Transaction wrap around warning WARNING:  database "dbname" must be vacuumed within 5463629 transactions
  HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
  You might also need to commit or roll back old prepared transactions.
  VACUUM


I have run vacuumdb --full -h 127.0.0.1 -U postgres dbname
It spits out that message galore.

I have checked active queries and there is nothing active.

It is a cluster with many DBs on Linux (postgres 9.5.10) but only this one small DB is giving this warning.

Any suggestion would be helpful.
Don't use "--full".  

Do you get any messages, other than the one you show, while vacuuming?

What is the output of select datname, age(datfrozenxid) from pg_database;