Human seasonal influenza occurs in the northern and southern hemispheres in epidemics in the colder months of the year. These vary annually in their mix of viruses (which constantly evolve), the severity of the illness and the ‘fit’ with the vaccine composition, which is reviewed annually by the World Health Organization (WHO) for each hemisphere [1]. Those in Europe responsible for influenza prevention and recommending immunisation for those at risk are always interested in how severe the coming winter season will be. That interest is highest in August and September, when preparations are being made for immunisation campaigns. One clue can come from the severity of the winter epidemics in the Southern Hemisphere, which is coming to an end at about that time.

There have been a few unusual and widely reported influenza cases in some parts of the Southern Hemisphere this season [2]. In Australia, several states have reported a higher number of cases this winter, with Queensland and New South Wales particularly affected. Six children under the age of five who contracted Influenza A this year have died – the cases were in Western Australia, Queensland and New South Wales. The predominant strain circulating in Australia is A(H3N2), although A(H1N1) has also been seen widely in some states. The country has only experienced low levels of influenza B activity this season.

In contrast, Chile has had a rise in cases from last year, but not as many as in 2004 – incidence has also been falling since week 28 [3]. The regular vaccination campaign there targeting the elderly, those with chronic diseases, public health workers, pregnant women, infants and workers in the avian industry was launched in March; 3.2 million doses were given. H3N2 has been the predominant strain in Chile this season. This has also been the case in Argentina: in week 20, 10 samples were characterised as A/Wisconsin/67/2005-like (H3N2); in week 21, seven samples were characterised as A/Wisconsin/67/2005-like (H3N2); and in week 22, 11 samples were characterised as A/Wisconsin/67/2005-like (H3N2) [4].

In equatorial regions (where influenza is less seasonal), the experience was also unexceptional. In China, the Philippines, Thailand and Malaysia, the predominant strain this season has been A (H3N2), with A(H1N1) and B also present [4]. According to the WHO’s summary of seasonal influenza activity in the world, last updated in March, Hong Kong saw a high level of A(H3N2) activity in weeks 8–11 [5].

Sri Lanka has seen a lot of A(H1N1) activity this season, while in Singapore it has been the B virus that has been most reported [3]. In contrast to Australia, New Zealand has been having a relatively mild influenza season in 2007 so far, with a mix of A(H1N1), A(H3N2) and some B viruses circulating [1]. South Africa, which saw a peak of activity in weeks 29 and 30, has had both A and B viruses in roughly equal proportions, with H1N1 viruses more common than H3N2 so far [4].

In summary, therefore, apart from the unusual cases in children in Australia, the 2007 influenza season in the Southern Hemisphere has not been exceptional, either in the number of cases being reported or the strains circulating [4]. Those strains that have been seen were also seen in the Northern Hemisphere in its last season, and most of them are included in the current vaccine for the Southern Hemisphere this season (an A/New Caledonia/20/99(H1N1)-like virus; an A/Wisconsin/67/2005(H3N2)-like virus and a B/Malaysia/2506/2004-like virus). As was seen in the Northern Hemisphere’s 2006/07 season, A(H3N2) strains in the Southern Hemisphere have not reacted well to antiserum A/Wisconsin/67/2005 [4]. The WHO will formally address the Southern Hemisphere experience in its coming regular consultation for selection of vaccine strains [6].