Influenza update - 21 April 2011

Update number 132

Table of contents

- Summary
- Countries in the temperate zone of the northern hemisphere
- Countries in the tropical zone
Countries in the temperate zone of the southern hemisphere
Virological surveillance


• Worldwide influenza activity is generally low and decreasing.
• Influenza activity in the northern hemisphere temperate regions is back to baseline levels in most areas, indicating the season is now ending.
• In countries of the tropical zone, influenza activity is generally low with a few recent localized areas of activity reported and ongoing transmission of H3N2 in central Africa.
• In southern hemisphere temperate countries influenza seasonal activity has not yet started.
• Viruses which have been characterized antigenically continue to be largely related to the lineages found in the current trivalent seasonal vaccine.

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Countries in the temperate zone of the northern hemisphere

North America:

Influenza activity continues to decrease in both Canada and the United States of America (USA). In Canada, the influenza-like illness (ILI) consultation rate declined compared to previous weeks and is now slightly below the expected rate for this time of year. The number of new virus detections, outbreaks and pediatric hospitalizations all decreased during epidemiological week (EW) 14. Ten percent of clinical samples tested were positive for influenza, of which influenza B now accounts for a greater proportion than influenza A. This represents a change in the relative proportions of the two virus subtypes since the beginning of the season. Overall in that time period, 89% of virus detections were influenza type A and 11% influenza type B. Notably, H1N1 (2009) has accounted for only 5% of all virus detections or 15% of all subtyped A viruses. Since the beginning of the season, 618 pediatric hospitalizations with laboratory-confirmed influenza have been reported from participating provinces; 16% confirmed as influenza A(H3N2), 4% A(H1N1)2009, 54% as unsubtyped influenza A, and 26% influenza B. This is somewhat different from the pattern of virus detections in the adult hospitalized cases in which A(H3N2) was detected in 21%, H1N1 (2009) in 5%, unsubtyped influenza type A in 67%, and influenza type B in only 6%. In the United States of America, the proportion of all outpatient visits due to ILI is below the national baseline of 2.5% and all ten regions reported ILI activity below regional baseline levels. The proportion of deaths attributed to pneumonia and influenza declined to near baseline levels after being at or above the epidemic threshold for 11 consecutive weeks. Overall, 9% of samples tested positive for influenza during EW 14, down from 14 % in the previous week. Of these, influenza type B was detected in 37% of the samples. Of the influenza A samples, 28% were influenza A(H1N1)2009, 41 % were influenza A(H3N2) and the remaining 31 % were not subtyped. In Mexico, there is a low number of virus detections but increasing proportion of influenza A(H1N1)2009. The H1N1 (2009) outbreak in the northern state of Chihuahua reported last month has primarily affected Juarez and Chihuahua. The outbreak has included six H1N1 (2009) associated deaths, all adults who were previously healthy and including one pregnant woman.


Influenza activity in the European Region continues to decline and activity has returned to baseline in 36 of 50 countries. Thirteen percent of sentinel specimens from patients with ILI or Acute Respiratory Infection (ARI) tested positive for influenza and 3% of specimens from sentinel SARI patients tested positive for influenza in EW14, down from 15 % in the previous. As overall detections of influenza virus decrease, the proportion of influenza type B virus detections has increased proportionately and now accounts for 70% of all the influenza virus detections in EW 14. Overall for the season, a total of 82,759 influenza virus detections have been reported since the beginning of the season in the last week of September 2010, of which 71% were influenza A and 29% were influenza B. Of 46,843 influenza A viruses that were subtyped, 96% were influenza A(H1N1), 4% were influenza A(H3N2). Seventy-two percent of 4,766 severe acute respiratory infection cases hospitalized since the beginning of the influenza season i n reporting countries of the European Union and European Economic Area were infected with an influenza virus. Of 3,072 typed and sub-typed influenza viruses from the hospitalized cases, 90% were influenza A(H1N1)2009, 1% were influenza A(H3N2) and 10% were influenza B viruses. As in North America, the large majority of viruses tested in Europe are antigenically similar to those in the in the current trivalent seasonal influenza vaccine.

North Africa and the Middle East:

Influenza virus transmission in North Africa has generally been similar to the pattern observed across Europe, primarily associated with H1N1 (2009) with smaller amounts of influenza type B. Peak transmission occurred somewhat later in Algeria and Tunisia but has now subsided to very low levels across the region.

Northern Asia:

Transmission in northern Asia has also returned to baseline or near baseline levels in Mongolia, northern China, Republic of Korea, and Japan. Of the very small number of viruses detected in those countries, most are influenza type B, though H3N2 and H1N1 (2009) have also been reported recently. Bhutan has reported a recent boarding school-associated outbreak of H1N1 (2009) in early April.

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Countries in the tropical zone

Influenza activity in the tropical zones of the world is largely localized to a few countries. In the Americas, there is no significant community transmission of influenza reported from Central America or the Caribbean, however, Venezuela recently reported an increase in cases of influenza A(H1N1)2009 in parts of the country associated with at least 12 deaths. All of the deaths are reported to have occurred in individuals with pre-existing medical conditions. The surveillance system in Venezuela reports that 33% of SARI admissions detected by the sentinel surveillance system were positive for H1N1 (2009). The epidemic appears to have peaked in early April. Other countries in tropical South America do not report similar increases. In sub-Saharan Africa, influenza A(H3N2) is circulating in the central part of the continent as evidenced by increases of virus detections in the Democratic Republic of Congo and Rwanda. Further to the east, Kenya H1N1 (2009) is circulating with influenza type B, primarily of the Victoria lineage. The previously reported transmission of H3N2 and influenza type B in Madagascar appears to have lessened somewhat, although both viruses continue to be detected there. Influenza activity in the tropical areas of Asia is at baseline levels in all of the countries reporting, indicating that transmission is at a very low level or absent in most of the zone.

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Countries in the temperate zone of the southern hemisphere

Influenza activity remains low in most regions in the temperate zone of the southern hemisphere. Australia continues to report on low but unusual levels of influenza A(H3N2) viruses, with the majority of the detections from the Northern Territory and Queensland, the tropical and subtropical areas of the country.

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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 19 April 2011, 12:40 UTC

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