Influenza Remains Elevated & Slight Uptick In Antiviral Resistance

Influenza activity remains elevated across the nation, marking the ninth consecutive week above the national baseline. Recent data from the CDC’s FluView report for Week 5 indicates a slight uptick in overall cases, driven in part by a rise in influenza B infections. However, a less prominent but potentially concerning trend is emerging: increasing signs of antiviral resistance in circulating influenza viruses.

Antiviral Resistance on the Rise

While the overall levels of resistance remain low, the CDC’s latest surveillance data reveals a small but notable increase in viruses exhibiting reduced susceptibility to antiviral medications. This development is being closely monitored, as similar patterns have been observed globally over the past two years, particularly in seasonal H1N1 and, less frequently, in HPAI H5 avian influenza.

Did You Know? Between September 28th and the end of December, testing of 193 viruses revealed zero instances of antiviral resistance.

Specifically, analysis of 233 additional H1N1 viruses tested since that period has identified 10 (4.29%) demonstrating varying levels of resistance to oseltamivir. Three A(H1N1)pdm09 viruses exhibited an amino acid substitution conferring highly reduced inhibition by both oseltamivir and peramivir. Seven more A(H1N1)pdm09 viruses showed reduced inhibition by oseltamivir due to different amino acid substitutions. Two B viruses displayed reduced inhibition by peramivir.

A Historical Context

This increase in resistance is occurring after a period of relative susceptibility. In contrast to the Week 53 FluView report, which showed zero resistance among 193 viruses tested, the current data signals a shift. The emergence of resistance is not unprecedented; in 2006, amantadine lost its effectiveness, and oseltamivir faced a similar challenge in 2008 – though it later regained efficacy with the arrival of a susceptible pandemic H1N1 strain in 2009.

Expert Insight: The cyclical nature of antiviral resistance underscores the ongoing evolutionary battle between viruses and the drugs designed to combat them. While current levels are not alarming, continued monitoring and research are crucial to anticipate and mitigate potential impacts on treatment effectiveness.

Reports from other countries, such as a March 2024 Lancet correspondence detailing a higher incidence of oseltamivir resistance in Hong Kong, and China’s Influenza Surveillance Weekly Report, which has reported roughly 4% resistance in H1N1 viruses since last summer, further highlight this global trend. The Chinese report indicated that 3.9% (32/829) of A(H1N1)pdm09 strains showed reduced or highly reduced sensitivity to neuraminidase inhibitors.

What Could Happen Next?

If these trends continue, we could see a gradual erosion of the effectiveness of oseltamivir, currently the most widely available and affordable antiviral. This could necessitate increased reliance on alternative treatments like baloxavir, which is not as readily accessible globally. It is also possible that the current trend could stabilize or even reverse, as has happened with oseltamivir in the past. However, the emergence of resistance underscores the need for continued surveillance and the development of new antiviral strategies.

Frequently Asked Questions

What is oseltamivir?

Oseltamivir is an antiviral medication used for the treatment and prevention of influenza A and B viruses.

What does it mean for a virus to show “reduced susceptibility” to an antiviral?

It means the virus is less easily inhibited by the antiviral medication, potentially requiring higher doses or alternative treatments for effective control.

Are adamantanes (amantadine and rimantadine) still effective against influenza?

No, high levels of resistance to adamantanes persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses, and their use is not recommended.

As antiviral resistance continues to evolve, what role will ongoing surveillance play in protecting public health?