(CNN) — If you're planning to fly for Memorial Day or a summer holiday, cinch up your seat belt. Air travel in the United States has recently approached pre-pandemic levels, according to data from the Transportation Security Administration.
That means a return to packed flights this summer -- along with no more empty middle seats -- and crowded air terminals as you take to the sky for that long overdue vacation. Only now you'll be traveling with both vaccinated and unvaccinated passengers, whether you travel domestically or internationally.
How safe is it to travel this summer? That depends on your vaccination status, your destination and your overall health and risk level if you were to be exposed to Covid-19, experts say.
The US Centers for Disease Control and Prevention recommends people refrain from nonessential travel until they are fully vaccinated -- meaning it's been 14 days since the second shot of the Pfizer or Moderna vaccine or one shot of the Johnson & Johnson vaccine. But the agency recently eased its domestic and international travel guidance for vaccinated people, stating that travelers who are fully vaccinated "can travel safely within the United States." Traveling internationally, even for vaccinated travelers, still poses a greater risk, according to the CDC. "The COVID-19 situation, including the spread of new or concerning variants, differs from country to country," the ageny advised. "All travelers need to pay close attention to the conditions at their destination before traveling."
A rush to open
The pandemic still rages across the world with the spread of new, more contagious variants of SARS-CoV-2, the scientific name for the novel coronavirus that causes Covid-19.
Over 140 countries are on the CDC's Level 4 "do not travel" list due to very high levels of Covid-19 transmission, including such popular vacation spots as Belgium, France, Greece, Italy, Ireland, Portugal, Costa Rica and Mexico.
The European Union, or EU, has only fully vaccinated some 15% of their population as of May 23, compared to 35% of Brits and 40% of Americans. Yet in the EU and other corners of the globe, officials are urgently considering ways to restart international travel and rescue tourist-starved economies.
But individual countries in the bloc may also have their own requirements, which could mean testing or quarantine measures.
The EU is also planning to launch Covid-19 vaccine certificates as soon as July 1 for travel within the bloc. This so-called "free pass" or "digital green certificate" will be available in paper and digital format once it is formally approved by the European Parliament.
Masks still required
The US has not developed a federal vaccine passport, but does require all passengers, regardless of vaccination status, to wear a mask over nose and mouth while "awaiting, boarding, disembarking, or traveling on airplanes, ships, ferries, trains, subways, buses, taxis, and ride-shares."
Mask are also required at transportation hubs such as "airports, bus or ferry terminals, train and subway stations, seaports, U.S. ports of entry, and other locations where people board public transportation."
This edict remains in place, the CDC said, despite the recent easing of restrictions on the use of masks indoors and out for fully vaccinated people.
"This doesn't include transportation," Association of Flight Attendants President Sara Nelson told CNN. "There's a mask order in place, and it will be enforced, period."
Travelers arrive for flights at O'Hare International Airport in Chicago on March 16, 2021. US airports are seeing pandemic-era record numbers of passengers.
Scott Olson/Getty Images
Despite the strict rule, airlines have reported 1,900 passengers who refused to comply with the federal face mask mandate to the Federal Aviation Administration; Civil penalties could run between $9,000 and $15,000.
Masks are key
Considering the mash-up of Covid variants, partially vaccinated populations and resistance to wearing masks, what's the safest way to travel right now, even if you're fully vaccinated?
Masks are a must, said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, a leading expert on the airborne transmission of Covid-19.
"Currently, we do not have a good way of knowing who is vaccinated versus unvaccinated," Marr told CNN. "The combination of crowds, close contact and long hours in these (travel) settings makes them risky if people do not wear masks.
"It is critical for everyone to wear masks on airplanes and in airports, as there will be unvaccinated people who might be infected or susceptible, including children," she added.
Wearing a mask onboard is still important for both vaccinated and unvaccinated passengers.
Michael Loccisano/Getty Images
Although rare, vaccinated people may still get "breakthrough" infections, such as those which recently struck the New York Yankees organization: One player, three coaches and five team staff were infected with Covid-19 after being fully vaccinated. "Vaccinated people could potentially still get COVID-19 and spread it to others," according to the CDC, as respiratory droplets fall onto surfaces or float in the air. "The risks of SARS-CoV-2 infection in fully vaccinated people cannot be completely eliminated as long as there is continued community transmission of the virus."
HEPA filtration helps -- but not all planes have them
Marr also stressed the need for HEPA or "high efficiency particulate air" filters in the circulation systems of all planes. HEPA filters are rated to remove 99.97% of dust, pollen, mold, bacteria and other airborne particles with a size of 0.3 microns. Covid-19 is thought to be between 0.06 to 1.4 microns.
All large commercial airliners -- the kind that fly travelers coast to coast and to farflung wonders across the sea -- have built-in HEPA filters to sanitize the breath passengers exhale. Called recirculated air, it is typically mixed 50/50 with cooled "bleed air" brought in off the engines, then fed back into the cabin several times an hour.
