Pasteurisation temperatures effectively inactivate influenza A viruses in milk

In late 2023 an H5N1 lineage of high pathogenicity avian influenza virus (HPAIV) began circulating in American dairy cattle¹. Concerningly, high titres of virus were detected in cows’ milk, raising the concern that milk could be a route of human infection. Cows’ milk is typically pasteurised to render it safe for human consumption, but the effectiveness of pasteurisation on influenza viruses in milk was uncertain. To assess this, we evaluated heat inactivation in milk for a panel of different influenza viruses. This included human and avian influenza A viruses (IAVs), an influenza D virus that naturally infects cattle, and recombinant IAVs carrying contemporary avian or bovine H5N1 glycoproteins. At pasteurisation temperatures, viral infectivity was rapidly lost and became undetectable before the times recommended for pasteurisation. We then showed that an H5N1 HPAIV in milk was effectively inactivated by a comparable treatment, even though its genetic material remained detectable. We conclude that industry standard pasteurisation conditions should effectively inactivate H5N1 HPAIV in cows’ milk, but that unpasteurised milk could carry infectious influenza viruses.