Influenza

Influenza update - 08 April 2011

Update number 131

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

Summary

Worldwide influenza activity is generally low.
Influenza activity in the northern hemisphere temperate regions is continuing to decline or back to baseline levels indicating the season is ending. In countries in the tropical zone, influenza activity is low in most areas. In southern hemisphere countries influenza 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, except for a small number of influenza B viruses of the Yamagata lineage.

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Countries in the temperate zone of the northern hemisphere

North America:

Influenza activity continues to decrease in Canada and the United States of America (USA).

In Canada influenza activity has declined in most of western Canada but persists in parts of Alberta, Ontario, Quebec and the Atlantic provinces. The national influenza-like illness (ILI) consultation rates decreased slightly and remained below the average rate for this time of year. The number of influenza related outbreaks and the number of adult and pediatric hospitalizations with laboratory-confirmed influenza decreased further this week. While the overall percentage of samples testing positive for influenza has again decreased from previous weeks to 11.3%, influenza B is increasing proportionately since January 2010 and now accounts for 44.4% of all influenza viruses detected.

In the USA, in epidemiological week 12, the proportion of outpatient consultations for ILI dropped below the national baseline for the first time since end of December, but the reported deaths due to pneumonia and influenza from the 122 city surveillance system remained still above the national baseline. Twelve influenza-associated pediatric deaths were reported, bringing the total for this season to 89. Thirty-three of the 89 deaths reported were associated with influenza B viruses; 21 were associated with influenza A(H1N1)2009 viruses; 17 deaths reported were associated with influenza A(H3N2) viruses, and 18 were associated with an influenza A virus for which the subtype was not determined.
Overall the proportion of samples testing positive for influenza for week 12 decreased further to 14% with 71% positive for influenza A and 29% positive for influenza B. Among the 301 influenza A viruses that were subtyped, 62% were influenza A(H3N2) viruses and 38% were H1N1 (2009). Of characterized influenza B viruses, 94.3% belong to the B/Victoria lineage, 5.7% to the B/Yamagata lineage. Mexico reported an influenza A outbreak in the state of Chihuahua, which affected primarily the cities of Juarez and Chichuahua. Between March 22 and April 4, 142 cases of ILI and severe acute respiratory infection were identified with 24% confirmed to be due to influenza A(H1N1)2009 virus. The confirmed cases included six deaths in healthy adults, including one pregnant woman). Genetic sequencing carried out by the Institute of Diagnosis and Epidemiological Reference of Mexico on samples from 3 cases showed that the virus is homologous to the currently circulating viruses. The outbreak has not been associated with excessive demand on the health care services. In the rest of Mexico, isolated influenza A(H1N1)2009 cases were reported but no other outbreaks.

Europe:

Activity for all influenza indicators in Europe are declining since the last report. Twenty-nine of the 46 reporting countries from the WHO European Region are now below the baseline for consultation rates for ILI or acute respiratory illness (ARI). All countries reported either medium or low influenza activity. In week 12, the proportion of samples testing positive for influenza among sentinel doctors was 22 %, a decrease from 46% in week 10. H1N1 (2009) is co-circulating with influenza B with proportional increase of influenza B. From the influenza positive samples from the sentinel physicians 34% were influenza A and 66% were influenza B. Among influenza A the majority (98%) were H1N1 (2009). 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 92% 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:

Several countries in North Africa and Middle East are reporting declining influenza activity.
Algeria is reporting a decrease of confirmed influenza cases in comparison with last update. Influenza A(H1N1)2009 is now predominating in comparison with influenza B which was more prevalent in the beginning of the influenza season. Tunisia is reporting continued but decreasing circulation of influenza A(H1N1)2009 and influenza B. Oman is reporting a steady decline in the proportion of outpatient visits for ILI. In Iran the number of confirmed influenza cases dropped to levels seen in the beginning of the season with co-circulation of influenza A(H1N1)2009 and influenza B.

Northern Asia:

ILI activity in northern hemisphere countries has been declining. In northern China ILI activity remained low and below that observed in the previous three seasons. During epidemiological week 12, only 7% of samples tested positive for influenza, majority (35/44) was influenza B. Japan also reported decreasing ILI activity with for the last weeks predominantly influenza A(H3N2) detections, followed by influenza B. And also in the Republic of Korea, ILI activity remains low. In Mongolia, ILI activity has been sustained through week 12 but majority of samples tested positive for respiratory syncitial virus.

