Luxembourg Prime Minister Xavier Bettel has been hospitalised a week after he tested positive for Covid-19.
He will be kept under observation as tests are carried out, the government said.
Mr Bettel, 48, is to stay in hospital for 24 hours as a precautionary measure.
He began ten days of self-isolation on 27 June, less than two days after attending a European Union summit in Brussels.
However, none of the other 26 EU leaders who attended the summit were considered a contact case.
The Luxembourg leader had already received one dose of the AstraZeneca vaccine on 6 May.
The country of 630,000 inhabitants has seen a surge of infections in the past ten days.
Yesterday, 136 people tested positive for the coronavirus, a level last seen in Luxembourg in mid-May.