A universal flu vaccine sounds almost too good to be true. Some experts still think it is, but that has not stopped a number of companies pursuing the holy grail.

Like Acambis, most have focused on the M2e peptide. Their hope is to hit on a vaccine that will produce long-lived protection from a couple of shots, thus eliminating annual boosters, the guesswork of creating a seasonal vaccine before the flu season begins, and millions of fertilised chicken eggs.

Adversity provokes ingenuity. The fear of a global pandemic that will kill millions has pumped life into a moribund corner of medicine that used outdated technology to make a highly perishable commodity. The idea of a universal vaccine is easy to grasp and good progress is being made, but there are still doubts.

The M2e peptide is an apparently unchanging feature of influenza A, but that could be because the flu virus has never been challenged in this way before. Experiments reported in 2005 show that such a challenge, in mice, does produce two mutations in M2e, in three weeks. That is quite encouraging, because if there are so few mutations it should be possible to cover them with a vaccine, too.

A second issue is how well M2e vaccines will protect against flu. The antibodies generated by the vaccine seem to work by binding to infected cells and clearing them, instead of stopping the virus infecting new cells. If this is borne out then the new vaccines may diminish the severity of illness and save lives, but not prevent disease.

The final question is how long protection would last. There is evidence that adults exposed to the H1N1 virus before the 1957 flu pandemic were partly protected from it, even though it was caused by H2N2. So that, too, is encouraging.

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