INFLUENZA PANDEMIC (H1N1) 2009 (25): AUSTRALIA AND UNITED KINGDOM, UPDATES

Date: Thu 6 Aug 2009
Source: Eurosurveillance, Volume 14, Issue 31, 6 Aug 2009 [edited]



Interim analysis of pandemic influenza (H1N1)
2009 in Australia: surveillance trends, age of
infection and effectiveness of seasonal vaccination

Between May and September each year, influenza
sentinel surveillance is conducted in general
practices in Melbourne and the state of Victoria
in southern Australia. We describe the 1st 11
weeks of sentinel surveillance in 2009 (weeks
18-28), during which time pandemic influenza
(H1N1) 2009 virus became established, and
investigate the protective effect of seasonal
influenza vaccine against laboratory-confirmed
infection caused by the pandemic virus. At the
time of reporting, the peak influenza-like
illness (ILI) activity in 2009 had been reached
and was similar to the peak recorded in 2007 but
below the peak of 2003. The proportion of cases
positive for any influenza virus increased from 6
percent in the 1st week of surveillance (week 18)
to 59 percent by week 28, during which time the
proportion of influenza viruses detected as
pandemic influenza increased from zero to 95
percent, with at least 91 percent of all
influenza viruses confirmed as pandemic influenza
by the 8th week of surveillance (week 25). The
median age of all 223 patients with pandemic
influenza for whom age was known was 21 years
(range 2-63 years) compared with the median age
of 53 patients with seasonal H1N1 influenza in
2007 or 2008 of 23 years (range 1-75 years).
There was no evidence of significant protection
from seasonal vaccine against pandemic influenza
virus infection in any age group.

Introduction
------------
Australia reported its 1st case of pandemic
influenza (H1N1) 2009 on 8 May 2009 in a
traveller returned from the United States [1]. 10
days later the state of Victoria in southern
Australia reported its 1st 3 cases, in 3 brothers
from one family, also recently returned from the
United States [2]. Victoria has used an existing
sentinel general practice network, established
with laboratory support in 1998 [3], to monitor
the pandemic. Sentinel monitoring is designed to
overcome the potential testing biases that arise
from monitoring all diagnosed cases, including
those identified from outbreaks and contact
tracing. During the current pandemic, sentinel
surveillance general practitioners have been
encouraged to test those patients who satisfied
the case definition of fever (reported or
observed), cough and fatigue/malaise [4], as they
have done in previous years [5-10].

We have previously demonstrated the feasibility
of estimating influenza vaccine effectiveness
(VE) using a case control study of patients
tested for influenza as a component of sentinel
surveillance [11]. We now aim to describe the 1st
11 weeks, from 27 Apr 2009 to 12 Jul 2009 (weeks
18 28), of sentinel surveillance in Victoria in
2009, during which time pandemic influenza (H1N1)
2009 virus became established. We compare
influenza-like illness (ILI) in 2009 with
previous seasons and compare our surveillance
system with ILI surveillance using the novel
Google Flu Trends. We investigate the protective
effect of seasonal influenza vaccine against
medically attended ILI due to
laboratory-confirmed infection caused by the pandemic virus in this period.

Methods
-------
The Victorian sentinel general practice network
Victoria is a southern Australian state with a
temperate climate. The influenza season occurs in
winter and often extends into the early months of
spring. Between May and September each year,
sentinel surveillance is conducted in general
practices scattered throughout Melbourne and
regional Victoria. Victoria's population is more
than 5 million, with 3.9 million people living in
the state capital, Melbourne. For each season,
participating general practitioners (GPs) report
weekly on the total number of consultations and
any patients presenting with ILI, defined as
fever (reported or observed), cough and fatigue/malaise [4].

Laboratory-confirmed influenza has been a
gazetted notifiable disease in Victoria since
2001. Because of the legal requirement for the
laboratory to notify positive cases, formal
ethics approval is not required for the
surveillance program. However written consent is
obtained from sentinel patients, indicating that
aggregate anonymous data will be used for
surveillance purposes and influenza positive
results will be notified to the state government
Department of Human Services, Victoria. After
consent is obtained GPs collect data on the age,
sex, symptoms and vaccination status (recording
the date of administering the vaccine) of the
sentinel patients. GPs collect a combined nose
and throat swab from consenting patients. The
swab is couriered to the Victorian Infectious
Diseases Reference Laboratory (VIDRL), a WHO
National Influenza Centre, for laboratory
testing. In 2009 sentinel surveillance commenced
on 27 Apr 2009 (week 18), with a network of 87 sentinel GPs, 60 in Melbour!
ne and 27 in regional Victoria. Optional online
data entry was introduced and we continued to use
surveillance data from the Melbourne Medical
Deputising Service (MMDS) [12]. We compared
publicly available ILI data from the Google website,

expressed as the Google search ratio, with our
surveillance data, expressed as ILI consultations per 1000 consultations.