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Countries in the tropical zone

Influenza activity in the tropics remains low. Most countries in the Caribbean, Central America and the Andean region reported low influenza activity and influenza detection rates.
In Sub-Saharan Africa, there is continuous circulation of influenza viruses. In Kenya and Uganda influenza A(H1N1)2009 and influenza B viruses are predominating since beginning of this year. Madagascar saw an increase in influenza activity in the last weeks with influenza A(H3N2) and influenza B co-circulating. In Ghana and Cameroun influenza A(H1N1)2009 was the predominant virus detected in last weeks.
Influenza activity in tropical Asia is low and declining. In southern China for the week 12 the percentage of visits for ILI was below that observed for the previous three seasons. Fourteen percent of the samples tested positive for influenza, but the relative proportion of influenza types was similar to that of northern China with 81% influenza B and 19% influenza A. In China, Hong Kong Special Administrative Region in week 12 reports declining ILI activity to baseline levels and the Center for Health Protection has stated that the influenza season is over. In Viet Nam, 29% of the samples tested were positive for influenza, lower than last month's rate (38%). The majority was influenza A(H1N1)2009 (88%). The Lao People’s Democratic Republic (PDR) and Cambodia reported both very low activity. In Lao PDR all 21 tested samples were negative for influenza and in Cambodia 2 of 99 tested specimens were positive for influenza (both influenza B) Singapore, too is reporting a decline in ARI activity, which is now below the warning level.

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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 reporting low activity of influenza, predominantly influenza A(H3N2) in the northern tropical areas of the country.

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Virological surveillance

During weeks 11 to 12, a number of countries reported laboratory confirmed detections of influenza A(H1N1)2009, A(H3N2) and B viruses. In the northern hemisphere, influenza activity due to influenza A(H1N1)2009 and B viruses continued to decease in Asia, most parts of Europe and the United States of America (USA). In Canada activity declined in the west but increased influenza B activity was reported in some eastern areas. Relatively few countries reported influenza A(H3N2) activity.
In the southern hemisphere and most tropical regions, influenza activity remained low with A(H1N1)2009, A(H3N2) and B viruses detected in a few countries. An increase in the number of positive influenza A(H3N2) detections was reported in Australia.
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.

FluNet reports

During weeks 11 to 12 (13 March 2011 to 26 March 2011), National Influenza Centres (NICs) from 76 countries, areas or territories reported data to FluNet*. A total of 7,614 specimens were reported as positive for influenza viruses, 4,824 (63.4%) were typed as influenza A and 2,790 (36.6%) as influenza B. Of the sub-typed influenza A viruses reported, 67.6% were influenza A(H1N1)2009 and 32.4% were influenza A(H3N2).

Influenza virus detection by type/subtype in countries, areas or territories:

  • Influenza A(H1N1)2009: Algeria, Armenia, Australia, Austria, Bangladesh, Belarus, Bosnia and Herzegovina, Cambodia, Cameroon, Canada, China, Croatia, Czech Republic, Denmark, Estonia, Fiji, France, Georgia, Germany, Ghana, Greece, Hungary, Iceland, Iran (Islamic Republic of), Italy, Jamaica, Japan, Kenya, Lao People's Democratic Republic, Latvia, Lithuania, Mexico, Netherlands, Norway, Poland, Romania, Russian Federation, Serbia, Slovakia, Slovenia, South Africa, Spain, Sweden, Switzerland, Thailand, Tunisia, Turkey, Ukraine, United Kingdom of Great Britain and Northern Ireland, United States of America, Viet Nam.
  • Influenza A(H1N1) (old seasonal virus): no report.
  • Influenza A(H3N2): Australia, Brazil, Canada, China, Costa Rica, Cuba, Democratic Republic of Congo, Denmark, France, France - French Guiana, Ghana, Japan, Kenya, Madagascar, Mexico, Republic of Korea, Romania, Russian Federation, Rwanda, Serbia, Sweden, Turkey, United Republic of Tanzania, United States of America.
  • Influenza A(H5): no report.
  • Influenza B: Austria, Belarus, Bulgaria, Cambodia, Canada, China, Croatia, Czech Republic, Denmark, Estonia, France, Georgia, Germany, Ghana, Greece, Hungary, Iceland, Iran (Islamic Republic of), Italy, Jamaica Japan, Kenya, Latvia, Luxembourg, Madagascar, Mali, Mauritius, Mexico, Mongolia, Morocco, Netherlands, Norway, Poland, Republic of Korea, Romania, Russian Federation, Rwanda, Serbia, Slovakia, Slovenia, South Africa, Spain, Sri Lanka, Sweden, Switzerland, Thailand, Tunisia, Turkey, Ukraine, United Kingdom of Great Britain and Northern Ireland, United States of America, Viet Nam.
  • No influenza activity reported: Angola, Argentina, Azerbaijan, Bangladesh, Central African Republic, Ethiopia, Honduras, India, Malta, Panama, Portugal, Uganda.

* Some NICs report to FluNet retrospectively leading to updates of previous summary data.

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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 07 April 2011, 9:50 UTC

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