There are a couple of points here about how to be polite if the Buddha sneezes or a monk sneezes:
Now at that time the Lord, surrounded by a large assembly, sneezed while he was teaching dhamma. Monks, saying: “Lord, may the Lord live (long), may the wellfarer live (long),” made a loud noise, a great noise; the talk on dhamma was interrupted by this noise. Then the Lord addressed the monks, saying: “Now, monks, when (the phrase) ‘Long life’ is spoken to one who has sneezed, can he for this reason live or die?”
“That is not so, Lord.”
“Monks, ‘Long life’ should not be said to one who has sneezed. Whoever should say it, there is an offence of wrong-doing.”
Now at that time people said “May you live (long), honoured sirs” to monks who had sneezed. The monks, being scrupulous, did not respond. People … spread it about, saying: “How can these recluses, sons of the Sakyans not respond when (the phrase) ‘May you live (long), honoured sirs’ is being spoken to them?” They told this matter to the Lord. He said: “Monks, householders like lucky signs. I allow you, monks, when (the phrase) ‘May you live (long), honoured sirs’ is being spoken to you by householders to say, ‘Long life’ (to them).”