Low Incidence of Rimantadine Resistance in Field Isolates of Influenza A Viruses

The spread of drug-resistant influenza viruses type A to close contacts in families, schools, and nursing homes has been well documented. To investigate whether drug-resistant influenza viruses circulate in the general population, 2017 isolates collected in 43 countries and territories during a 4-year period were tested for drug susceptibility in a bioassay. Drug resistance was confirmed by detection of specific mutations on the M2 gene that have been shown to confer resistance to amantadine or rimantadine. Sixteen viruses (0.8%) were found to be drug-resistant. Only 2 of these resistant viruses were isolated from individuals who received amantadine or rimantadine treatment at the time the specimens were collected. For 12 individuals use of amantadine or rimantadine could be excluded, and from the remaining 2 patients information about medication was unavailable. These results indicate that the circulation of drug-resistant influenza viruses is a rare event, but surveillance for drug resistance should be continued.