1. Introduction
Influenza is a highly contagious, airborne respiratory tract
infection associated with a significant disease burden. The
annual “mild” influenza epidemics caused by antigenic drift
of the virus affects 10–20% of the world’s population with
up to 5 million cases of serious illness and 500,000 deaths
(http://www.who.int/vaccine research/diseases/ari/en). At
∗ Corresponding author. Tel.: +45 3532 7885; fax: +45 3532 7696.
E-mail address: s.buus@immi.ku.dk (S. Buus).
various intervals, new influenza subtypes emerge against
which no immunity exists in the human population and these
may cause global pandemics with an even higher disease toll.
The current outbreak of a new influenza subtype A (H5N1),
which can be directly, although at this time rarely, transmitted
from birds to humans [1], is an example of a potential pandemic
flu threat, which has currently reached phase 3 of 6 in
theWHOdelineation of a flu pandemic (http://www.who.int/
csr/disease/avian influenza/phase/en/index.html). It is highly
pathogenic in birds, and when transmitted to humans it
carries a mortality of over 50% [2]. So far, all the resulting