Preliminary implications for Europe of the 2011 influenza season in five temperate southern hemisphere countries

The 2011 influenza season (May to October) in the southern hemisphere was dominated by the A(H1N1) viruses that emerged during the 2009 influenza A(H1N1) pandemic and influenza B viruses, although the proportion of these two varied between and within countries. Some influenza A(H3N2) viruses were also seen. We discuss here the preliminary implications for Europe of the 2011 influenza season in five temperate southern hemisphere countries.

The 2011 influenza season (May to October) in the southern hemisphere was dominated by the A(H1N1) viruses that emerged during the 2009 influenza A(H1N1) pandemic and influenza B viruses, although the proportion of these two varied between and within countries.Some influenza A(H3N2) viruses were also seen.We discuss here the preliminary implications for Europe of the 2011 influenza season in five temperate southern hemisphere countries.
Since 2009, the European Centre for Disease Prevention and Control (ECDC) has been monitoring the patterns of human influenza infection in five temperate southern hemisphere countries in their winters (May to October) as this gives some indication of what can be expected in the following northern hemisphere winter [1][2][3][4][5][6][7][8].
The pattern of influenza in the southern hemisphere is one of the many factors that the Centre takes into consideration in formulating its risk assessment in relation to severity and impact for epidemics of influenza [8].
From May to October 2011, ECDC monitored what was occurring in the five southern hemisphere countries in terms of virology, epidemiology and impact on healthcare of influenza and other respiratory viruses.One important source was the reports that the countries place regularly on the websites of their ministries of health and public health institutes [1][2][3][4][5].In addition, more specific analyses and reports -especially related to the impact (in the sense of pressures on primary and secondary healthcare services) -as well as information on unusual features were sought from influenza experts in the countries by a simple questionnaire to get information that was not otherwise available.The aim of the questionnaires was to gather details on the impact on the healthcare service, risk factors associated with severe cases, observed complicating conditions, vaccine coverage among the general population and anything unusual that could have been observed.Three reference time-points were indicated for comparisons: before the 2009 influenza A(H1N1) pandemic, during the pandemic, and the first post-pandemic winter season (2010).

Findings and their implications for Europe
The findings for the five countries are shown in the Table, combining information from the questionnaire, the national websites and an earlier summary of the epidemiology and virology from the World Health Organization [9].
The observed respiratory virus pattern was mixed although no pre-pandemic seasonal influenza A(H1N1) viruses were seen in the southern hemisphere in the 2011 season.In 2011, in Argentina and Chile, respiratory syncytial virus (RSV) was the most frequent isolate, followed by influenza A(H1N1)pdm09 virus.South Africa also reported a predominance of RSV during 2011.In Australia, the most frequently isolated strains were influenza A(H1N1)pdm09 and influenza B viruses.New Zealand observed a pattern of influenza B viruses (Victoria lineage) dominating in 2011.This has been seen at intervals, approximately once every three seasons.All countries reported some influenza A(H3N2) circulation, although it was not the predominant influenza A subtype in any country.
The match with seasonal vaccines was found to be good overall [10].Australia reported a regional cluster of oseltamivir-resistant influenza A(H1N1) viruses which were collected from patients without oseltamivir exposure (only one of the 29 cases infected with the resistant virus had received oseltamivir treatment).The individuals were not known to be immunosuppressed [11][12][13].The viruses remained sensitive to zanamivir but were resistant to adamantanes.All the resistant influenza A(H1N1) viruses were found to carry a point mutation in their neuraminidase genes which encoded a histidine to tyrosine substitution at residue 275 (H275Y) of the neuraminidase active site.
Argentina reported higher burden on the healthcare system in 2011 than during the 2010 season and Chile noted higher pressure than usual on child healthcare  services from illness among children, but mostly due to RSV (Chile is one of the few countries in the world outside the European Union that routinely reports on RSV detections).Australia and New Zealand reported less burden on the healthcare system in 2011 than in 2010, and much less than during the 2009 influenza A(H1N1) pandemic.In hospitals, the only unusual impact was high burden on the secondary healthcare system in Argentina and the burden through childhood RSV in Chile.Some geographical differences were reported in the burden of respiratory illness on the primary healthcare system in Chile and on the secondary healthcare system in Argentina.
Australia reported that surveillance data on severe disease remained consistent with people with co-morbidities being at higher risk of severe disease but that the age groups with severe disease had reverted to the pattern seen in the period before the 2009 pandemic.However, during 2011, three other countries noted a similarity with the pandemic pattern of severe disease in younger people (Table ).There were fewer reports of acute respiratory distress syndrome than during the 2009 influenza A(H1N1) pandemic.
Three of the four countries that reported information on vaccine coverage, Argentina, New Zealand, and South Africa, indicated that vaccine coverage for seasonal influenza among the recommended groups was higher than before the 2009 influenza A(H1N1) pandemic, whereas Chile reported that coverage was lower than during the 2009 influenza A(H1N1) pandemic and in 2010 (Table ).

Discussion and limitations
The influenza virological pattern seen in the southern hemisphere in 2011 was not consistent enough to make a clear prediction for the 2011/12 in Europe.However, it was different from what was seen in 2010/11 in the northern hemisphere for Europe (predominance of influenza A(H1N1)pdm09 and, to a lesser extent, influenza B viruses), North America and North Asia (predominance of influenza A(H3N2) virus).
In relation to the seasons before 2011, the overall impact of influenza in the southern hemisphere was lower in 2010 than in 2009, with some exceptions, e.g.locally in New Zealand [14,15].The reports of circulation of oseltamivir-resistant influenza A(H1N1) viruses are concerning, although these were also observed during the 2009 influenza A(H1N1) pandemic and in Europe in 2010/11 [16,17].This indicates a particular need to monitor these viruses in Europe in the 2011/12 season to detect any rise in prevalence as was observed for the pre-2009 influenza A(H1N1) seasonal viruses in the 2007/08 season [18].
The main limitation of this survey lies in its descriptive character.In addition, the selection of the contributors did not follow a systematic procedure.Data derived from more thorough quantitative and statistical analysis would render the information more meaningful but cannot be generated while there are such differences in the surveillance systems in the countries concerned.
The findings on the impact of influenza in the southern hemisphere in 2011 are reassuring for Europe before the influenza season reaches its peak, usually around January.The differences in the impact of influenza observed within the 2011 season between Australasia, South Africa and the southern cone of South America may become more apparent in future seasons.This was the case in the last inter-pandemic period, when large differences existed between continents for both the southern and the northern hemisphere [19].This may reduce, but not eliminate, the utility of this kind of surveillance for Europe in the future.

RSV: respiratory syncytial
virus. a None of the preceding seasonal influenza A(H1N1) viruses was observed in any of the five countries [9].b As compared with the 2010 season and the 2009 influenza A(H1N1) pandemic.c As compared with the 2010 season, the 2009 influenza A(H1N1) pandemic, and prior to the pandemic.d As compared with the 2010 season and prior to the 2009 influenza A(H1N1) pandemic.

Table
Characteristics of the influenza season in five temperate southern hemisphere countries and their implications for Europe, 2011