In its latest weekly review of influenza in the world published on 30th July WHO reported few important recent changes. The global picture is heterogeneous but compared to 12 months ago transmission of pandemic influenza A(H1N1) is much reduced. All countries that report to WHO are indicating only low to moderate occurrence of respiratory infection even in the countries experiencing their winter.
Active circulation of pandemic virus has reappeared at low levels with other influenza viruses in the temperate Southern Hemisphere countries and is persisting at low levels in some areas in the tropics, particularly South and Southeast Asia, the Caribbean and West Africa. Increasing influenza activity had recently been observed in several countries of Central America and South America, including the pandemic strain with differing levels and trends even in countries close to each other.
The significance of this influenza activity in tropical countries nearly a year and a half after the emergence of the pandemic strain is unclear since in many of the equatorial countries the surveillance has been greatly intensified since the pandemic and normal influenza activity, the baseline, has yet to be defined. In tropical settings where surveillance is established the pattern of transmission across the months is quite different from what is seen in the temperate countries. For example in Hong Kong and Southern China peaks in the early Spring and June-August are usually reported.
However, importantly, WHO finds that in all the reporting countries the activity was well below that seen in the pandemic year of 2009. WHO also continues to publish its Weekly update of virological data including an update on oseltamivir resistance which indicates that overall influenza activity has remained at low levels in most parts of the world but that there is a co-circulation of pandemic A(H1N1) and seasonal A(H3N2) viruses reported from some countries while influenza type B virus detections have decreased. There are hardly any detections of the previous seasonal A(H1N1) viruses. Almost all the pandemic A(H1N1) viruses are resistant to the adamantanes. There are a few detections of pandemic viruses resistant to oseltamivir from countries with stronger surveillance, all are of the type A(H1N1) H275Y but very few represent person to person transmission of resistant virus, i.e. while there have been cases of oseltamivir-resistant pandemic strain infection these have not achieved the ability to transmit efficiently.
Southern Hemisphere Temperate Countries Following the recommendations of an Advisory Forum group that oversaw the development of ECDC’s Forward Look Risk Assessment this winter’s experiences with influenza in the temperate countries of the Southern Hemisphere are being monitored especially carefully by ECDC during the Northern Hemisphere summer. This is being done using epidemic intelligence techniques, that is consulting routine and non-routine published sources in countries with developed long-term epidemiological and virological influenza surveillance systems: Argentina, Australia, Chile, New Zealand and South Africa.
Sources of Epidemic Intelligence Information for Southern Hemisphere Temperate Countries:
The special interest this year is because these will be the first countries to experience the second winter of transmission with the 2009 pandemic virus. Hence they will give the best indication of what Europe can expect in its winter of 2010/2011. In addition since these countries were the first to experience a full winter of the pandemic they may also be the first to show patterns of infections consistent with previous seasonal influenza. According to the WHO criteria this would then indicate entering the post pandemic phase i.e. whether the “Levels of influenza activity have returned to the levels seen for seasonal influenza in most countries with adequate surveillance.“ However, only WHO and specifically its Director General can declare a phase change, and that following advice from the pandemic Emergency Committee and other consultations.
Normally seasonal influenza transmission starts to rise after May or June in the Southern Hemisphere temperate countries. Influenza-like illness has risen this year about that time but as can be seen in the Figures influenza activity has remained low during the early part of the current winter season in the temperate zone of the Southern Hemisphere. Low levels of seasonal influenza (H3N2 and type B) viruses were detected from June 2010 onwards in South Africa, while Chile, Australia, and New Zealand, as were low levels of pandemic influenza virus. During week 28, the above mentioned four countries have shown a mixed but mild overall picture (see figures below). None of the four are experiencing anything quantitatively like that of the last 2009 Southern Hemisphere winter. In Australia and especially New Zealand there are only modest epidemics so far of influenza predominately affecting younger age-groups. They are dominated by the pandemic influenza A(H1N1) in New Zealand while in Australia the viruses are more mixed with also A(H3N2) and B viruses. The picture is similar in Chile with only modest epidemics and both pandemic influenza A(H1N1) and A(H3N2). There has been a rise in Chile of respiratory syncytial virus (RSV) and that is causing more of the respiratory illnesses at present than influenza viruses. South Africa is having the same clinical experience (modest epidemics) but is different virologically with a mix of A(H3N2) and B viruses, and very little A(H1N1). All five countries are reporting few cases of severe disease associated with influenza and neither health services nor critical services are stressed, i.e. the pattern of illness is looking more like seasonal influenza than the pandemic pattern seen in the winter of 2009 in those countries.
(Influenza Surveillance Report No. 28 for the week ending 10 to 16 July 2010 (PDF 259 KB)