UK Foreign Secretary Dominic Raab has indicated that vaccine certificates are "under consideration", but some UK government officials, including Health Secretary Matt Hancock, have contradicted him.

Israel, which has vaccinated more than 40 percent of its population, the highest rate in the world, has agreed to test a travel bubble with EU member states Greece and Cyprus to open the door for vaccinated travelers.

The United States government hasn't decided whether it will approve vaccine certificates for US citizens. But US President Joe Biden has issued an executive order asking multiple government agencies to "assess the feasibility" of linking coronavirus shots to international vaccination certificates and producing digital versions of them.

Biden also has said the US will have enough vaccines available for everyone in the country by May, two months ahead of his previous schedule. That could open up travel for millions of US residents.

The US Centers for Disease Control and Prevention hasn't issued guidance on vaccine certificates and noted that there is no international standard on vaccines or documentation of a vaccination.

"Until then, all air passengers traveling to the US, regardless of vaccination or antibody status, are required to provide a negative COVID-19 test result or documentation of recovery," CDC spokeswoman Caitlin Shockey said.