Laboratory methodologies for testing the antiviral susceptibility of influenza viruses:
M2 ion channel inhibitor

November 2012

Current approaches to M2 inhibitor resistance testing

M2 inhibitor resistance is associated with amino acid substitutions, due to single-nucleotide polymorphism (SNP) between residues 26-34 in the transmembrane domain of the M2 protein. The most commonly detected M2 amino acid substitution that confers M2 inhibitor resistance is S31N.

The frequency of M2 inhibitor resistance in currently circulating seasonal influenza A viruses is very high (>99%) (see box below). And, the M2 inhibitors are ineffective against influenza B viruses. Consequently, WHO is not recommending the use of the M2 inhibitors for the treatment of patients infected with circulating seasonal influenza viruses.

The WHO Collaborating Centres in GISRS continue to monitor circulating viruses closely for M2 inhibitor resistance. Results are provided to GISRS on a biannual basis and ad-hoc on any event that may warrant an immediate response.

Currently M2 inhibitor resistance testing is not considered a priority activity for National Influenza Centres in GISRS.

Current frequency of M2 inhibitor resistance (as of November 2012)

Data from publicly accessible gene sequence databases, e.g. GISAID, demonstrate that virtually all recently circulating influenza A viruses are resistant to M2 inhibitors associated with M2 S31N amino acid substitution.

  • A(H3N2): >99% of strains from 2009-2011 contain the M2 S31N substitution
  • A(H1N1)pdm09: >99% of strains from 2009-2011 contain the M2 S31N substitution
  • A(H5N1): the frequency of M2 inhibitor resistance differs by clade. The following data are the frequency of the M2 S31N mutation in A(H5N1) viruses from 2008-2011
    • Clade 1 → 100% S31N (n=24)
    • Clade → 0% S31N (n=4)
    • Clade 2.2.1 → 67% S31N (n=36)
    • Clade 2.2.2 → 8% S31N (n=37)
    • Clade 2.3.2 → 8% S31N (n=59)
    • Clade 2.3.4 → 26% S31N (n=69)