Tamiflu Falls Short Against Bird Flu as CDC Study Backs Newer Antiviral

Tamiflu Shows Limited Effectiveness Against H5N1

A study published by CDC researchers in Communications Biology has found that oseltamivir, the antiviral medication sold as Tamiflu, provided little or no clinical or virologic benefit to ferrets infected with H5N1 bird flu compared to untreated animals. The findings raise serious questions about current pandemic preparedness strategies, given that Tamiflu comprises approximately eighty percent of global antiviral stockpiles.

Xofluza Emerges as Superior Treatment

The research evaluated two antiviral drugs against the D1.1 genotype of H5N1 clade 2.3.4.4b virus, the strain currently infecting poultry farm workers in the United States. Ferrets treated with baloxavir, marketed as Xofluza, showed significant reductions in clinical signs including fever, weight loss, and viral replication when compared to both untreated animals and those receiving Tamiflu.

The study tested both drugs individually and in combination, with treatment beginning either twenty four or forty eight hours after infection. While Xofluza alone performed well, some ferrets experienced a late rise in fever and viral shedding after four days of treatment, suggesting a potential viral rebound effect.

Combination Therapy Prevents Rebound

Combination therapy using both drugs provided similar clinical protection to Xofluza alone but crucially prevented the rebound effect. The researchers noted that combination treatment administered before or after symptom onset resulted in significant improvements compared to Tamiflu monotherapy.

Resistance Concerns Add Urgency

Adding to concerns about current stockpile strategies, researchers in Canada recently identified an H275Y mutation conferring Tamiflu resistance in H5N1 viruses from eight poultry farms. This substitution, described as exceptionally rare among clade 2.3.4.4b viruses, spread rapidly despite presumed fitness costs.

Stockpile Implications

The limited proportion of Xofluza in current stockpiles reflects its more recent regulatory approval, higher cost, and narrower paediatric labelling compared to Tamiflu. While ferrets serve as valuable models for influenza research, the CDC researchers cautioned that results may not be fully applicable to humans. Current CDC guidance still recommends Tamiflu as the first line empiric treatment for patients with suspected H5N1 infections.