Vaccine 26S (2008) D31–D34
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Influenza vaccine strain selection and recent studies on the global migration
of seasonal influenza viruses
Colin A. Russella, Terry C. Jonesa,b,c, Ian G. Barrd, Nancy J. Coxe, Rebecca J. Gartene, Vicky Gregoryf,
Ian D. Gustd, Alan W. Hampsond, Alan J. Hayf, Aeron C. Hurtd, Jan C. de Jongb, Anne Kelsod,
Alexander I. Klimove, Tsutomu Kageyamag, Naomi Komadinad, Alan S. Lapedesh, Yi P. Linf,
Ana Mosterina,c, Masatsugu Obuchig, Takato Odagirig, Albert D.M.E. Osterhausb,
Guus F. Rimmelzwaanb, Michael W. Shawe, Eugene Skepnera, Klaus Stohri,
Masato Tashirog, Ron A.M. Fouchierb, Derek J. Smitha,b,∗
aDepartment of Zoology, University of Cambridge, Cambridge CB2 3EJ, United Kingdom
bDepartment of Virology, Erasmus Medical Centre, Rotterdam, Netherlands
cUniversitat Pompeu Fabra, Barcelona, Spain
dWHO Collaborating Centre for Reference and Research on Influenza, Melbourne, Australia
eWHO Collaborating Center for Influenza, Centers for Disease Control and Prevention, Atlanta, GA, USA
fWHO Collaborating Centre for Influenza, National Institute for Medical Research, London, United Kingdom
gWHO Collaborating Center for Influenza, National Institute for Infectious Diseases, Tokyo, Japan
hTheoretical Division, Los Alamos National Laboratory, Los Alamos, NM, USA
iNovartis Vaccines and Diagnostics, Cambridge, MA, USA
a r t i c l ei n f o
Received 29 July 2008
Accepted 29 July 2008
Vaccine strain selection
Seasonal influenza viruses
a b s t r a c t
Annual influenza epidemics in humans affect 5–15% of the population, causing an estimated half million
deaths worldwide per year [Stohr K. Influenza—WHO cares. Lancet Infectious Diseases 2002;2(9):517].
The virus can infect this proportion of people year after year because the virus has an extensive capacity
the human population in 1968 the A(H3N2) component of the influenza vaccine has had to be updated
almost 30 times to track the evolution of the viruses and remain effective. The World Health Organization
Global Influenza Surveillance Network (WHO GISN) tracks and analyzes the evolution and epidemiology
of influenza viruses for the primary purpose of vaccine strain selection and to improve the strain selection
process through studies aimed at better understanding virus evolution and epidemiology. Here we give
an overview of the strain selection process and outline recent investigations into the global migration of
seasonal influenza viruses.
© 2008 Published by Elsevier Ltd.
The influenza viral surface glycoprotein hemagglutinin (HA) is
the primary target of the protective immune response. HA is thus
the focus of influenza virus surveillance and the primary compo-
nent of currently licensed vaccines. The antigenic properties of HA
of human influenza viruses change substantially over time, a pro-
cess known as antigenic drift [2,3]. The degree to which immunity
induced by one strain is effective against another is mostly depen-
is thus both the root cause of the enormous public health burden
of influenza epidemics, and a primary reason why the virus is such
bridge CB2 3EJ, United Kingdom. Tel.: +44 1223 330933; fax: +44 1223 336676.
E-mail address: firstname.lastname@example.org (D.J. Smith).
a fascinating pathogen from a scientific perspective. The analysis
of antigenic differences among strains is therefore critical for the
selection of vaccine strains.
The data used for the WHO influenza vaccine strain selection
process are collected by the WHO GISN . The network currently
consists of more than 120 national influenza centers in over 90
countries and is continually expanding to strengthen virological
of locally circulating viruses for analysis and for communication
to one of the four WHO Collaborating Centers for Reference and
Research on Influenza: The Centers for Disease Control and Preven-
tion, Atlanta, USA; The National Institute for Medical Research, Mill
on Influenza, Melbourne, Australia; or The National Institute of
Infectious Disease, Tokyo, Japan.
0264-410X/$ – see front matter © 2008 Published by Elsevier Ltd.
C.A. Russell et al. / Vaccine 26S (2008) D31–D34
is characterized antigenically using the hemagglutination inhibi-
tion (HI) assay. The HI assay is a binding assay based on the ability
of HA to agglutinate red blood cells and the complementary ability
of antisera raised against the same or related strains to block this
agglutination. Thus an HI titer gives information about the affinity
of an antiserum for a virus strain and can be used to compare the
antigenic similarity of viruses. Currently, the WHO Collaborating
Centers generate antigenic data on an average of ∼3000 A(H3N2)
viruses per year.
