International tourism arrivals are set to stagnate this year, except in some Western markets, causing up to $2.4 trillion in losses, a U.N. study said on Wednesday, adding the sector is not expected to rebound fully until 2023.

COVID-19 vaccination and certificates are key to restoring confidence in foreign tourism, which provides a lifeline for many countries, especially small island states that rely heavily on the sector to provide jobs, it said.

In 2020, international arrivals plunged by 73% from pre-pandemic levels in 2019, causing estimated losses of $2.4 trillion in tourism and related sectors, according to the report by UNCTAD and the UN's World Tourism Organization (UNWTO).

"The outlook for this year doesn't look much better," Ralf Peters of UNCTAD's trade analysis branch, told a news conference.

"The first three months were again bad, there was not much travelling happening," he said.

"There is an expectation of a certain recovery in the second half of the year, at least for North America and Europe to a certain extent," he told Reuters, crediting vaccinations.

The report sets out three scenarios for 2021, showing international tourism arrivals forecast to drop by between 63% and 75% from pre-pandemic levels, resulting in losses of between $1.7 trillion and $2.4 trillion.

"In international tourism we are at levels of 30 years ago, so basically we are in the '80s ... Many livelihoods are really at threat," said Zoritsa Urosevic, Geneva representative of the Madrid-based UNWTO.

"What we are looking at in the long run is...meeting the 2019 numbers after 2023," she said.

Sandra Carvao, chief of market intelligence at UNWTO, said that it would be a "very diverse recovery", varying by region and by country.

The European Union's digital COVID-19 certificate, due to come into force on Thursday, represents the only regional harmonisation to date, she said.

Carvao, referring to travel corridors, said: "We see for example Asia-Pacific is still one of the most closed regions in the world at this moment - most of the borders in the countries are either totally closed or with significant restrictions."