The world is now in a post pandemic phase that is, according to the WHO criteria “Levels of influenza activity have returned to the levels seen for seasonal influenza in most countries with adequate surveillance.“ Post pandemic seasonal influenza can be different from the pre-ceeding influenza and therefore careful attention has to be paid to what is happening with the ‘new’ seasonal influenza pattern. This is highlighted in a recent editorial by ECDC.
Southern Hemisphere Temperate Countries Following the recommendations of an Advisory Forum group that oversaw the development of ‘ECDC’s Forward Look Risk Assessment ‘ the experiences with influenza in the temperate countries of the Southern Hemisphere are being monitored especially carefully by ECDC during the Northern Hemisphere summer and autumn. The special interest this year is because these are the first countries to experience a second winter of transmission with the 2009 pandemic virus. Hence they give one, maybe the best indication of what Europe can expect in its winter of 2010/2011. This is being done using epidemic intelligence techniques consulting routine and non-routine published sources in the five temperate Southern Hemisphere countries with developed long-term epidemiological and virological surveillance systems: Argentina, Australia, Chile, New Zealand and South Africa.
As Argentina and South Africa have already ceased publishing weekly reports on their Ministry of Health websites this update focuses especially on recent information from Chile, Australia and New Zealand (see graphs below).
As in most years seasonal influenza transmission started to rise in the Southern Hemisphere temperate countries after May and June (see Figures below). However the patterns of transmission and the viruses responsible have been diverse, much more diverse than in the pandemic winter of 2009.
South Africa experienced low levels of influenza like illness H3N2 and type B) viruses but hardly any influenza A(H1N1) were detected from June 2010 onwards. In contrast in the other four countries (Argentina, Australia, Chile and New Zealand) the pandemic influenza A(H1N1) strain initially predominated but with some A(H3N2) and B viruses. As elsewhere in the world there have been none of the previous seasonal A(H1N1) viruses. Chile among the five countries in that also routinely tests for and reports on respiratory syncytial viruses (RSV) and these viruses resulted in early epidemics of respiratory infection in children (see figures for Chile). With the exception of New Zealand the countries have all reported levels of transmission in the community (as reflected in consultations) and hospitalisations that were quantitatively lower than the last 2009 Southern Hemisphere winter when those countries experienced pandemic waves. Now Argentina, Australia, Chile and South Africa are not reporting many cases of severe disease associated with influenza and generally neither the health services nor the 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.
Most recently though both Australia and Chile are experiencing unusual late seasonal rises (see figures).
Australia: In the reporting period 04th-10th September 2010 (week 36), data from a number of surveillance systems indicated that influenza activity has been rising late in the season. Though overall national rates of ILI consultations in primary care have remained well below levels observed during the 2009 winter pandemic wave, there is now widespread ILI activity in South Australia, West Australia and Victoria and regional activity in the rest of country except in the capital county where transmission is classified as local; in addition, in the island of Tasmania there has been sporadic activity. The proportion of sentinel respiratory samples testing positive for influenza virus has slightly increased in the last two weeks, and it is now 16%, a figure considerably higher than earlier in the season. The majority (70%) of recent influenza virus isolations have been characterized as H1N1 2009, though seasonal H3N2 and B viruses have also been detected. It will be important to determine what viruses are driving these late epidemics. There are no data indicating to what extent the transmission has been affected by attempts at immunisation in 2009 and 2010 with first pandemic and then seasonal influenza vaccines in Australia.
Chile: In the latest influenza report from Chile (reporting from 5th - 11th September 2010) it is noted that an increase in respiratory consultations has also been observed during the previous 6 weeks. Conversely RSV circulation, which was responsible for early local epidemics in children, has decreased during the last weeks with RSV now accounting for only 30% of the total of viruses, followed by influenza B and parainfluenza viruses. In addition, the co-circulation of 2009 pandemic influenza A(H1N1) and H3N2 viruses continues with the latter becoming more important; the proportion of all influenza viruses accounted for by the A(H3N2) viruses is three-times that of 2009 pandemic influenza A(H1N1) virus.
New Zealand: This country has also shown an unusual pattern of infection this winter. The weekly surveillance report for week 37 recorded how the reported national rate for influenza-like illness (ILI) during the last two weeks had continued to decrease to be lower than it was at the same time in 2008 and during the 2009 pandemic and under the national baseline level (see New Zealand figure below). However overall influenza activity (as indicated by rates of ILI, hospitalizations and absenteeism) has been notably uneven geographically. The New Zealand Ministry of Health reports that transmission has been focally intense in some areas, even higher for those areas than in the pandemic winter. It has suggested by the authorities that areas that experienced less transmission during the pandemic winter wave of 2009 may now be experiencing more transmission. There is some support for this in the seroprevalence data that were reported by the New Zealand authorities at the end of last winter where seroprevalence rates by locality varied from 20% to 30% (1). However overall rates of ILI and numbers of severe and fatal cases in New Zealand remain below levels seen during the winter 2009 pandemic wave and the current seasonal epidemics shows evidence that they have passed its peak. Equally through there are some localities in New Zealand where levels of reported influenza like illness are above those seen in 2009 pandemic winter. To date the Ministry of Health reports there have been 18 deaths linked to influenza though the risk factors for these deaths are yet to be reported. Fourteen of these deaths have so far been confirmed as being due to A(H1N1). As of September 23rd there have been 702 hospitalisations of laboratory-confirmed cases of pandemic influenza H1N1. So far this year, a total of 104 people with confirmed H1N1 have been admitted to intensive care (these figures do not include influenza-like illness among people admitted to hospital without a positive H1N1 laboratory test result). While in the other countries there is some heterogeneity in the clinical reports from state by state reports it is only in New Zealand that this heterogeneity has driven the all age incidence rate of ILI to such high levels. There are no data indicating to what extent the local transmission has been affected by intensive attempts at immunisation with first pandemic and then seasonal influenza vaccine.
