Hi Elizabeth,

Yes!  It can be a bit tricky.

To write du Covid-19 suggests that Covid-19 is a grammatically "specific noun" and the "du" represents the preposition "de" followed by "le" meaning "of the"   (definite article).

"En cas d'opposition" --> in case "of" opposition where d' (de) is a preposition.

This is how I understand it  --  hope that it may help?

I got an error message when trying to access your link.

Bonne journée

Jim

Note though that officially in France it is “la Covid 19” although before the Académie decreed such (based on being ‘la maladie … “),  le Covid was in common popular use (I know from experience of being locked down there in the first prolonged confinement), and it remains in colloquial use also. 

The confusion is not helped by Canada decreeing it to be le Covid, nor the fact that it is le coronavirus (as virus is masculine). 

 In terms of your example, in metropolitan French it should be ‘de la Covid 19’, not “du Covid”.

https://www.diplomatie.gouv.fr/fr/le-ministere-et-son-reseau/actualites-du-ministere/informations-coronavirus-covid-19/faq-covid-19-les-reponses-a-vos-questions/ 

https://www.theguardian.com/world/2020/may/13/le-la-covid-coronavirus-acronym-feminine-academie-francaise-france

Thank you Jim. I realised there could be prepositions too. I guess with practice it gets easier to be able to differentiate. Here is the link of the article that I was referring to: https://rfi.my/7RJm 

Thank you Maarten for pointing the difference out on the masculin and féminin. 

Thanks for the link, Elizabeth. A French news article confirming that it is still common to use ‘le COVID’ , despite the Académie arriving late on the scene to recommend otherwise !