# 6042
A correspondence appears in today’s NEJM that provides some detailed information on a story we began to follow last August (see Australia Reports Cluster Of Antiviral Resistant H1N1); the detection of an unusual number of oseltamivir (Tamiflu ®) resistant H1N1 viruses in and around the Newcastle area of New South Wales.
First, a little background.
During 2008 and early 2009 -prior to the emergence of the 2009 H1N1 pandemic virus - the old seasonal H1N1 virus developed nearly complete resistance to the antiviral drug oseltamivir.
The H1N1pdm09 virus which replaced the old H1N1 – while resistant to the older amantadines – has remained largely sensitive to oseltamivir. The concern is, that over time, this newer strain might one day develop resistance as well.
During the first two years, only 1%-2% of samples tested have shown the most common mutation known to convey oseltamivir resistance; H275Y, where a single amino acid substitution (histidine (H) to tyrosine (Y)) occurs at the neuraminidase position 275.
(Note: some scientists use 'N2 numbering' (H274Y) and some use 'N1 numbering' (H275Y))
Most of the resistant cases we’ve seen reported have been isolated and sporadic, with no apparent epidemiological links. They have often occurred in patients under therapeutic or prophylactic treatment with oseltamivir, and are assumed to have been the result of spontaneous resistance.
Which brings us to today’s report in the NEJM that looks at the:
Community Transmission of Oseltamivir-Resistant A(H1N1)pdm09 Influenza
N Engl J Med 2011; 365:2541-2542 December 29, 2011
What the authors found was evidence for the sustained community transmission of a resistant strain of the H1N1pdm09 virus.
After analyzing viral samples pulled from 182 patients seen in emergency departments, intensive care units, and doctor’s offices in New South Wales between May and August of 2011, they found 29 (16%) carried the H275Y resistance mutation.
Most of the patients lived within 50km of Newcastle, and while 10 of the cases could be epidemiologically linked (2 in 4 households, 2 in a shared car trip), the rest could not.
Only one had been treated with oseltamivir prior to testing.
The good news is that while 7 of these cases were hospitalized (24%), none ended up in the Intensive care unit, and none died. This resistant strain also appears to be antigenically similar to the vaccine strain.
The authors recommend:
As winter approaches in the Northern Hemisphere, it remains important to ensure that A(H1N1)pdm09 strains from early in the season are analyzed rapidly for any indication that this transmissible oseltamivir-resistant variant has spread.
So far, this appears to be regional phenomenon, and numbers like these have not been reported in other parts of the world.
For more on this evolving story, you may wish to revisit:
WER: Update On Anti-Viral Resistant Influenza
ECDC: Risk Assessment On Australia’s Antiviral Resistant H1N1 Cluster
