Euro 2020's fate will not be decided until March but UEFA is hoping vaccination efforts will allow the governing body to stick to its original plan.
The 24-nation showpiece, scheduled to begin on June 11, was postponed by 12 months after last year's COVID-19 outbreak.
The 2020 edition was planned to be the first staged across the continent, rather than by a single nation or joint hosts.
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The host cities are Glasgow, Dublin, Bilbao, Amsterdam, Copenhagen, Munich, Rome, St Petersburg, Bucharest, Budapest and Baku, with the semis and final set for London's Wembley Stadium.
UEFA had hoped that a year-long wait would have allowed fans to be present at games but, with most football continuing to be held behind closed doors, that prospect depends on an improvement in the situation.
UEFA president Aleksander Ceferin is hoping vaccination programmes, moving at different speeds across the various host countries, will be the key to sticking to the original plan.
"Vaccination has started and I think we will be able to have full stands in the summer," Ceferin said in a recent interview with Serbian newspaper Informer.
"For now, the plan is to play in all 12 countries. Of course, there are backup options in case a country has a problem."
But Ceferin accepts that there may need to be some juggling of the hosting plan and perhaps different levels of stadium capacity use.
"We are ready to organise competitions in 11, 10 or nine cities... and even only in one country, if necessary. However, I am 99.9 per cent sure that we will have the European Championship in all 12 cities, as planned," he said.
UEFA confirmed it has been working on four operational scenarios - for full stadiums, 50-100 per cent capacity or 20-30 per cent capacity and games behind closed doors.
The host cities have been asked to come up with two to three plans out of those options and it is possible different approaches will be taken in each venue.
"A decision on which scenario will be applied individually in each city during the tournament will be made on 5 March 2021," UEFA said in a statement.