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Drug-Resistant H5N1 Influenza Viruses May Retain Fitness

By Eric Toner, M.D., November 1, 2007

Should widespread resistance to antiviral drugs occur in a pandemic influenza strain, the chance of limiting the disease outbreak, either locally or globally, or treating the sick with antiviral medications would be greatly diminished. This has been of particular concern because many isolates of H5N1, the virus currently considered to have the greatest pandemic potential, are known to be resistant to one class of antivirals, the M2 inhibitors, and a few isolates have evidenced resistance to the only other class of influenza antivirals, the neuraminidase inhibitors (NAI). It has been posited, however, that the risk of NAI resistance becoming widespread might be relatively low because it was thought that as influenza viruses acquire NAI resistance, they become less fit and as a result, cannot be transmitted efficiently. 

Now, a  study by Yen, Ilyushina, and colleagues from Robert Webster’s laboratory at St. Jude’s Childrens’ Research Hospital, reveals that although some mutations conferring NAI resistance are indeed associated with a diminished capacity to replicate, other such mutations do not lead to loss of fitness.

The Current Study

To investigate the fitness (pathogenicity and replication efficiency) of NAI-resistant H5N1, Yen and colleagues used reverse genetics to create recombinant H5N1 influenza viruses with either wild-type neuraminidase (NA) or one of four single amino acid changes found in nature and known to be associated with NAI resistance: E119G, H274Y, R292K, and N294S. For comparison, they created recombinant H1N1 viruses with the same mutations.1

Effect on Viral Replication

The researchers found that two of the four mutations (E119G and R292K) resulted in H5N1 and H1N1 viruses that did not grow well in vitro and could not be stably maintained. The other two mutations (H274Y and N294S) resulted in viruses that replicated as efficiently in vitro as wild-type H5N1 and H1N1 viruses. H274Y resulted in markedly reduced susceptibility to oseltamivir but not zanamivir in both H5N1 and H1N1. N294S resulted in moderately reduced susceptibility to oseltamivir in both viruses but also slightly reduced susceptibility to zanamivir.

Effect on Pathogenicity

The H5N1 viruses with either of the two mutations were just as lethal to mice as wild-type H5N1; however, the H1N1 viruses with either mutation did show decreased lethality. The explanation for this may be that although both mutations resulted in decreased NA enzymatic activity in both H5N1 and H1N1, the NA enzymatic activity of the wild-type H5N1 was significantly higher than that of the wild-type H1N1. Therefore, the reduced NA function did not affect the H5N1 virus fitness but did affect that of the H1N1. The function of NA is to enable release of the newly created virus from the host cell; therefore, reduced NA activity would be expected to lead to a reduced ability to replicate and cause disease.

The Frequency of Resistance

Resistance to the M2 inhibitor class of antivirals develops rapidly during use of these drugs, and the drug-resistant viruses remain pathogenic and readily transmissible. Today, as a result,  most human H3N2 and many avian H5N1 isolates are resistant to this older class of influenza antiviral drugs, which includes amantadine and rimantidine.2 Resistance to the other class of influenza antivirals, the NAIs, which includes oseltamivir (Tamiflu®) and zanamivir, is much less frequent (0.5% of 2287 community isolates tested between 1999 and 2002).3  This low frequency has been explained by the fact that although resistant variants emerge in up to 18% of children and 1% of adults during therapy, NAI-resistant strains of H3N2 have compromised fitness compared with non-resistant strains and thus do not  become widespread in the community.2

NAI-Resistant H5N1 in Humans

There have been three case reports (totaling 5 patients) of NAI-resistance present in H5N1 isolates from humans. Two of the patients had viruses with H274Y mutations, two had viruses with N294S mutations and one had both. Of the two patients with N294S mutated viruses, the mutation was present in specimens taken before the start of antiviral therapy, suggesting that the mutated virus had been acquired and thus was able to be transmitted.1 


The use of NAIs for prophylaxis and treatment is one of the main pillars of pandemic influenza response. This study indicates that the risk of an influenza pandemic in which antivirals have limited efficacy may be greater than previously thought. 


  1. Yen HL, Ilyushina NA, Salomon R, et al.  Neuraminidase inhibitor-resistant recombinant a/vietnam/1203/04 (H5N1) influenza viruses retain their replication efficiency and pathogenicity in vitro and in vivo. J Vir 2007:81(22); 12418-12426. Accessed October 31, 2007.

  2. Hayden F. Antiviral resistance in influenza viruses—implications for management and pandemic response. NEJM 2006: 354;8. Accessed October 31, 2007.

  3. Monto AS, McKimm-Breschkin JL, Macken C. Detection of influenza viruses resistant to neuraminidase inhibitors in global surveillance during the first 3 years of their use. Antimicro Agents Chemo 2006:50(7); 2395–2402. Accessed October 31, 2007.