The threat of the deadly H5N1 bird flu virus adapting to humans has been inching closer towards becoming a reality all year, and we may be nearing an inflection point. Three H5N1 cases in humans have happened in the very scenarios that public health officials are concerned could lead to a pandemic in people.
“There is no clear evidence yet of human-to-human transmission, but I will not be surprised if we obtain conclusive data of such an event soon,” says at Emory University in Atlanta, Georgia. “The more the virus spills over into humans, the more likely it is to gain the potential for sustained transmission.”
The US has been a hotbed of H5N1 activity, with the virus spreading through dairy cows, bringing it into close proximity with people. Since January, 53 people in the US have tested positive for H5N1 after interacting with sick cows or chickens. Even more worrisome, however, is that an additional two people in the country have contracted it despite reporting no exposure to animals.
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The Missouri Department of Health & Senior Services announced the first of these cases on 6 September in someone who had been hospitalised for other underlying health conditions – routine flu surveillance detected the virus. Genetic sequencing showed a rare mutation in the virus, which early analysis suggests may modestly improve H5N1’s ability to bind to influenza receptors that are more common in humans than birds.
Another person in their household also developed flu-like symptoms on the same day. While they weren’t tested for influenza, blood tests showed they had antibodies for H5N1, raising the possibility of human-to-human transmission. However, the US Centers for Disease Control and Prevention (CDC) says that given the timing of their symptoms, it is more likely that the two people were exposed to the same source, such as a wild bird.
A child in California had the second case in a person without animal exposure, which was . The child tested positive for other common respiratory viruses as well, illustrating a worrisome scenario. If one of these viruses were another type of influenza, it could swap genetic material with H5N1, potentially making it more transmissible to humans, says at St. Jude Children’s Research Hospital in Tennessee. There is no reason to think this occurred, but the likelihood of it happening in the future will increase as the US enters flu season.
However, the most concerning case of H5N1 was in a teenager in British Columbia. The adolescent, who had no underlying health conditions, is in critical condition. It isn’t clear how they contracted the virus. A pet dog in their household that was sick around the same time tested negative for influenza, said at the British Columbia Ministry of Health in a press briefing on 26 November.
Genetic sequencing of the virus infecting the teenager revealed crucial for receptor binding. We don’t know the impact of these mutations yet, though shows that changes along these sites can improve a virus’s ability to bind to influenza receptors that are more prevalent in humans than birds. “This does show that under a natural infection, this virus can make changes at those critical positions,” says Webby. “I always had a bit of comfort prior to this that we really hadn’t seen any changes in these receptor binding domains as far as other cases.” That no longer seems to be the case.
All three of these infections appear to have been a dead end for the virus. Yet they underscore just how close H5N1 has come to developing mutations that could enable it to transmit between people. Of the more than to have reportedly had the virus since 2003, roughly half of them died from it, so it must be taken seriously. If the virus does gain the ability to pass from person to person, we may be in for more social distancing, lockdowns and another push to create vaccines to stave off a full-blown pandemic.
