Influenza update - 06 May 2011

Update number 133

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 currently low.
• Influenza activity across the entire temperate Northern Hemisphere is generally back to baseline or pre-seasonal levels
• As levels of influenza activity decrease, influenza type B has become more commonly detected compared to influenza A viruses across the Northern Hemisphere temperate areas and in much of the tropics.
• Transmission in tropical areas of the world is also generally low with some transmission reported in countries of Sub-Saharan Africa with a mixture of viruses and slight predominance of influenza type B.
• In the temperate areas of the southern hemisphere the influenza season has not yet started.
• Nearly all influenza A viruses tested continue to be antigenically similar to those found in the current trivalent vaccine. More than 90% of the influenza type B viruses are also of the lineage found in the vaccine (Victoria lineage); however, a small number of B viruses of the Yamagata lineage are also being reported.

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Countries in the temperate zone of the northern hemisphere

North America:

Influenza activity continued to decline both in Canada and the United States of America (USA). In Canada the influenza-like illness (ILI) consultation rate has decreased. Both adult and pediatric hospitalizations with influenza were similar to previous weeks. Most provinces and territories report no or sporadic activity though there has been a slight increase in reported influenza outbreaks, primarily in the Atlantic provinces. The overall proportion of samples testing positive for influenza during week 16 was 7.2% (1.9% influenza A, 5.3% influenza B), which represents a continued decreased from previous weeks. In the USA, the proportion of outpatient ILI visits was 1.3%, which is below the national baseline. Laboratory-confirmed influenza-associated hospitalizations and deaths have decreased and were almost back to pre-seasonal levels; however the percentage of all deaths due to pneumonia and influenza reported from the 122 cities surveillance system remained at or above the epidemic threshold for the 13th consecutive week. The percentage of specimens testing positive for influenza declined further from previous weeks to 4.3%, with 43% due to influenza B. Of the influenza A positive samples, 64% were influenza A(H3N2), 26% unsubtyped, and 10% influenza A(H1N1)2009. In Mexico the percentage specimen testing positive for influenza increased slightly from 9% to 14%. This increase was primarily H1N1 (2009) related and follows a recent report of an H1N1 (2009) outbreak in the northern state of Chihuahua.


ILI activity was below baseline or at pre-seasonal levels in most countries. There was a further decrease in SARI hospitalizations in countries with SARI sentinel surveillance. Georgia is an exception with an increase mainly in the 0-4 year old age group, but none of the specimens tested were positive for influenza. The numbers of ILI and severe acute respiratory infection (SARI) samples that tested positive for influenza further decreased to 5% and 3% respectively. Of the few influenza samples that tested positive, 55% were due to influenza B, and 35% due to influenza A(H1N1)2009.

North Africa and the Middle East:

Influenza activity continued to decline, with very low numbers of influenza type B and H1N1 (2009) reported in Algeria, Tunisia, and the Islamic Republic of Iran.

Northern Asia:

In temperate areas of Asia, influenza activity was also low and primarily associated with influenza B and a smaller proportion of influenza A(H3N2). In Northern China, ILI activity remained low with 3.2% of visits to sentinel hospitals being for ILI, which is noted to be below that observed during the previous three seasons. The proportion of specimens testing positive for influenza was 5% and all were due to influenza B. ILI activity was also declining in Mongolia and no influenza virus was detected. In the Republic of Korea, ILI activity remained low with low level of circulation of influenza A(H3N2) and influenza type B. In Japan, where ILI activity remained low, of the 184 influenza viruses detected during the month, 1% were influenza A(H1N1)2009, 40% were influenza A(H3N2) and 59% were influenza type B.

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Countries in the tropical zone

Influenza activity in the tropical zones is generally low. In sub-Saharan Africa, Ghana and Kenya both report ongoing transmission of a mix of H1N1 (2009) and influenza type B. Madagascar reports decreasing detections of influenza viruses which have been a mix of influenza type B and H3N2 with type B slightly more common. In tropical countries of Asia, ILI activity has been low and the few viruses detected have been a mixture of primarily H1N1 (2009) and influenza type B.

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Countries in the temperate zone of the southern hemisphere

Influenza activity has not increased in the temperate zone of the southern hemisphere. Australia continues to report on low but decreasing levels of influenza, primarily A(H3N2) viruses primarily in 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.

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