Threatwatch is your early warning system for global dangers, from nuclear peril to deadly viral outbreaks. Debora MacKenzie highlights the threats to civilisation – and suggests solutions

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H7N9 first started infecting people in China in 2013. Like its cousin H5N1, the virus that drew attention to bird flu in 2004, it mainly infects birds and doesn’t readily pass from human to human – but should it acquire this ability a deadly pandemic could ensue.

H7N9 seems to jump to people from poultry more easily than H5N1, staging regular winter outbreaks in the last 4 years. By mid-2016 there were 798 known cases, and around 40 per cent of the people died. But since last October alone, there have been 424, the most ever seen in one season – and it isn’t over yet.

“I suspect the spike in cases of H7N9 is real,” says Malik Peiris of Hong Kong University, and not due to better diagnosis. He thinks the jump is due to an increase in poultry infections. Tests in poultry markets are finding H7N9 more often, he says, and it is spreading: this winter has seen human cases in 18 provinces of mainland China, including for the first time in southern Yunnan province, and it could spread to Vietnam from there.


When people fall ill

But we only know this because someone in Yunnan became severely ill with the virus. H7N9 spreads in poultry without making birds visibly sick. It is often only discovered when people fall ill.

Most were exposed to the virus in live poultry markets. Despite calls to close them, public demand for freshly killed chicken keeps markets open – although four of the hardest-hit provinces in China have now temporarily closed some markets.

But H7N9 could be coming out of hiding. This week both mainland China and Taiwan reported human cases in which the virus’s haemagglutinin surface protein had a mutation that makes it lethal to chickens. This would make it a “highly pathogenic” bird flu like H5N1 and its descendants such as H5N8, which is killing birds across Eurasia.

While the mutation doesn’t affect illness in people, it allows the virus to replicate much faster in chickens. If the mutation spreads in poultry, as it has with other kinds of bird flu, H7N9 will rip through flocks, making its presence much easier to spot.

But the trouble is these sick birds will shed much more of the virus, meaning more cases in people and perhaps other mammals such as pigs, each an opportunity for H7N9 to adapt to mammals and learn to spread from person to person. H7N9 already has some of the mutations thought to be required before bird flu can do this, and it is already capable of limited spread between ferrets, the best animal model for human flu.

There could also be more cases of H7N9 in people than we think, says Ab Osterhaus of the Research Centre for Emerging Infections and Zoonoses in Hannover, Germany. Usually, only people sick enough to require a trip to hospital are tested to see which virus they have. Two of the cases reported by the World Health Organisation this week were mild, but the individuals were tested because of exposure to known cases. There could be many more mild cases.

Our only real defence, say the virologists, is a vaccine. The WHO has approved eight vaccine strains of H7N9, and last week China launched clinical trials of four strains by a state-owned vaccine company.

But even if the trials are successful, WHO officials admit that we still have no means of making enough flu vaccine in time to protect large numbers of people, should H7N9, or any other flu virus, go pandemic.