Health

Keeping swine flu at bay (Image: Leander Baerenz/Getty)

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Common cold may hold off swine flu

• 12 November 2009 by Debora MacKenzie
• Magazine issue 2734. Subscribe and get 4 free issues.
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A VIRUS that causes the common cold may be saving people from swine flu. If this intriguing idea turns out to be true, it would explain why swine flu's autumn wave has been slow to take off in some countries and point to new ways to fight flu.

"It is really surprising that there has not been more pandemic flu activity in many European countries," says Arnold Monto, an epidemiologist at the University of Michigan, Ann Arbor.

It is really surprising that there has not been more pandemic flu activity in many European countries

In France, flu cases rose in early September, then stayed at about 160 per 100,000 people until late October, when numbers started rising again. The delayed rise was puzzling, says Jean-Sebastien Casalegno of the French national flu lab at the University of Lyon.

He reports that the percentage of throat swabs from French respiratory illnesses that tested positive for swine flu fell in September, while at the same time rhinovirus, which causes colds, rose (Eurosurveillance, vol 14, p 19390). He told New Scientist that in late October, rhinovirus fell - at the same time as flu rose. He suspects rhinovirus may have blocked the spread of swine flu via a process called viral interference.

This is thought to occur when one virus blocks another. "We think that when you get one infection, it turns on your antiviral defences, and excludes the other viruses," says Ab Osterhaus at the University of Rotterdam in the Netherlands.

How important such interference is in viral epidemics is unclear, however: there are also cases in which there is no interference, and people catch two viruses at the same time. Normally, we don't get a chance to see how rhinovirus affects flu, as flu epidemics usually strike in winter, whereas rhinovirus hits when schools start (late summer in the northern hemisphere).

But this year the pandemic meant flu came early - and France isn't the only country in which rhinovirus seems to have held it at bay. In Eurosurveillance last month, Mia Brytting of the Swedish Institute for Infectious Disease Control in Solna reported a rise in rhinovirus coupled with a swine flu lull just after school resumed in Sweden at the end of August (see graph). She too says rhinovirus has now fallen, as flu has climbed. Researchers in Norway report rhinovirus rose there as flu fell in August, while Ian Mackay at the University of Queensland found the same trend in Australia.

What's more, in March, Mackay reported that people with rhinovirus are less likely to be infected with a second virus than people with other viruses, and are just one-third as likely to have simultaneous seasonal flu (Journal of Clinical Virology, DOI: 10.1016/j.jcv.2009.03.008).

So why hasn't the US, for example, seen a dip in pandemic cases during a back-to-school rhinovirus outbreak? Mackay speculates that interference from rhinovirus may not be enough to fend off flu if someone is exposed repeatedly. There were far more cases of swine flu in the US in September than in Europe.

The effects of rhinovirus, often dismissed as "only" a cold, are too poorly understood, say all the researchers. Its seeming ability to block swine flu may already have saved lives in France by buying the nation time before the vaccine arrived. It may even lead to a drug that induces the antiviral state, but without the sniffles.

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