Influenza

Influenza update - 30 December 2010

Introduction

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), FluID 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.

Description: Displayed data reflect the most recent information reported to FluNet, WHO regional offices or on ministry of health websites in the last 2 weeks.

The percent of specimens tested positive for influenza includes all specimens tested positive for all influenza subtypes. The pie charts show the distribution of virus subtypes among all specimens that were tested positive for influenza.

The available country data were joined in larger geographical areas (Influenza transmission zones)with similar influenza transmission patterns in order to present an overview.

Influenza - Update 124

30 December 2010 - Summary:

The winter influenza season is now under way in parts of the Northern Hemisphere. North America is seeing increases of influenza-like illness (ILI) now above baseline levels in parts of Canada and the United States of America (US) associated primarily with influenza viruses A(H3N2) and type B. The United Kingdom of Great Britain and Northern Ireland (UK) has been experiencing a surge in both mild and severe cases for the last three weeks which has not yet peaked and is primarily associated with influenza A(H1N1) 2009 virus and to a lesser extent influenza type B. The pattern of illness associated with H1N1 (2009) virus infection in the UK is similar to last season primarily affecting young adults, particularly those with underlying chronic illness or pregnancy although a number have had no known risk factors. On the European continent, the Middle East and in northern Asia rates of influenza-like illness are low but recent increases have been noted in some areas. In tropical regions, very little activity is noted in most of the world, however Sri Lanka has reported a marked increase in the number of both mild and severe cases related to H1N1 (2009) virus, including 22 deaths. As in the UK, the deaths in Sri Lanka have been predominantly in people under the age of 60 years and most have had preexisting medical conditions. No significant influenza transmission has been reported in Southern Hemisphere temperate regions. Notably, the large majority of viruses that have been characterized from North America and the UK have been antigenically similar to those contained in the current trivalent influenza vaccine. WHO continues to recommend vaccination for those at high risk of complications, where it is available, and early treatment of those at high risk or with severe or rapidly deteriorating disease.

Countries in the temperate zone of the Northern Hemisphere

North America

The US and Canada both continue to experience increases in all indicators of influenza activity. Consultation rates for influenza-like illness (ILI) are still below the epidemic threshold in the US nationally; however, several south eastern states have crossed the threshold and are reporting high rates of ILI. In Canada, the highest levels of ILI activity are currently being reported in the eastern and central provinces. Rates of death related to pneumonia and influenza in the US were noted to be below the epidemic threshold. Nationally, more than 15% of clinical specimens tested in the US and Canada are now positive for an influenza virus; which is increased in both countries over the previous week. In the US, influenza type A viruses account for about 56% of virus detections and influenza type B, 44%. In contrast, 99% of influenza viruses in Canada are type A. In both countries, of the influenza A viruses that have been sub-typed, about 90% have been H3N2 with influenza H1N1 (2009) accounting for the other 10%. All influenza viruses that have been characterized in the US and Canada are antigenically related to the viruses that make up the current vaccine, including the H3N2, H1N1, and type B viruses. Data from Canada indicates that the age distribution of severe H1N1 cases is somewhat younger than those associated with H3N2 virus, with H1N1 continuing to predominantly affect younger adults and children and H3N2 being more evenly distributed and causing severe disease in older age groups.

Europe

The increase in ILI and severe respiratory disease noted in the UK in recent weeks has not yet peaked. The UK reports that severe cases related primarily to influenza H1N1 (2009) continue to occur and a total of 17 have now required Extra Corporeal Membrane Oxygenation, a form of advance respiratory support. The rate of reported "all-cause" deaths in the UK passed the expected levels for this time of year in week 49. Severe cases have been predominantly associated with H1N1 (2009), have been less than 65 y.o., and largely unvaccinated. On the European continent, rates of respiratory disease are still relatively low but the number of countries reporting influenza detections and the rates of positivity in sentinel samples are increasing. Of the 13 countries presenting calculated baseline thresholds, France, Ireland, the Russian Federation and Ukraine reported clinical consultation rates above their thresholds. Overall in week 50/2010, 39% of sentinel samples tested positive for influenza compared with 20% in the previous week. Of 465 samples positive for influenza virus, 318 (68%) were influenza A and 147 (32%) were influenza B. Since week 40/2010, 161 influenza viruses have been characterized antigenically. The type A viruses are all similar to those found in the current seasonal vaccine: 90 were A/California/7/2009 (H1N1)-like; 15 A/Perth/16/2009 (H3N2)-like. However, the type B viruses were mixed: 5 were B/Florida/4/2006-like (B/Yamagata/16/88 lineage); and 51 were B/Brisbane/60/2008-like (B/Victoria/2/87 lineage). The influenza B virus in the vaccine is Brisbane-like. Respiratory Syncytial Virus (RSV) is also circulating and 14 countries reported detections during week 50.

Middle East:

The Islamic Republic of Iran has noted a sharp increase in H1N1 (2009) detections in the past week with smaller numbers of influenza type B. Oman, which has reported increases in influenza virus detections for the past 3 to 4 weeks, continues to detect predominantly H1N1 (2009) with lower numbers of type B. Algeria has detected primarily influenza type B in recent weeks.

Asia:

ILI rates are beginning to increase in the temperate countries of Asia as well. Mongolia has noted a sharp increase in the proportion of outpatients with ILI in sentinel clinics, which crossed the epidemic threshold 2 to 3 weeks ago. China has noted slight increases recently in the northern part of the country, where 18% of specimens tested have been positive for influenza virus. In both countries, influenza A (H3N2) virus has been the predominant virus detected with China also reporting smaller numbers of influenza type B. The Republic of Korea and Japan have also noted low level but increasing rates of ILI. In Republic of Korea, 35% of specimens tested were positive for influenza virus in week 50. H1N1 (2009) is the predominant virus circulating in both countries although in Japan this represents a shift from recent weeks in which H3N2 virus was more commonly detected.

Countries in the tropical zone

Influenza activity in the tropical areas of the Americas is quite low with only sporadic activity being reported in most countries. Previously noted influenza A (H3N2) transmission in Paraguay and Bolivia has decreased to near baseline levels. In tropical Asia, Sri Lanka has experienced a marked increase in influenza cases in the last 3 to 4 weeks almost entirely associated with influenza H1N1 (2009) virus with small numbers of influenza type B. Sri Lanka reports that 22 influenza related deaths have occurred so far this season, only two of which were in individuals 60 years of age or older. Limited data from Viet Nam indicates community transmission of influenza there as well, predominantly influenza type B, but the remainder of southern Asia including India, Bangladesh, Thailand, Cambodia, Singapore, China Hong Kong Special Administrative Region, Southern China and Chinese Taipei report very little or no influenza activity. The small numbers of viruses detected in these areas have been a mixture of all three circulating types. Data from the tropical areas of Africa are limited but indicate that no active transmission is occurring in Western Africa and continuing transmission of H1N1 (2009) in central Africa as evidenced by detections of the virus in Cameroon. In Eastern Africa, Kenya and Madagascar both report ongoing transmission of H3N2 virus with occasional detections of influenza type B and A(H1N1) 2009.

Countries in the temperate zone of the Southern Hemisphere

The influenza season has ended in the temperate countries of the southern hemisphere and no appreciable community transmission is being reported from any of the countries monitored there.

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