By Dorothy H. Crawford
“It’s only a virus.” How often do GPs utter those words over the course of a working day? They mean, of course, that your symptoms are mild, non-specific, and don’t warrant any treatment. If you just go home and rest you’ll recover in a few days. But that does not apply to all viruses, and the flu is one that can vary enormously in severity. It may be relatively mild like the 2009 pandemic strain, but it can also be a killer. Just consider the H5N1 bird flu that emerged in 1997, which kills around 60% of those it infects. Now a new bird flu virus is on the loose — H7N9 — that seems to be almost as deadly as its predecessor.
H7N9 was first reported this February in China. By the beginning of June, it had caused 132 known infections, including 37 deaths. This has galvanised flu experts around the world into action. While the virus hunters are trying to locate the origin of the outbreak and stop the virus from spreading any further, other scientists are racing to make a vaccine against it.
So far there is both good news and bad news to report. One piece of good news is that, to date, H7N9 has not succeeded in spreading effectively from one person to another. At present, each new infection seems to be acquired either directly or indirectly from an infected animal — presumably some kind of domestic poultry. But the bad news is that flu viruses mutate quickly and this one already has many of the mutations it needs to spread easily between humans — a recipe for a pandemic.
Again, on the positive side, H7N9 has not yet spread outside China, and at the time of writing, there seems to be a lull in new cases. However, cases are scattered widely across China with no obvious source. Scientists must find its origin in order to stop it from spreading further, generally by closing local live poultry markets and culling infected flocks. But so far they are baffled. While they have detected infected birds in several markets, they can’t find the virus on any of the farms that supply the birds. Until they pinpoint the source, the question of whether to close markets remains an open one; while on the one hand this would prevent transport of infected birds, on the other it risks the trade going underground.
Scientists trying to make a vaccine also face a dilemma. Flu vaccines take several months to prepare so it’s best to get ahead of the game, but start too early and the virus may mutate to become unrecognisable to immunity generated by the vaccine. Generally the gap between the start of an epidemic and vaccine availability is plugged by using anti-viral drugs. But the Chinese often give anti-virals to poultry and so, unsurprisingly, some cases are already resistant to drugs like Tamiflu. Another piece of bad news comes from early stage vaccine development. Apparently, H7N9 only stimulates a weak immune response. This means that around 13 times more of the vaccine would be needed to protect against this virus than is required with other flu strains. So if there is a pandemic there will be less vaccine to go around. Conventional flu vaccine strains are laboriously grown in hens’ eggs, making the whole process difficult to scale up. However, on the plus side, a synthetic vaccine is being developed which could cut the production time by up to a month.
With all the uncertainty about the origin of H7N9, the speed and randomness of its mutations and the unpredictability of its spread, we are left with many unanswered questions to discuss and debate.
Dorothy H. Crawford is the Robert Irvine Professor of Medical Microbiology and the Assistant Principal for Public Understanding of Medicine at the University of Edinburgh. She is the author of Virus Hunt: The search for the origin of HIV/AIDs, as well as The Invisible Enemy: A Natural History of Viruses , Deadly Companions: How Microbes Shaped our History, and Viruses: A Very Short Introduction.