processunderstand

Putting the pieces together

May 28th 2009 | NEW YORK
From The Economist print edition

A better understanding of how the new strain of influenza arose

YOU are now officially permitted to blame the pigs. When a strain of influenza with pandemic potential struck in April, it was generally referred to as “swine flu” because it seemed similar to an existing group of strains, known as A/H1N1, which are commonly found in pigs. But when it became clear that the new bug was being spread by people, not porkers, the pig-breeding industry complained that it was being unfairly maligned. It also became apparent that the new virus contains bits and pieces derived from avian and human strains of influenza, as well as porcine ones, further muddying its origins.

A new study, however, suggests pigs really were to blame. Several dozen researchers, led by Rebecca Garten of America’s Centres for Disease Control and Prevention (CDC), sequenced full or partial genomes of 76 samples of the new virus, which has afflicted almost 13,000 people around the world so far. In a paper published in Science, they confirm that the closest genetic relatives of the new virus are swine-flu strains from both North America and Eurasia. The virus is made of eight gene segments of known provenance but which have not previously been seen in this combination. The genetic material in them is indeed a hotchpotch derived from avian, human and swine sources, but all eight segments come most recently from pigs.

On the positive side, the researchers discovered that the antigens (proteins that provoke an immune response) found in different samples have remained similar to each other. That, according to Derek Smith of Cambridge University, one of the paper’s authors, may make it easier to design a vaccine against the new virus. The World Health Organisation and a number of European governments are now talking to manufacturers about expediting the development of such a vaccine and, on May 22nd, American officials announced a $1 billion scheme with the same goal.

The worrying news is that the study found the genetic material in the new influenza is significantly different from that in its known close relatives. This, they conclude, means the bug was circulating and evolving undetected in swine for quite some time before the first people were infected. In other words, the early-warning systems did not work.

Another cause for concern is that Canadian officials recently confirmed that the new strain has hopped from humans back into pigs in Alberta. Because swine are natural mixing vessels for influenza strains from different species (hence the hotchpotch found by Dr Garten and her colleagues), the fear is that a deadlier form could emerge and jump back into humans.

This new study does not answer the big questions of how, exactly, the virus crossed over to humans and why it kills some people and not others—in particular, why it hits the young (and thus, presumably, healthy) harder than the elderly. A different study by the CDC has found that nearly two-thirds of swine-flu infections in America have been in people aged between five and 24, whereas only 1% of cases affected those over 65. This is the reverse of the pattern seen in seasonal flu, which kills thousands of old people every winter. One possible explanation, according to Anne Schuchat of the CDC, is that “older adults might have been in contact a long time ago with a virus similar to the one we see now.” That, she surmises, might give them an immunity to this new menace that young people lack.