Published at www.nejm.org December 9, 2009 (10.1056/NEJMc0910448) To the Editor: Oseltamivir-resistant infection with the 2009
pandemic influenza A (H1N1) virus has so far been described
only rarely and is conferred by the H275Y substitution in the
neuraminidase enzyme.
1 Only 3 of the 32 patients with oseltamivir-resistant
infection reported on as of this writing were not receiving
oseltamivir when the resistant viruses were detected, and ongoing
community transmission has not yet been shown.
1 However, the
emergence of oseltamivir-resistant 2009 H1N1 influenza remains
a grave concern, since widespread oseltamivir resistance has
been observed in seasonal H1N1. This resistance was unrelated
to selective drug pressure, and the H275Y substitution did not
appear to reduce transmissibility or severity.
2,3 We report
on a cluster of seven cases of oseltamivir-resistant 2009 H1N1
infection in Vietnam.
In July 2009, during a 42-hour journey, 10 students socialized
together in the same train carriage. None of the students knew
each other before the journey, none had contact with a person
with suspected influenza in the week before the trip, none were
symptomatic during the journey, and none were previously or
currently receiving oseltamivir. Fever developed in four of
the students within 12 hours after arrival and in two more students
within 48 hours after arrival (Fig. 1 in the
Supplementary Appendix,
available with the full text of this letter at NEJM.org). An
additional case was identified in a traveler in a different
carriage (Patient G). Nasal swabs, throat swabs, or both from
all seven persons were positive for 2009 H1N1 RNA when tested
with reverse-transcriptase–polymerase-chain-reaction (RT-PCR)
assays, and viruses were successfully cultured from specimens
obtained from three of the persons. The H275Y substitution was
detected retrospectively in diagnostic specimens obtained from
all seven subjects before any oseltamivir treatment. The concentrations
of oseltamivir carboxylate required for a 50% inhibition of
neuraminidase activity of the isolated viruses in a fluorometric
neuraminidase-inhibition assay were 323.6, 429.5, and 889.2
nM; these concentrations confirmed resistance
4 (see the
Supplementary Appendix).
Six patients were admitted to a hospital for isolation, one
patient was isolated at home, and all were treated with oseltamivir
phosphate at a dose of 75 mg twice daily (Fig. 1 in the
Supplementary Appendix), since resistance testing had not yet been performed.
All patients recovered uneventfully, although one patient (Patient
F), with the highest 50% inhibitory concentration, continued
to test positive on RT-PCR until day 9, despite receiving oseltamivir
from the day of the onset of illness (Fig. 1 in the
Supplementary Appendix). An extensive public health investigation did not
identify additional patients or the index patient.
In this cluster, infection developed in at least 6 of the 10
people who were probably exposed to the index patient; this
shows that resistant 2009 H1N1 viruses are transmissible and
can replicate and cause illness in healthy people in the absence
of selective drug pressure. Ongoing transmission from the cluster
was not detected, but the tracing of all contacts was not possible,
so ongoing transmission cannot be ruled out.
However, only three
other resistant cases have been detected in Vietnam as of this
writing, and all were due to selection of resistant viruses
during treatment rather than person-to-person transmission.
Although data are limited, it is likely that the detected levels
of oseltamivir resistance are clinically relevant.
5 The loss
of oseltamivir as a treatment option for severe 2009 H1N1 infection
could have profound consequences. To minimize this risk, the
use of oseltamivir should be restricted to prophylaxis and treatment
in high-risk persons or the treatment of people with severe
or deteriorating illness, antiviral stockpiles should be diversified,
and optimal dosages and combination therapies should be urgently
studied.
Close monitoring and reporting of resistance to neuraminidase
inhibitors are essential.
Le Quynh Mai, M.D., Ph.D.
National Institute of Hygiene and Epidemiology Hanoi, VietnamHeiman F.L. Wertheim, M.D., Ph.D.
Oxford University Clinical Research Unit Hanoi, VietnamTran Nhu Duong, M.D., Ph.D.
National Institute of Hygiene and Epidemiology Hanoi, VietnamH. Rogier van Doorn, M.D., Ph.D.
Oxford University Clinical Research Unit Ho Chi Minh City, VietnamNguyen Tran Hien, M.D., Ph.D.
National Institute of Hygiene and Epidemiology Hanoi, VietnamPeter Horby, M.B., B.S., F.F.P.H.
Oxford University Clinical Research Unit Hanoi, Vietnampeter.horby@gmail.com for the Vietnam H1N1 Investigation Team
Supported by grants from the Wellcome Trust United Kingdom (081613/Z/06/Z and 077078/Z/05/Z, to Drs. Wertheim, van Doorn, and Horby) and the South East Asia Infectious Disease Clinical Research Network (N01-A0-50042, to Drs. Wertheim, van Doorn, and Horby). Financial and other disclosures provided by the authors are available with the full text of this letter at NEJM.org. This letter (10.1056/NEJMc0910448) was published on December 9, 2009, at NEJM.org.
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