A senior World Health Organization official said that so-called "vaccine passports" for COVID-19 should not be used for international travel because of numerous concerns, including ethical considerations that coronavirus vaccines are not easily available globally.

At a press briefing on Monday, WHO emergencies chief Dr. Michael Ryan said there are "real practical and ethical considerations" for countries considering using vaccine certification as a condition for travel, adding the U.N. health agency advises against it for now.

"Vaccination is just not available enough around the world and is not available certainly on an equitable basis," Ryan said. WHO has previously noted that it’s still unknown how long immunity lasts from the numerous licensed COVID-19 vaccines and that data are still being collected.

Ryan also noted the strategy might be unfair to people who cannot be vaccinated for certain reasons and that requiring vaccine passports might allow "inequity and unfairness (to) be further branded into the system."

Some countries, including in the European Union, are considering the possibility of demanding certificates of COVID-19 vaccination for foreigners trying to enter.

Travel with certificates raised legal questions, because those last in line for vaccinations could argue their freedom of movement was unjustly restricted by the often monthslong queues.