Trying to resolve an "invalid use of Null" in VBA

In VBA/VB6, strings cannot be set to Null; only Variants can be set to null. In addition, when you declare variables inline comma-separated like in the question, only the last one will be typed as string; all of the others are typed as variants. To declare them on one line as a type you have to include the type

Dim a As String, Dim b As String ...

That's why it makes sense to just declare them on a single line.

(Btw, it should be noted that deceased, resolved are also typed as variants for the same reason.)

The reason that it succeeds for givenNames, etc. is that you have unwittingly defined them as Variant type. It fails for patientNumber, because you successfully defined that as a String, and strings do not accept Null values.

Within a Dim statement, the As <type> clause applies to each individual variable in the list, so by putting it only at the end of the list, you applied the explicit type only to the last-listed variable. The implicit type of Variant is applied to the others.