Study details reduced Tamiflu susceptibility in H1N1 flu cases

A study yesterday in Emerging Infectious Diseases spotlights recent H1N1 flu cases with reduced susceptibility to oseltamivir (Tamiflu), the most common antiviral used to limit symptoms from seasonal flu. 

The reduction in susceptibility has been seen in viruses collected from five continents—with most cases in Europe—from May 20203 to February 2024. The viruses showed a 13-fold reduced inhibition by oseltamivir while retaining normal susceptibility to other antiviral drugs, the authors said. 

The detections were made as part of standard genetic sequencing of influenza viruses, and included 2,039 H1N1 viruses from the United States (1,274) and 38 other countries (765).

Most detections made in Europe 

The viruses that lost susceptibility to Tamiflu had acquired a novel combination of neuraminidase mutations, NA-I223V + S247N. Overall detection frequency of these two mutations was low (0.67%). Of 101 viruses with this double mutation, 67 were found in Europe. 

“The first dual mutant was collected from Canada in May 2023, and the latest were collected from 4 countries (France, the Netherlands, Spain, and the United Kingdom) during January–February 2024,“ the authors wrote. 

The Netherlands had the most detections (30), followed by France (24), Bangladesh (11), Oman (9), and the United Kingdom (9). Hong Kong had 4 detections, followed by Niger (3), Australia (2), Spain (2), and the United States (2). One dual mutant each was detected in Canada, Ethiopia, Maldives, Norway, and Sweden.

Our study highlights the need to closely monitor evolution of dual mutants .

“Our study highlights the need to closely monitor evolution of dual mutants because additional changes may further affect susceptibility to antiviral drugs or provide a competitive advantage over circulating wild-type viruses,” the authors concluded.