ECDC Comment (27th September 2010): As in 2009 the findings from the Southern Hemisphere countries deserve continuing attention. While the global picture was sufficiently like seasonal influenza to allow WHO to declare that the world is in a post pandemic phase that does not mean that everywhere in Europe there will be a benign 2010-2011 season. We do not know what the pattern of the ‘new’ seasonal influenza will be especially the picture of who the risk groups are (2,3). On the basis of the pandemic, at least one EU country has decided to proactively add pregnant women to their usual recommendations for immunisation (4). The finding of heterogeneous transmission in New Zealand and perhaps also Australia will need careful watching and interpretation. The late season rises associated with influenza A(H3N2) and B viruses indicates the wisdom of using trivalent vaccines. Of course seasonal influenza can show a heterogeneous pattern geographically, however, some of the New Zealand transmission has been sufficiently intense to cause the Ministry concern in that country. The implication could be that parts of European countries that were less affected in 2009 may be more affected in the 2010/2011 season. There are serological data which suggest heterogeneous coverage following the early waves in Europe but no analyses as yet following the full 2009/2010 winter (5,6). Certainly, these findings emphasise the importance of the autumn immunisation campaigns when the new trivalent seasonal vaccines become available.
1. Bandaranayake D, Huang S Seroprevalence of the 2009 influenza A (H1N1) pandemic in New Zealand ESR May 2010
2. Nicoll A, Sprenger M. The end of the pandemic – what will be the pattern of influenza in the 2010-11 European winter and beyond? . Euro Surveill. 2010;15(32):pii=19637.
3. Nokleby H, A Nicoll A Risk groups and other target groups – preliminary ECDC guidance for developing influenza vaccination recommendations for the season 2010-11. Eurosurveillance March 25th 2010
4. United Kingdom Department of Health, England The influenza immunisation programme 2010/2011 May 28th 2010
5. Weekly Epidemiological Record (WER); 11 JUNE 2010, 85th YEAR; No. 24, 2010, 85, 229–236. Seroepidemiological studies of pandemic influenza A(H1N1) 2009 virus.
6. Miller E, Hoschler K, Hardelid P, Stanford E, Andrews N, Zambon M Incidence of 2009 pandemic influenza A H1N1 infection in England: a cross-sectional serological study The Lancet, Early Online Publication, 21 January 2010 doi:10.1016/S0140-6736(09)62126-7
Influenza elsewhere in the world: WHO’s most recent global review of influenza was published on 10th September (it is now published every two weeks). Apart from the New Zealand and Australian situations already described, WHO reports how India is still experiencing epidemics of A(H1N1) 2009 with active transmission and a numbers of fatal cases being reported in several states across the country. In Africa, one country, the Central African Republic reported its first detection of H1N1 (2009).
The significance of this influenza activity in tropical countries like India is unclear since in many of the equatorial countries surveillance and testing has improved during and following the 2009 pandemic. Hence normal patterns of influenza activity (the baseline) have yet to be defined. In tropical settings where surveillance is established the pattern of transmission across the months can be 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 while in India it is suggested that influenza transmission usually intensifies when the monsoon starts as happened in July in parts of India.
WHO also continues to publish its weekly update of virological data including an update on antiviral resistance to August 18th. The former confirms that overall influenza activity has remained at low levels in most parts of the world but that there is 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 adamantanes which is also the case with the A(H3N2). 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 have been shown to 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.
Figures for emergency consultations and hospitalizations for influenza and pneumonia by age-group; Source: Chilean Ministry of Health (weeks 1-36)
Unusually among the five countries Chile reports on the isolation of a number of viruses and this can be seen in the diagram below which shows the importance of respiratory syncytial virus (VRS) in both 2009 and 2010 and this is thought to partially explain the surge in referrals for acute respiratory infections in the <15 year old age group seen above.
Figure confirmations of viruses in Chile in 2009 and 2010
Influenza Surveillance Report No. 36 for the week 04 to 10 September 2010
CAPTIONS – ASPREN: Australian Sentinel Practices Research Network; VIDRL: Victorian Infectious Diseases Reference Laboratory; NT: Northern Territory
Figures from New Zealand Influenza Weekly Update 2010/37
CAPTION – DHB: District Health Board