Portugal's northern health authority has warned anyone who watched the Champions League in person over the weekend to monitor COVID-19 symptoms.
Saturday's final between Chelsea and Manchester City drew big crowds of English fans who were largely maskless and did not social distance.
Many took over the city of Porto along the riverside to drink and chant team slogans.
Some 16,500 people were allowed into the stadium but many others traveled to Porto to support their teams from the sidelines.
There has been concerns the event could lead to another outbreak after authorities decided to relax COVID-19 rules for the match.
All fans had to present a negative COVID-19 test on arrival.