New: in vitro testing of more drugs against the 2.3.4.4b (most common) strain of bird flu

Just in case a deadly mutant appears, it is good to know there exist other drugs to try

Oseltamvir (Tamiflu) is 10x less active (in vitro) against bird flu compared to human influenza A viruses, in whom it barely works. But CDC says to use it. Luckily, it seems there are a number of other drugs that may be effective, based on in vitro studies this month.

https://www.cdc.gov/bird-flu/spotlights/h5n1-response-03192025.html

https://wwwnc.cdc.gov/eid/article/31/4/24-1820_article

Antiviral susceptibility

CDC regularly performs sequencing of seasonal influenza A and B viruses and novel influenza A viruses, including A(H5N1) viruses, to assess for genetic changes known to be associated with antiviral resistance. In a new CDC study published in Emerging Infectious Diseases on March 7, 2025, CDC scientists assessed the antiviral susceptibility of clade 2.3.2.1c A(H5N1) viruses and clade 2.3.4.4b A(H5N1) viruses collected from humans in Cambodia, United States, and Chile. The study found that except for two viruses isolated from humans in Cambodia, all viruses were susceptible to M2 ion channel-blockers [amantadine and rimantadine] in cell culture-based assays. All viruses were susceptible to the PA cap-dependent endonuclease inhibitor class of antiviral drugs, baloxavir and tivoxavir, and to the polymerase basic 2 (PB2) inhibitor antiviral drug, pimodivir. All viruses also displayed susceptibility to neuraminidase inhibitor class of antiviral drugs, which includes oseltamivir, zanamivir, peramivir, laninamivir, and AV5080. Oseltamivir was approximately 10-fold less active at inhibiting the neuraminidase activity of clade 2.3.4.4b viruses and approximately 3-fold less active against clade 2.3.2.1c viruses, when compared to seasonal influenza A viruses. The clinical significance of these laboratory findings, however, is unknown. Significant reduction in antiviral susceptibility is considered to be greater than 100-fold reduction. The laboratory findings in this study, therefore, indicate that these A(H5N1) viruses are likely to retain susceptibility to oseltamivir. Additionally, these findings do not support changing the current recommendations for antiviral treatment of human infections with novel influenza A viruses, including A(H5). CDC continues to recommend prompt treatment with oseltamivir for people with confirmed or suspected A(H5N1) virus infection. Flu antiviral drugs, including oseltamivir, work best when started as soon as possible, ideally within two days after flu symptoms begin.

* One additional case was previously detected in Colorado in a poultry worker who experienced mild illness in 2022.

According to CDC, Oseltamivir resistance has been reported in hospitalized patients with HPAI A(H5N1) virus infection, resulting in fatal outcome. CDC cites several studies on the benefits of oseltamvir in bird flu cases, but all were published 12-17 years ago and studied Asian outbreaks, with older strains.