Is the definite article necessary before "treatment" in these sentences?

Question

Can I use the definite article before the word "treatment" in a generic sense? If not, what's the difference in the meaning of these sentences?

1. Several therapeutic agents have been evaluated for (the) treatment of Covid-19, but none have yet been shown to be efficacious.
   

2. The optimal dose and duration of (the) treatment of COVID-19 with Remdesivir are unknown.
   

3. The cost-effectiveness of (the) treatment of Covid-19 was calculated.
   

4. Individualizing (the) treatment of patients is being considered.
   

Which one is grammatically correct?

5. The use of Remdesivir for (the) treatment of Covid-19 was approved.
   

6. The use of Remdesivir for a treatment of Covid-19 was approved.
   

7. The use of Remdesivir as a treatment of Covid-19 was approved.
   

8. They have used Remdesivir for (the) treatment of Covid-19.
   

9. They have used Remdesivir for a treatment of Covid-19.
   They have used Remdesivir for a treatment of Covid-19.