Interpretation of the tables of HI assay data works well in expert
hands for distinguishing drift variants; however, fine-grain differ-
ences are difficult to judge quantitatively. Thus the HI data on
to The Center for Pathogen Evolution at the University of Cam-
maps allow a quantitative interpretation of antigenic differences to
an average accuracy of 0.8 of an antigenic unit (a twofold difference
in HI titer), and also produce a simple visualization of antigenic
differences among strains.
An antigenic map is automatically created for each new HI table
mentally added to the master antigenic map for the laboratory in
which it was made, and to a composite antigenic map for the data
from all laboratories. The automated processing and updated mas-
ter maps are typically available for analyses by all the laboratories
involved about 24h after submission. Although the system is cur-
rently operational for a limited number of laboratories, the system
and for other pathogens, and the software will be made available
as free and open source software.
Currently, ∼10% of the viruses analyzed antigenically are also
The viruses selected for sequencing have been shown to be a rep-
resentative subset of the total sample including both the dominant
circulating variants and outliers .
The core components of influenza vaccine strain selection
are assessing the match between the vaccine strain and the
currently circulating strains and the identification of emerging
antigenic variants. If the vaccine does not match the currently
circulating strains, or an emerging variant is judged likely to
be the major variant in the upcoming influenza season, the
vaccine is updated to contain a representative of the new vari-
ant. A fourfold difference in HI titer, a distance of two units
in an antigenic map, between the vaccine strain and the cur-
rently circulating or emerging strains is generally considered
a sufficient antigenic difference to warrant a vaccine strain
Because influenza A viruses continue to evolve the vaccine must
be updated frequently. To ensure that the vaccine tracks the evo-
lution of the virus the WHO Consultation on the Composition of
Influenza Vaccine is held twice a year to analyze the antigenic and
together with human serological data and epidemiological data,
to assess the need for an update of the vaccine. The Consultation
consists of representatives from WHO, the four WHO Collaborat-
ing Centers, the National Institute for Biological Standards and
Control, the Therapeutic Goods Administration, the Center for Bio-
logics Evalutation and Research, and the University of Cambridge.
Each February the Consultation is held to recommend which of
the then circulating strains should compose the vaccine for the
upcoming northern hemisphere influenza season. The recommen-
dation is made in February to allow time for the ∼300 million
doses of vaccine to be manufactured in time for people to be vac-
cinated in October and November in preparation for the influenza
season which typically peaks sometime between December and
March. The process is similar for the southern hemisphere with the
recommendation made in September. Effectively, vaccine strains
currently have to be selected almost 1 year before the influenza
season in which they will be used.
The influenza virus vaccine works well most years, but if a new
variant emerges after the vaccine strain has been selected the vac-
cine may be sub-optimally matched to the circulating strains and
potentially result in decreased vaccine efficacy. Substantial mis-
match in which cross-reactivity is very low between circulating
and vaccine strains is infrequent but could be reduced further by
increasing the “prediction horizon” for influenza virus evolution.
The more that is understood about how influenza viruses evolve
and spread around the world, the more predictable the evolution
and epidemiology of the virus could become and the more the
vaccine strain selection process could be optimized.
1. The global migration of seasonal influenza A(H3N2)
For over 60 years, the global migration patterns of influenza
viruses have been a mystery. There are several hypotheses about
the travel patterns of influenza viruses and there has previously
not been enough evidence to judge among them. The hypotheses
include migration between the northern and southern hemi-
spheres following the seasons, migration out of the tropics where
influenza viruses were thought to circulate continuously owing to
the lack of seasonal temperature variation to drive epidemics, and
by a further hypothesis that influenza viruses continue to circu-
late locally between epidemics and that subsequent epidemics are
caused by these locally circulating viruses or by external viruses,
according to the hypotheses above, only in seasons when there is
substantial antigenic drift.
and A(H1N1) influenza viruses, Nelson et al. [14,15] showed that
seasonal influenza A epidemics in New York State, Australia, and
New Zealand during the study period were seeded by an exter-
nal source and not by strains that had caused epidemics the year
before. This evidence against local persistence of viruses from one
season to the next implied that influenza viruses circulate glob-
ally each year and Nelson et al. found evidence compatible with
either northern-to-southern hemisphere migration or migration
from tropical regions, including Southeast Asia. Simonsen et al.
