Real Madrid and Chelsea could be without their unvaccinated players for their respective Champions League last-16 away legs due to a law change in France.
Real are set to face Paris Saint-Germain at Parc de Princes on 15 February while the Blues travel to Lille on 16 March.
However, both Thomas Tuchel and Carlo Ancelotti could be posed some selection headaches after French lawmakers altered the way that the country's Covid-19 passport is accessed.
Under the new rules only people who have been vaccinated or those who have recently recovered from the virus will be permitted to enter large areas. This includes football stadiums and the French sports minister has insisted that elite athletes will not be granted exemptions.
A UEFA statement, relayed by ESPN, seemed to reiterate this message.
"UEFA is liaising with the relevant stakeholders across Europe ahead of the resumption of UEFA's club competitions in February," it read.
"The specific conditions under which matches are played in each country are subject to the decisions of the competent authorities in that country.
"Each team will in principle be required to comply with the applicable rules in the country where the match takes place, but the specific competition Annexes -- still under elaboration -- which contain special rules due to COVID-19 applicable to the knockout phase, may provide further guidance in this regard."
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