The present invention provides modified viral genomes for use as vaccines or vectors, which are improved in their ability to retain attenuating mutations. The genomes are from viruses that replicate by way of an RNA-dependent RNA or DNA polymerase. The genomes are modified in the pol gene to encode polymerases that catalyze slower replication, have increased transcriptional fidelity, or are otherwise altered such that the reversion rate of the modified virus to a non-attenuated form is decreased as compared to an equivalent, unmodified virus. In particular, modified coxsackievirus genomes are disclosed.