Influenza update - 25 March 2011
Update number 130
Influenza activity in most areas of the northern hemisphere temperate regions appears to have peaked and is declining. Although the level of pneumonia and influenza mortality in the United States of America (USA) is above the epidemic threshold and many states still are reporting on widespread activity, most indicators on influenza activity in North America are indicating decreasing influenza activity. As activity in the Americas declines, influenza A(H1N1)2009 has increased proportionately and now accounts for 38% of all virus detections. In Europe the peak has been passed in most countries and all countries now report medium or low influenza activity. Cases of Severe Acute Respiratory Infections in Europe are decreasing but still above baseline in some parts of Eastern Europe. Influenza viruses in Europe continue to be primarily influenza A(H1N1)2009, about 70% of all viruses characterized, and influenza type B, making up about 28% of all viruses. Data from parts of Northern Africa show that there is ongoing community transmission of both influenza A(H1N1l)2009 and influenza type B in Tunisia and Algeria. The large majority of the viruses characterized are closely related to the vaccine strains included in the current seasonal vaccines. Viruses which have been characterized antigenically continue to be largely related to the lineages found in the current trivalent seasonal vaccine except for a small number of influenza B viruses of the Yamagata lineage.
Influenza activity in North America is decreasing in both Canada and the USA. In Canada, most regions report either declining or stable activity. The influenza like-illness (ILI) consultation rate fell from 37.4 to 25.3 per 1000 patient visits in epidemiological week 10 and is now below the expected rate for this time of year. New hospitalizations with laboratory-confirmed influenza among both adults and children also declined in the same period. Since the beginning of the season, 93% of all influenza viruses have been type A. Of the influenza A viruses subtyped, 86% have been influenza A(H3N2). Detections of influenza type B have been increasing proportionately since late January and now accounts for 28% of all influenza positive specimens. Seventy-eight percent of 185 fatal influenza cases reported from nine provinces and territories were in patients aged 65 years or older. Ninety-one percent of the subtyped influenza A viruses in the fatal adult cases have been H3N2, and less than 3 % influenza type B. In the USA, the ILI consultation rate fell 3 % but is still above the national baseline. Reported deaths due to pneumonia and influenza from the 122 city surveillance system also remain above the epidemic threshold for this time of the year. During epidemiological week 10, eleven children were reported to have died of influenza related illness, making a total of 71 pediatric deaths since October 2010. The overall proportion of samples testing positive for influenza in week 10 has decreased from 27% to 21%. Virus detections in the USA were primarily H3N2 and type B early in the season, however, the proportion of detections due to influenza A(H1N1)2009 have steadily increased and now account for 43% of all the subtyped influenza A viruses. Mexico reports a low number of influenza virus detections, with influenza type B as the most frequent virus.
All influenza indicators in Europe continue to decline since the last report. Thirty-seven of the 53 Member States in the WHO European Region have passed a peak of the ILI or Acute Respiratory Illness (ARI) surveillance in recent weeks. All countries reported either medium or low influenza activity, and only Bulgaria reported increased activity. Cases of Severe Acute Respiratory Infections (SARI) in the European Economic Area (EEA) peaked at the end of December 2010 but still remain above pre-season levels in the Eastern European countries (Georgia, the Republic of Moldova, Romania, Serbia and Ukraine). In the EEA, the most frequently reported risk factor for severe influenza infections is obesity. In week 10 the proportion of samples testing positive for influenza among sentinel doctors was 46 %. Of all influenza virus detections that were reported this season, 72% were influenza A and 28% were influenza B. Of the influenza A viruses that were subtyped 97% were influenza A(H1N1)2009 and 3% were influenza A(H3N2). Since week 40, 2010, and similar to North America, nearly all influenza A viruses characterized this season in Europe have been antigenically similar to the H1N1 and H3N2 strains included in the current trivalent seasonal influenza vaccine. Approximately 93% of influenza type B viruses characterized are also of the same lineage as those in the current vaccine (Victoria) with the remainder being of the Yamagata lineage.
North Africa and the Middle East:
Influenza activity in North Africa and the Middle East is moderate with influenza A(H1N1)2009 and influenza B viruses co-circulating. The number of confirmed cases in Algeria has increased over the last several weeks, with nearly 60 cases of influenza A(H1N1)2009 reported during 7-12 March. Tunisia has had continued activity during the last few weeks with nearly 30 cases of co-circulating influenza A(H1N1)2009 and influenza B reported. In the Middle East, influenza activity in Oman and Iran remain moderate with approximately 50 confirmed influenza cases in each country, while the number of confirmed cases in Pakistan has decreased to less than 20.
Overall, influenza activity in northern Asia continues to decline. In north China, ILI activity has remained low and below the activity level observed during the last three seasons. ILI activity in Japan has seen a slight increase after several weeks of decreasing activity. The proportion of ILI patients has remained stable in the Republic of Korea after a declining trend seen over the last several weeks. The majority of cases involve influenza A(H1N1)2009, with influenza A(H3N2) and influenza B circulating in lower numbers.