We used data from all surveillance sources to
describe the 1st 11 weeks of the influenza season
and compared features of the 2009 season with
previous influenza seasons. Seasonal thresholds
were based on the proportion of ILI cases per
1000 consultations. Baseline activity, normal
seasonal and higher than expected seasonal
activity were defined as below 2.5, between 2.5
and <15, n="147)" n="43)." p="0.17)." n="69)," n="10)," n="22)" n="6)," or =" 1.65)" org="" 2009="" 06="" html="">
so that retrospective similarity of data is
expected. The prospective similarity is
interesting. Unfortunately there is no detailed
published information on the approach used by
Google for ILI surveillance in the southern
hemisphere, preventing a more detailed comparison.

With complete subtyping, influenza in sentinel
patients was shown to be exclusively due to
pandemic influenza in weeks 30 and 31 date available from:
.
However, considering only patients for whom
subtyping data were complete in previous weeks
when these patients comprised at least 90 percent
of all influenza detections, influenza in these
sentinel patients was entirely due to pandemic influenza from week 25.

We have previously suggested the median age of
patients infected with influenza A(H1N1) was
similar for patients infected with seasonal and
pandemic influenza H1N1 strains [15, 16] and the
surveillance data presented here confirm these
original observations. Infections with influenza
A(H3N2) tend to occur in older people [15, 17]
and comparisons of the age of infection with
pandemic H1N1 influenza with the age of infection
of all seasonal influenza may be misleading if
previous seasons were dominated by influenza
A(H3N2). A younger median age of infection with
pandemic H1N1 influenza is likely to reflect the
age of infection with influenza A(H1N1) viruses.
We detected no sentinel patients with pandemic
influenza over the age of 63 years, consistent
with some protection afforded to older people as
demonstrated by the detection of cross-reacting
antibodies to the pandemic H1N1 virus in people aged 60 years and above [18].

We found no evidence of protection against
medically attended laboratory-confirmed pandemic
influenza from receipt of the seasonal vaccine in
age-stratified or age-adjusted analyses. However,
we do not collect data on co-morbidities and
could not adjust for potential confounders, other
than age. The ILI case control observational
study design has limitations, some of which may
bias the VE estimate towards the null. Sampling
of patients is not systematic and the sampling
proportion increased to 69 percent in 2009 from
40 percent in the 5 influenza seasons from 2003
to 2007 [11]. Seasonal influenza infection may be
asymptomatic or afebrile [19] and the same is no
doubt true for infection with pandemic H1N1
influenza. Sentinel patients therefore represent
the mid-range of the influenza morbidity
spectrum, although this is likely to be true for
both seasonal and pandemic infections. Given the
high level of community concern, patients may
have been more likely to attend their general
practitioner with an ILI in 2009, compared with
previous seasons, and GPs may have been more
likely to swab patients. However the proportion
of 44 percent of sentinel patients positive for
influenza in the 1st 11 weeks of surveillance in
2009 is not significantly different to the
proportion of 42 percent positive in the 5
influenza seasons between 2003 and 2007 [11].

Because of the high workload in the early weeks
of the pandemic in Victoria, not all influenza
positive specimens have been definitively
subtyped. However, the distribution of
vaccination status and pandemic influenza
infection in the weeks where subtyping is
incomplete would need to be remarkably different
to the distribution in the weeks with almost
complete data for this lack of data to bias our
estimate of VE. Because of low case numbers in
the early weeks, we did not adjust for week of
presentation in the interim analysis, but
performed an analysis restricted to the 4 weeks
when subtyping data were almost complete and in
which pandemic influenza comprised at least 90
percent of all influenza detections. There was no
significant difference in VE estimates comparing
these 4 weeks with the entire period. We did not
adjust for time between symptom onset and date of
specimen collection since GPs are instructed to
collect a specimen only within 4 days of symptom onset.

While there are potential limitations with
interim analyses of VE from observational studies
using routinely collected data, the results
reported here, showing no protection from
seasonal vaccine against laboratory confirmed
medically attended infection due to pandemic
influenza (H1N1) 2009, are not unexpected.

[Byline: H Kelly and K Grant
At: Epidemiology Unit, Victorian Infectious
Diseases Reference Laboratory, Melbourne, Australia]