RetireJapan

I have an old iPhone (iPhone 6, I think) without the NFC function, so I am unable to download and use the 新型コロナワクチン接種証明書アプリ.

I will need to travel abroad soon from Japan to Korea for work.

What are my options?
Can I get some kind of paper certificate to prove I have been vaccinated and even received my third booster shot?
Or, am I bang out of luck, and I will have to buy a new, more modern smartphone (I really don't want to do this!).

Thanks!

Your city hall/health center should be able to give you a paper certificate. I got one from mine. You should check their website to figure out the process. Mine required me to mail them a photocopy of my vaccine coupons with a self addressed envelope back.