This story has been updated.

The coronavirus vaccines are welcome news to restless travelers. But if you’re planning to take a trip as soon as you get your shot or shots, depending on which vaccine you receive, not so fast: Vaccinated individuals should wait at least two weeks after their last shot, according to doctors and the Centers for Disease Control and Prevention. In addition, some countries and cruise lines are echoing the call for vaccinated travelers to delay their visits.

Medical experts say the two-week time frame allows the vaccine to do its job. “Our bodies need that time to develop both antibody and cellular immune responses, which are critical for protection,” said Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital in Boston.

Sten Vermund, dean of the Yale School of Public Health in New Haven, Conn., said that while the first shot can provide partial protection, it is necessary to wait the prescribed amount of time to reach the highest level of immunity. “To maximize the protection you get from the vaccine,” he said, “you should be two weeks beyond your second dose before you expose yourself in travel or other somewhat higher-risk circumstances.” The recommendation applies to Pfizer and Moderna’s vaccines, which entail two shots, and Johnson and Johnson’s, which features a single dose.

In addition, the vaccine’s side effects might temporarily dampen any strong urge to leave the couch, much less run off to explore the world. The CDC warns that the vaccine can trigger headaches, muscle pain, chills, fever and nausea, among other discomforts. The effects can be more intense after the second shot in a two-shot regimen, it notes.

“Some people are going to feel not-great after the vaccine,” said Emily Landon, an infectious diseases specialist at the University of Chicago Medicine. “That is really intended. It is the vaccine putting your immune system through its paces, like doing a trial run of combating covid.”

Proof of vaccination to travel or attend school is not new, but the coronavirus has introduced a potential need to modernize outdated paper standards. (Jonathan Baran/The Washington Post)

The CDC changed its long-held position against all but essential travel with an April 2 announcement that fully vaccinated people — meaning those who have received two Moderna or Pfizer shots or a single J&J shot and waited the requisite amount of time afterward — can safely move about the country. (Transportation Security Administration figures suggest the public has been ignoring that advice: The agency has screened more than 1 million air passengers daily for the past three weeks.)

For domestic trips, the CDC said people can skip bookended testing as well as self-quarantining. “You do NOT need to get tested or self-quarantine if you are fully vaccinated or have recovered from COVID-19 in the past 3 months,” the agency stated. For international travel, visitors must follow the country’s entry guidelines and will need to present a negative test result to return to the States.

Health-care professionals still advise caution when traveling because none of the vaccines is 100 percent effective against infection. “There are millions of people who have been vaccinated, and if 5 percent of them get covid, that would be a lot of people with covid,” Landon said. “It’s just not negligible yet.” The rise in variants also poses a concern, because it’s not yet clear how much protection the vaccines provide against those more highly transmissible strains.

It’s also not clear whether fully vaccinated individuals can contract and transmit covid-19 asymptomatically. Even though a recent study released by the CDC found that the Pfizer and Moderna vaccines cut down on asymptomatic transmission, the CDC continues to urge vaccinated individuals to wear masks and socially distance when out in public. Masks are required on all forms of U.S. public transportation, such as planes and trains, and inside transportation hubs, such as airports and bus stations. “You don’t know if the person sitting in seat 16B or 14A is at risk,” said Daniel Kuritzkes, chief of the Division of Infectious Diseases at Brigham and Women’s Hospital in Boston. “We can’t make such fine distinctions yet, so everyone needs to wear a mask.”

When planning a vacation, travelers also will want to take into consideration is how long the vaccines will protect them from covid-19. Because the inoculations are so new, their lasting power remains a mystery. “There is no way of predicting how long the vaccine will be effective for,” Kuritzkes said. However, clinical study participants who received the vaccine months before the general public can provide a clue, according to Landon. So far, protection appears to last at least several months. “They had a head-start advantage,” Landon said. “We can see if their antibodies start to come down.”

Based on this timeline, Landon said travelers can safely book a vacation for the summer and a few seasons out. But, she advised, every Plan A should come with a Plan B, or at the very least, travel insurance. “I would feel pretty confident in making plans for trips that are six to 12 months out,” she said. “But I would not get nonrefundable tickets.”

A handful of countries and cruise lines are opening up to vaccinated travelers who have waited after their last shot. Visitors must show proof of the inoculation, which includes the date it was administered, at the start of their trip.

Travelers to Iceland, for example, must present documentation before boarding the plane and upon arrival at Keflavík Airport. Belize is also asking incoming visitors for proof, as is Slovenia, though the wait time varies by vaccine type. Before planning any travel, visit your destination’s tourism authority website for entry requirements.

Crystal Cruises, which will begin sailing in the Bahamas in July, is requiring proof at embarkation. Cruisers with American Cruise Lines must also present their health record at the time of boarding. However, the U.S. company, which extended its vaccine mandate for departures through April, only requests passengers wait two weeks after one shot, not both.

Please Note

Potential travelers should take local and national public health directives regarding the pandemic into consideration before planning any trips. Travel health notice information can be found on the Centers for Disease Control and Prevention's interactive map showing travel recommendations by destination and the CDC's travel health notice webpage.

The coronavirus pandemic has disrupted travel domestically and around the world. You will find the latest developments on The Post’s live blog at www.washingtonpost.com/coronavirus