Avian influenza virus (H5N1) has infected wild and domestic birds worldwide since the 1990s. The first transmissions from chickens to humans were recorded in 1997, and of 600 human infections there was 30-50% mortality. In recent years the virus has been transmitted to several mammalian species including minks and sea lions, and it spread to cattle in 2024 causing renewed concern for transmission to humans. There have been several cow-to-human virus transmission events reported in the last year, and one death caused by an H5N1 variant reported in December. So far there have been no reports of human-to-human transmission of these H5N1 flu viruses.
The annual flu shot that you get each year (right?) changes because the flu virus mutates rapidly, and this year’s strain is likely to be different from last year’s.The changes are in the two influenza virus proteins, one that mediates binding to human epithelial cells, H for hemagglutinin, and the second that causes the release of virus from infected cells, N for neuraminidase. The influenza viruses currently circulating in humans are subtype A(H1N1) and A(H3N2). Spread of the H5N1 bird flu variant would require a new vaccine.
A recent paper from the Scripps Research Institute published in the December 9th, 2024 issue of the journal Science raised widespread concerns that a virulent H5N1 mutant virus could cause a pandemic in humans. The title of the paper is “A single mutation in bovine H5N1 hemagglutinin switches specificity to human receptors.” The single amino acid change was in the receptor binding domain of the influenza hemagglutinin (H) protein that recognizes sugar-like molecules on the surface of avian or human epithelial cells lining the respiratory tract. These sugar-like molecules are different between birds and humans, and the single mutation switched binding specificity from the bird sugar alone to both the bird and the human versions. The influenza H protein used in these studies was from a patient in Texas who had been infected by contact with a bovine virus, so the starting virus was capable of human infection but still retained the H5 avian sugar specificity.
The possibility that only one mutation might lead to human-to-human transmission surprised many flu experts who thought that 2 or more mutations would be required for efficient airborne transmission. One common concern is that bird flu could infect someone who also was sick with the current circulating flu viruses, resulting a hybrid strain with properties of both.
It is impossible to predict when and if the required mutations in the bird influenza virus will occur. The fewer mutations needed, the more likely that they will occur. Each transmission event from bird or cow to humans poses a new risk, so reducing these exposures seems prudent. Unfortunately, we are playing Russian Roulette with a flu virus that could be quite deadly.Until a flu vaccine against H5N1 viruses is available, the best advice is to avoid contact with birds and cattle, or to wear an N95 mask if contact is unavoidable. Do not drink raw milk as it could contain H5N1 virus.