The Netherlands confirmed 13 cases of the new omicron variant of the coronavirus on Sunday and Australia found two cases, the latest to find in travelers from South Africa.

Countries around the world impose restrictions to limit the spread of the variant. On Sunday, Israel decided to ban foreign nationals, the most extreme measure yet.

Confirmed or suspected cases of the new variant have already emerged in several European countries, in Israel and Hong Kong, just days after researchers in South Africa identified it. The “do first, ask later” approach reflected growing concern about the emergence of a potentially more contagious variant nearly two years after the COVID-19 pandemic that has killed more than 5 million people, disrupting lives and economies around the world.

While there is still much to learn about the new alternative, researchers worry that it may be more resistant to the protection offered by vaccines and could mean the epidemic will last longer than expected.

In the Netherlands, health authorities confirmed that 13 people who traveled from South Africa on Friday had tested positive for Omicron. They were among 61 people who tested positive for the virus on Friday after arriving on the last two flights at Amsterdam’s Schiphol airport before a flight ban was imposed on South African countries. They were immediately isolated, mostly in a nearby hotel, while the sequence was in progress.

Australian authorities say two foreign travelers arriving in Sydney from Africa have become the first to test positive for the omicron variant in the country. Now arrivals from nine African countries must self-isolate in a hotel upon arrival.

Israel has taken steps to ban entry to foreigners and has quarantined all Israelis coming from abroad.