De or het massavaccinatie?

The correct Dutch article is:

de massavaccinatie

Rule Words ending in -tie and -sie: almost always de. See the rule

How to use massavaccinatie

the massavaccinatiede massavaccinatie
this massavaccinatiedeze massavaccinatie
that massavaccinatiedie massavaccinatie
our massavaccinatieonze massavaccinatie
your massavaccinatiejouw massavaccinatie
every massavaccinatieelke massavaccinatie
which massavaccinatie?welke massavaccinatie?
a big massavaccinatie / the big massavaccinatieeen grote massavaccinatie, de grote massavaccinatie