The selection for cidofovir-resistant mutant vaccinia viruses is inhibited by coinfecting cidofovir-sensitive wildtypes
Recent years have seen two outbreaks of zoonotic mpox, highlighting the need for additional therapies to treat these infections and curb the spread of the virus. However, there are only two antiviral drugs available for treating mpox, and drug resistance is a potential threat to intervention. Here we investigate how drug resistance emerges in a population of viruses where the sensitive wildtype strain vastly outnumbers the drug-resistant mutant. Using cidofovir treatment and resistance as a model system during vaccinia virus infection, we found that cidofovir-resistant poxviruses rapidly outcompete cidofovir-sensitive viruses when the two strains replicate in complete isolation. However, when the two strains infect the same cell, the drug-resistant virus lost its competitive advantage over the sensitive strain. We found that when the two viruses replicate in separate cells treated with cidofovir, the resistant strain displays faster genome replication kinetics compared to the sensitive strain. However, when the two viruses replicate in the same drug-treated cell, the strains replicated at the same rate. This showed that the drug-resistance trait was functionally shared between the two genetically-distinct strains, allowing the sensitive strain to parasitize the growth advantage of the resistant strain and preventing the rapid outgrowth of the resistant strain. These observations suggest that poxvirus coinfections negatively affect the selection for a rare trait like cidofovir-resistance. Considering that poxviruses actively prevent superinfection, we hypothesize that superinfection exclusion could maintain fitness in a population and facilitate the outgrowth of rare, advantageous traits in poxvirus populations.