Attenuation - Attenuation of the virus must be a stable characteristic, so that reversion to a severe strain does not occur. Choice of a virus that is known to have a low mutability in the field should help in this regard.

It might also be important to use a virus which has been debilitated in its natural host range, or in its ability to be transmitted naturally (by insects, for example).

Since viruses that are mild on one host can still be severe on another, these modifications would reduce potential damage resulting from the maintenance of a reservoir of a virus in a crop plant. Another potential objection to the use of viral vectors comes from the observation that some viruses in combination are more severe than either alone.