But not all planes in service today have HEPA filtration. Regional airlines around the world may use older jets, turboprops and piston engine powered planes originally built without HEPA filters to fill out their fleets, shuttling thousands of travelers to regional destinations or larger airline terminals to catch connections.
Many of these smaller planes, including many regional jets, don't need HEPA filters to purify cabin air because it is never recirculated -- air is constantly refreshed from outside the plane while in flight. Often called "fresh air systems," air is heated by the engines, destroying many impurities, before being cooled and entering the cabin.
But several smaller jets do recirculate passenger exhalations, leaving airlines without HEPA filtration on some airplanes scrambling to retrofit their fleets.
"Air travel safety could be improved if airlines and manufacturers install and use HEPA filters properly in more planes. Proper filtration is a key to safe air travel," Marr said.
Boarding and deplaning can pose ventilation challenges
As long as everyone remains masked, short flights on small planes or jets without HEPA filtration would likely be low risk.
But what about when the plane (with or without HEPA filtration) is boarding or deplaning -- or every flier's worst nightmare -- stuck on the tarmac for hours due to bad weather or last-minute maintenance?
"During boarding that's when there's usually no ventilation -- the planes don't have their auxiliary power units going, they're not often tied into the gate-based ventilation systems," said Joseph Allen, who directs the Healthy Buildings program at the Harvard T.H. Chan School of Public Health, in a prior interview.
"We've done measurements on airplanes when people are boarding, and we see high levels of carbon dioxide, which is an indicator that there's insufficient ventilation," Allen said.
Over the last year, Marr has been flying with a carbon dioxide monitor, conducting a one-woman unscientific survey of air quality during boarding and deplaning. She's found some high levels -- which are only a concern if the plane's HEPA filters were not functioning or if the aircraft didn't have any HEPA filters.
Getting on and off airplanes often poses a greater risk because the air is generally not as thoroughly filtered.
Hector Retamal/AFP via Getty Images
One flight involved a deicing procedure, which kept the passengers sitting on the tarmac for an hour. Marr recorded carbon dioxide levels of 3,000 parts per million, far above the 800 parts per million considered normal.
Travel is less risky for vaccinated travelers, but everyone should still wear masks and socially distance.
Michael M. Santiago/Getty Images
Marr told CNN: "A CO2 (carbon dioxide) level of 3,000 ppm means that for every breath I take in, about 7% of the air is other people's exhaled breath...like drinking someone else's backwash! Let's hope the HEPA filters were running!"
What to do to fly more safely
While there may not be much you can do about CO2 levels or HEPA filtration (other than ask the airline and make your concerns known), experts say there are key actions you can take to make your flight safer.
Unvaccinated people are urged to get tested one to three days before traveling domestically, the CDC says. Avoid crowds while traveling, and stay at least two arm lengths from anyone who is not traveling with you. Wear a mask over your nose and mouth at all times and wash your hands often.
Plan your ride to the airport.
If you have to take an Uber, Lyft or taxi to the airport, make sure you, your family and the driver are all masked throughout the journey -- and be sure to roll down the windows to encourage air flow, Allen said.
"We've done some modeling on this, we're showing that even rolling down the windows just a couple inches can really help with airborne transmission," Allen said. "And you want to put down the windows even if it's inclement weather. A little bit will really help."
Doublemask and watch the fit.
The mask should have a nose wire, the CDC says. Bend it tightly across the nose to keep the mask close to your face. That also helps with fogging if you wear glasses. Stay away from bandanas and gaiter masks unless that's all that's available. A study last year found both types to be the least effective in terms of protection.
Carry the essentials.
Airlines have been implementing additional cleaning methods since the pandemic, but it never hurts to bring disinfecting wipes and a 3-ounce bottle of hand sanitizer with greater than 60% ethanol or 70% isopropyl alcohol -- the level needed to kill most coronaviruses, according to the CDC.
For extra protection, consider using sanitizing wipes on your seat, tray table and arm rest on the plane, and don't forget your seat belt, the filtration nozzle and light buttons above your head, the video monitor and the back of the seat in front of you.
Don't forget to bring snacks from home you can eat quickly, to minimize the time you're unmasked. Dried fruit, nuts, or cheese and crackers are good choices. Most airlines have reduced meal service to a minimum.
Stay in your seat if you can.
Getting up and moving around puts you closer to others on the plane, and visiting the bathroom opens up a whole new set of potentially germ-covered things to touch. If you must take a bathroom break on the plane, have hand sanitizer at the ready.
What's best, experts say, is to prepare in advance by having your meals and bathroom breaks before or after the flight.
Caution is best.
If you have a weakened immune system, such as might happen from taking chemotherapy or drugs for an autoimmune disorder, the CDC says "you may NOT be fully protected" even if you are fully vaccinated. Talk to your healthcare provider about your travel plans.