 showed that amantadine resistance spread globally in a man-
ner also consistent with global circulation of influenza viruses. A
recent phylogenetic study of ∼300 influenza A(H3N2) viruses in
Hong Kong  showed further evidence for global circulation and
against persistence from season to season, with phylogenetic evi-
low-level circulation, but which also could have been seeded from
Using the same full-genome dataset as used by Nelson et al.,
Rambaut et al.  found that the evolution of A(H3N2) and
frequent reassortment and periodic selective sweeps. Rambaut et
al. conclude that their results suggest a sink–source model of viral
migration. In this model new lineages are seeded from a source
reservoir. They hypothesize this reservoir is located in the tropics
and from there seeds the temperate regions.
In a complementary global migration study, we investigated
the antigenic differences among the ∼13,000 influenza A(H3N2)
C.A. Russell et al. / Vaccine 26S (2008) D31–D34
Fig. 1. Schematic of the dominant migration routes of seasonal influenza A(H3N2) viruses. The structure of the network within E-SE Asia is unknown. (World map image
courtesy of NASA Visible Earth. Image credit Kristin Wuichet.)
viruses collected worldwide by the WHO GISN between 2002 and
2007, and the HA1 domain sequences of the HA gene of about ∼10%
of these viruses .
viruses have migrated out of what we call the “East and Southeast
Asian circulation network” – which includes tropical, subtropi-
cal, and temperate countries – and then spread around the world
(Fig. 1). The dominant pattern was that new strains emerged in
East and Southeast (E-SE) Asia and then reached Europe and North
America, on average, 6–9 months later. Several months later still,
strains arrived in South America potentially due to South America’s
relative isolation from E-SE Asia.
We found that instead of local persistence in any one country,
seasonal influenza A(H3N2) viruses circulated year-round in E-SE
Asia by passing from epidemic to epidemic in the region. For rea-
sons that are not yet well understood influenza epidemics typically
occur during the winter months in the temperate regions of the
northern and southern hemisphere whereas in tropical countries
influenza epidemics often coincide with the rainy season [8–13].
In E-SE Asia there is a large amount of variation in the timing of
the rainy season. For example, Bangkok and Kuala Lumpur are only
∼1100km apart but their influenza epidemics typically occur at
opposite times of year. Thus, along with the wintertime epidemics
in the temperate parts of the region, the overlap in the timing of
epidemics in the region gives the opportunity for influenza viruses
to circulate year round. This temporal overlap of epidemics com-
bined with substantial travel among E-SE Asian countries results
in the East and Southeast Asian circulation network that allows
this region to serve as the source of influenza A(H3N2) viruses for
epidemics in the rest of the world.
The two-way nature of air travel makes the transport of
viruses both to and from E-SE Asia equally likely. However, in the
left E-SE Asia they rarely caused epidemics on their return to E-SE
Asia. We propose two hypotheses to explain this phenomena: (1)
population immunity is more advanced within the E-SE Asian cir-
culation network due to new variants causing epidemics there first
the returning strains, (2) the number of viruses being introduced
to countries within the E-SE Asian circulation network from other
duced from external regions and thus are more likely to be strains
that seed epidemics. In the 2002–2007 A(H3N2) dataset used in 
there was antigenic advancement from year to year and thus those
data cannot be used to test between these two hypotheses.
These studies highlight the value of antigenic, HA sequence, and
demiology of seasonal influenza A viruses, and the importance of
existing and new surveillance and collaborations in E-SE Asia to
further optimize the influenza vaccine strain selection process.
We are thankful for the enormous contributions of individu-
als throughout the WHO Global Influenza Surveillance Network,
particularly those in National Influenza Centers. This work
was supported by an NIH Director’s Pioneer Award to DJS,
part of the NIH roadmap for medical research, through grant
number DP1-OD000490-01. RAMF is supported by NIAID-NIH
contract HHSN266200700010C, and the De Nederlandse Organ-
isatie voor Wetenschappelijk Onderzoek Netherlands Influenza
Vaccine Research Centre. The Melbourne WHO Collaborating
Centre for Reference and Research on Influenza is supported
by the Australian Government Department of Health and Age-
ing. The WHO Collaborating Centre for Influenza, NIMR, UK
is funded by The Medical Research Council (UK). The con-
clusions presented in this paper are those of the authors
and do not necessarily reflect those of the funding agen-
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