I sent one, my format was:
"Dear PD,
I hope this email finds you well during this busy time! My name is TelemarketingEnigma, and I am a current applicant to [program]. I am writing to express my continued interest in training with your program. I know that you have many incredible candidates this year, but should you be offering further invitations to interview, I hope that you will keep my application in consideration among them.
[very brief paragraph about why their program specifically - near home + academic/community service interests]
[very brief update on accepted publications]
Thank you for your consideration, and I wish you the best of luck with your recruitment this year."
It was either successful or I embarrassed myself by sending it just before they were gonna invite me anyway, because I got the invite later that day. But either way it didn't get me uninvited lol