I thought so too, but later on in the paper they mention that they performed necropsy on the mice that survived.

> samples from surviving mice on day 12 were also obtained through necropsy to measure viral titers.

Not only that, but just a month earlier, South Korean scientists published another Virology Journal paper revealing that they had engineered a chimeric H5N1 virus using hallmark gain-of-function (GOF) techniques, combining gene segments from three different influenza viruses to increase the virus's heat resistance, alter host targeting, and enhance human cell entry.

"Recombinant viruses were generated using a pHW2000 plasmid-based reverse genetics system."

"Combining the R90K and H110Y mutations (22W_KY) resulted in a synergistic increase in thermal stability and maintained HA activity without measurable reduction even after 4 h at 52 °C."

"22 W HA and 22 W NA genes, along with six internal genomic segments (PB2, PB1, PA, NP, M, NS) from PR8 and a PB2 gene from 01310 containing the I66M, I109V, and I133V (MVV) mutations"

The study also confirmed enhanced antigen uptake and intracellular penetration in human cells:

"The highest level of intracellular entry was observed for BEI_22W_KY, confirming its superior effectiveness in penetrating cells."

Ref: https://virologyj.biomedcentral.com/articles/10.1186/s12985-...