Dec 12, 2008 (CIDRAP News) – With this year's US influenza epidemic barely getting started, there are already signs of increased viral resistance to oseltamivir (Tamiflu), the most widely used antiviral drug, the Centers for Disease Control and Prevention (CDC) said today.
In its flu surveillance report for Nov 30 to Dec 6, released today, the CDC said 45 of 46 influenza A/H1N1 viruses tested so far have shown resistance to oseltamivir.
All the H1N1 viruses were susceptible to zanamivir (Relenza), the other antiviral drug in the neuraminidase inhibitor class, the agency reported. All tested influenza A/H3N2 and B viruses were susceptible to both oseltamivir and zanamivir. Sixty-three percent of all the viruses tested came from only two states.
The CDC said all the H1N1 viruses also were susceptible to the adamantanes (amantadine and rimantadine), the older class of flu antivirals. However, 5 of 5 H3N2 viruses tested were resistant to the adamantanes (which do not work against type B viruses). The CDC has been recommending against using the adamantanes since January 2006 because of high resistance rates in H3N2 viruses.
With only three states reporting more than scattered flu cases so far, the CDC said it's too early to assess how common resistance to either class of drugs will be.
"Limited data on antiviral resistance, as well as the uncertainty regarding which influenza virus types or subtypes will circulate during the season, make it too early to make an accurate determination of the prevalence of influenza viruses' resistance to oseltamivir or the adamantanes . . . nationally or regionally at this time," the report said.
Increased H1N1 resistance to oseltamivir cropped up during the last flu season in the United States and a number of other countries. By the end of the season, 10.9% (106 of 969) of H1N1 viruses tested had shown the mutation associated with resistance, the CDC reported in May. All the isolates remained sensitive to zanamivir.
After the resistance problem emerged last winter, the World Health Organization said there was no evidence that the resistant H1N1 viruses caused more severe illness or spread more easily than susceptible H1N1 viruses.
In its update today, the CDC said only Hawaii, Massachusetts, and Texas reported local flu activity in early December. Another 24 states, with Washington, DC, and Puerto Rico, reported sporadic cases, and 23 states reported no flu activity.
In addition, only 1.3% of patient medical visits reported through the CDC's Outpatient Influenza-like Illness Surveillance Network were due to flu-like illness, which is well below the national baseline of 2.4%, the agency said.
The CDC said it has tested 36 isolates so far to determine how well they match up with the strains used in this year's flu vaccine. The 20 H1N1 and 3 H3N2 isolates tested were all found to be similar to the corresponding strains in the vaccine.
Of the 13 type B isolates tested, 4 belonged to the Yamagata lineage, the type used in the vaccine. However, the other 9 B isolates—8 of which were from one state—belonged to the Victoria lineage, the CDC said.
The agency said "limited to no protection" may be expected when the circulating and vaccine strains belong to different lineages. However, it said the data available so far must be interpreted cautiously because so few isolates were tested and most of them came from only two states. Also, the report said, the results may not correlate with clinical protection conferred by the vaccine because the testing involves ferret serum.
The B strain in the seasonal vaccine was changed from the Victoria lineage to the Yamagata lineage for this season. The switch was prompted by a mismatch last year, when most of the type B viruses in circulation were of the Yamagata lineage.
CDC flu surveillance report for Nov 30 through Dec 6
CDC flu surveillance report for the week ended May 10
Feb 1 CIDRAP News story "Europe says Tamiflu-resistant virus seen in 9 countries"
Feb 14 CIDRAP News story "WHO advises total makeover for 2008-09 flu vaccine"