Influenza activity in the tropics remains low in most areas with co-circulation of influenza B and influenza A(H1N1)2009. For the most part, sporadic activity is reported in regions of Central America, the Caribbean and the Andean Region. However, in Guatemala, the proportion of positive respiratory viruses among the tested samples has increased to 37% during 28 February - 4 March, compared to 6% in the previous week. In Ecuador, the percentage of samples positive for respiratory viruses increased slightly from the previous week to approximately 30%. In Sub-Saharan Africa, limited available data indicate low levels of co-circulating influenza A and influenza B viruses. However, influenza activity has been increasing in Kenya, with 17 influenza B cases, 11 influenza A(H1N1)2009 cases, and 1 influenza A(H3N2) case during epidemiological week 10, compared to a total of 18 cases the previous week. Madagascar also reports low level persistent activity primarily associated with influenza type B. Influenza activity in tropical Asia remains low with the majority of cases involving influenza B and influenza A(H1N1)2009. In south China, the percent ILI of sentinel hospital visits decreased to 3% during week ten from 3.3% the previous week. This is lower than the level seen during the 2008-2009 and 2009-2010 seasons (3.4% and 3.3% respectively).
Influenza activity remains low in most regions in the temperate zone of the southern hemisphere. However, Australia reports persistent low-level of out-of-season influenza activity, primarily in the northern tropical areas of the country. Influenza A(H3N2) is the predominant virus in Australia.
During weeks 9 to 10, laboratory detections of influenza A(H1N1)2009, A(H3N2) and B viruses continued to be reported in many parts of Asia, Europe and North Americas, though virus activity in general was low.
In the northern hemisphere and tropical regions, a general decline in activity was observed for both H1N1 (2009) and B viruses that co-circulated in some countries. In Europe, decreased activity was observed for both H1N1 (2009) and B viruses that co-circulated. In Asia, influenza A(H1N1)2009 viruses continued to predominate, while influenza B viruses were slightly predominant in Africa with co-circulation of A(H3N2) viruses. In the United States of America, influenza A(H1N1)2009, A(H3N2) and B viruses co-circulated, while in Canada, the predominant viruses continued to be influenza A(H3N2). In the southern hemisphere, influenza activity was low with influenza A(H1N1)2009, A(H3N2) and B viruses detected in few countries.
The vast majority of characterized viruses from the 2010-2011 influenza season remain antigenically similar to the viruses WHO recommended for the 2010-2011 northern hemisphere influenza vaccines.
During weeks 9 to 10 (27 February 2011 to 12 March 2011), National Influenza Centres (NICs) from 71 countries, areas or territories reported data to FluNet*. A total of 13,496 specimens were reported as positive for influenza viruses, 9,379 (69.5%) were typed as influenza A and 4,117 (30.5%) as influenza B. Of the sub-typed influenza A viruses reported, 76.7% were influenza A(H1N1)2009 and 23.3% were influenza A(H3N2).
Influenza virus detection by type/subtype in countries, areas or territories:
- Influenza A(H1N1)2009: Albania, Algeria, Argentina, Armenia, Australia, Austria, Belarus, Bosnia and Herzegovina, Bulgaria, Cameroon, Canada, China, Croatia, Cuba, Czech Republic, Denmark, Estonia, Finland, France, France - French Guiana, France - Martinique, Georgia, Germany, Ghana, Greece, Hungary, Iran (Islamic Republic of), Italy, Jamaica, Japan, Kenya, Lao People's Democratic Republic, Latvia, Lithuania, Luxembourg, Madagascar, Mali, Mongolia, Morocco, Netherlands, Norway, Poland, Portugal, Romania, Russian Federation, Rwanda, Serbia, Slovakia, Slovenia, Spain, Sri Lanka, Sweden, Switzerland, Tunisia, Turkey, Ukraine, United Kingdom of Great Britain and Northern Ireland, United Republic of Tanzania, United States of America, Viet Nam.
- Influenza A(H1N1) (old seasonal virus): no report.
- Influenza A(H3N2): Australia, Brazil, Canada, China, Cuba, Ecuador, Ethiopia, France, France - French Guiana, Germany, Greece, Guatemala, Iran (Islamic Republic of), Italy, Japan, Kenya, Kyrgyzstan, Madagascar, Morocco, Norway, Paraguay, Russian Federation, Rwanda, Serbia, Spain, Sweden, Turkey, United Republic of Tanzania, United States of America, Viet Nam.
- Influenza B: Albania, Australia, Austria, Belarus, Belgium, Brazil, Bulgaria, Cambodia, Canada, China, Costa Rica, Croatia, Czech Republic, Denmark, Estonia, Finland, France, France - French Guiana, Georgia, Germany, Ghana, Greece, Hungary, Iceland, Iran (Islamic Republic of), Italy, Jamaica, Japan, Kenya, Kyrgyzstan, Lao People's Democratic Republic, Latvia, Lithuania, Luxembourg, Madagascar, Mexico, Mongolia, Morocco, Netherlands, Nicaragua, Norway, Poland, Romania, Russian Federation, Serbia, Slovakia, Slovenia, Spain, Sri Lanka, Sweden, Switzerland, Tunisia, Turkey, Ukraine, United Kingdom of Great Britain and Northern Ireland, United Republic of Tanzania, United States of America, Viet Nam.
- No influenza activity reported: Afghanistan, Azerbaijan, Central African Republic, Colombia, Dominican Republic, El Salvador, Ethiopia, France - Guadeloupe, Honduras, India, Mauritius, South Africa, Uganda.
* Some NICs report to FluNet retrospectively leading to updates of previous summary data.
* Some NICs report to FluNet retrospectively leading to updates of previous summary data.
Source of data
The Global Influenza Programme monitors influenza activity worldwide and publishes an update every two weeks.
The updates are based on available epidemiological and virological data sources, including FluNet (reported by the Global Influenza Surveillance Network) and influenza reports from WHO Regional Offices and Member States. Completeness can vary among updates due to availability and quality of data available at the time when the update is developed.
FluNet data as of 22 March 2011, 14:50 UTC