A virus that causes the common cold may be saving people from swine
flu [pandemic (H1N1) 2009 influenza virus infection]. 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. 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. 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 2 viruses at the same time. Normally, we don't get a
chance to see how rhinovirus affects flu, because 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 arrived early -- and France isn't the only country in which
rhinovirus seems to have held it at bay.
In Eurosurveillance last month [see reference below}, 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 . 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 2nd virus than
people with other viruses, and are just one-third as likely to have
simultaneous seasonal flu (Journal of Clinical Virology, DOI:
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.
[Byline: Debora MacKenzie]
[The reference for the Eurosurveillance publication cited above is
the following: Eurosurveillance, Volume 14, Issue 40, 08 October 2009
This paper by A Linde et al. is titled: Does viral interference
affect the spread of influenza. The summary reads: "This short
communication hypothesis that rhinovirus epidemics occurring after
start of school may interfere with the spread of influenza during the
period when warm and humid climate decreases the influenza spread by
aerosol. Limited laboratory data supporting this hypothesis are
included in the article, but the report is written mainly to
stimulate interest and research concerning the possibility that viral
interaction may affect influenza epidemiology."
There are many viral respiratory pathogens and it is not unreasonable
to hypothesize that some or all of these might interfere with the
replication of influenza virus, which is itself susceptible to
auto-interference by the generation defective interfering particles
during replication (Indeed such defective interfering particles may
in time be harnessed as specific therapeutic agents). The
identification of rhinoviruses as interfering agents is partly a
consequence of the surveillance activities associated with this
virus. Interference by other viral respiratory pathogens will remain
unrecognised in the absence of systematic surveillance.