Can OPV with stage three or four VVM be used if the expiration date is still valid?
No. A late stage VVM is an indication that the vaccine has been exposed to heat long enough that it should no longer be used. There is a margin of safety built into the VVM so that even a late stage VVM might still be potent, but the vaccine has had enough exposure to heat that it should be discarded because it is potentially weakened. Using a vaccine with a late stage VVM is not harmful, but it might not offer immunity.
