BRUSSELS — The omicron variant had a foothold in multiple countries in Europe before travel restrictions were imposed, new genetic sequencing data has revealed.

Dutch officials said Tuesday that they had detected the variant, with its unusually high number of mutations, in a sample collected on Nov. 19 and another on Nov. 23 — well before Dutch authorities panicked over two flights from South Africa carrying infected passengers.

The earliest known cases are still from southern Africa. The first identified samples were collected Nov. 9, from a 34-year-old man and a 23-year-old man in Johannesburg, according to the GISAID global database. On Nov. 11, five samples of the variant were collected in Botswana.

Experts caution, though, that omicron could have originated elsewhere. Even countries with world-leading sequencing programs only assess a portion of their cases. Labs in many countries are now combing through samples collected not just in recent days but over the past month.

Coronavirus variants like omicron, delta and mu are an expected part of the virus's life cycle, but vaccines can prevent more infectious variants from forming. (John Farrell, Hadley Green/The Washington Post)

As of Tuesday, cases of the variant had been reported in 20 countries, with 13 of those in Europe, the European Union’s public health body said.

Hong Kong, Sweden, Israel, Britain, Italy, Canada, Belgium and the Netherlands all traced their first cases to samples collected before South Africa warned the world late Thursday of a potentially more contagious variant, with mutations of the sort that might evade vaccines.

The next day, as the World Health Organization assessed that omicron was a “variant of concern,” two planes carrying about 600 passengers from South Africa landed in the Netherlands with 61 people testing positive for the virus, 14 of whom had the omicron variant.

The E.U., Britain, the United States and others quickly imposed restrictions on travel from southern African countries. South African leaders decried the travel bans as unnecessary — and as punishment for the vigilance of their scientists and the transparency of their public health officials.

While sequencing data underscores that travel restrictions came too late to prevent transmission, a spokesperson for the European Commission defended the moves on Tuesday, saying the restrictions may still slow the spread.

“A certain number of precautionary measures had to be taken — not to stop the variant from reaching Europe or other parts of the world — I think everybody recognizes that is not possible,” spokesman Eric Mamer said. “But to ensure that we slow down its spread as much as possible, in order to give European health authorities [and] our pharmaceutical suppliers of vaccines the time to adapt and ensure that we can then roll out an appropriate response.”

The E.U. health authority said Tuesday that no severe cases or deaths had been reported in connection to the variant — and that the cases it had information about were mild or asymptomatic.

“The majority of confirmed cases have a history of travel to African countries, with some having taken connecting flights at other destinations between Africa and Europe,” the European Center for Disease Prevention and Control added in a statement.

The Dutch public health institute said it did not know if the two first people to test positive there had been in southern Africa.

Europe had been experiencing a surge of coronavirus infections before the omicron variant was detected, and countries have continued to enact measures to mitigate a second pandemic winter.

Switzerland said Monday that anyone — vaccinated or unvaccinated — coming from the growing list of countries where the variant has been detected must quarantine for 10 days on arrival. Polish media reported that Poland had enacted similar measures.

And in Greece, Prime Minister Kyriakos Mitsotakis said on Tuesday that his country will make vaccination against the coronavirus mandatory for everyone over age 60 and will apply 100 euro monthly fines for noncompliance. That money, he said, will be used to support the hospital system.

“It is not a punishment,” Mitsotakis said. “I would say it is the price for health.”

The executive director of the European Medicines Agency told lawmakers Monday that it could take two weeks to learn whether current vaccines are effective against the omicron variant.

If omicron does require a new vaccine formula, Emer Cooke said, approval for use in the E.U. could take up to four months.

“We are prepared,” Cooke told E.U. lawmakers. “We know that at some stage there will be a mutation that means we have to change the current approach.”

Chico Harlan in Rome and Quentin Aries in Brussels contributed.