viruses is essential for public health because oseltamivir is the most
widely used antiviral medication.
Development of resistance to zanamivir or oseltamivir also has
been identified during treatment of seasonal influenza (116–120).
One study reported that oseltamivir-resistant seasonal influenza A
viruses were isolated from nine (18%) of 50 Japanese children during
treatment with oseltamivir (121). Transmission of neuraminidaseinhibitor–resistant
influenza B viruses has been reported among
household contacts (122). Development of resistance to oseltamivir
during treatment was more common among seasonal influenza A
(H1N1) virus infections (27%) compared with seasonal influenza A
(H3N2) (3%) or B (0) viruses in another study (123). Sporadic cases
of resistance to oseltamivir have been observed among persons with
2009 H1N1 virus infection (e.g., immunosuppressed patients with
prolonged viral replication during oseltamivir treatment and persons
who developed illness while receiving oseltamivir chemoprophylaxis)
(114,124). Emergence of oseltamivir-resistant 2009 H1N1 virus
strains within 48 hours after initiation of treatment has been reported
(125). Transmission of oseltamivir-resistant influenza B virus strains
or 2009 H1N1 virus strains acquired from persons treated with
oseltamivir is rare but has been documented (112,122). Isolation
of influenza A viruses with reduced susceptibility to zanamivir have
been reported rarely, although the number of posttreatment isolates
tested is limited (117–119,126). Clinical isolates with reduced
susceptibility to zanamivir have been obtained occasionally from
immunocompromised children on prolonged therapy (118,127).
Prolonged shedding of oseltamivir- or zanamivir-resistant virus
by severely immunocompromised patients, even after cessation
of oseltamivir treatment, has been reported (118,127–129). Rare
cases of infection with 2009 H1N1 virus resistant or with reduced
susceptibility to multiple neuraminidase inhibitors in severely immunosuppressed
pediatric patients with prolonged viral replication have
been reported