is that enough?
Wen: Proof of a negative test is definitely better than nothing, but it doesn't replace the need for vaccination. After all, the person could still be harboring the virus but it's not picked up yet by the test, and they could still become infected during the journey.
Some cruises are allowing unvaccinated people to join, with limitations. For example, they are supposed to still wear masks in public places, and they could be restricted from certain dining hours or indoor areas or activities. I still don't think this is sufficient. I wouldn't bring unvaccinated kids to a cruise right now.
CNN: There's a lot of talk about variants, especially the Delta variant that's become dominant in some countries and is likely to become dominant in the US. Should concern for these variants factor into the decision to travel?
Wen: For the vaccinated, no. The vaccines we have in the US are very protective against the variants identified thus far.
People who are partially vaccinated -- who received only one dose of the Pfizer-BioNTech vaccine or the Moderna vaccine -- should be concerned. That's because one dose provides only about 33% protection against the Delta variant. And those who are unvaccinated should certainly be worried about this variant that is more transmissible and possibly deadlier than the other strains of coronavirus.
Dining outside is safer for unvaccinated travelers.
CNN: Since capacity restrictions have been lifted in many places, is it really safe to eat inside a crowded restaurant and go to a crowded indoor concert if you're vaccinated?
Wen: None of these activities are risk-free. If any of these settings require proof of vaccination, that immediately reduces the risk substantially. Concerts with only vaccinated people will be a lot safer than otherwise -- think about the exposure risk if you're standing shoulder to shoulder with dozens of unvaccinated, unmasked and very excited fans who are screaming for hours.
There are two other factors to consider. First, what's the transmission rate in the community you're going to? Some parts of the US have such high levels of vaccination that the level of Covid-19 is pretty low, but other parts are going through active surges.
Second, look at each individual circumstance. A restaurant that has some spacing between tables and good ventilation will be safer than a bar that's standing room only and so loud that people have to shout to hear one another.
If you're unvaccinated, you should stick with outdoor dining. If you must go into indoor spaces with other people of unknown vaccination status, you should wear a mask and limit your time in these higher-risk settings.
Ultimately, deciding which activities to bring back after you're vaccinated depends on the value of that activity to you, as well as your risk tolerance.
Restrictions may be lifted and people can go back to many aspects of pre-pandemic normal if they wish, but we should also keep in mind that just because we can do something doesn't mean that we must. Everyone should make the decisions that are most suitable to them and their families.