Influenza update - 28 January 2011
Update number 126
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
Influenza transmission in North America appears to be levelling off or decreasing according to several national influenza indicators, though not all regions have experienced a peak yet. Influenza activity in North America has been related primarily to influenza A (H3N2) virus with some co-circulation of influenza type B in the United States (U.S.). Influenza transmission in the United Kingdom (U.K.), predominantly related to influenza A (H1N1) 2009, is now decreasing. Influenza activity on the European continent is increasing, particularly in the west, and countries are increasingly reporting severe and fatal cases. Severe cases have been reported in association with all three influenza viruses, H1N1 (2009), influenza A (H3N2), and influenza type B, but H1N1 (2009) appears to be disproportionately over-represented among severe cases when compared to the distribution of viruses in the community. The large majority of the viruses characterized from North America and Europe continue to be of the same lineages as those found in the current seasonal trivalent vaccine. Transmission in Northern Africa and Northern Asia has peaked recently and is declining. In the tropics, several countries of southern Asia have seen increasing trends recently mainly due to H1N1 (2009). Other tropical areas of the world and the temperate countries of the Southern Hemisphere are currently reporting very little influenza circulation.
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Countries in the temperate zone of the northern hemisphere
North America: Several national-level indicators of influenza transmission indicate that influenza may be levelling off or decreasing in North America, however, there are many areas that have not yet experienced widespread circulation and some that are now reporting increases in activity. Rates of influenza-like illness (ILI) and hospital admissions due to influenza have decreased nationally in the last two weeks in Canada, though one region of Ontario has reported an increased rate. The national rate of ILI reporting is similar to influenza seasons prior to 2009. About 23% of specimens tested in Canada are positive for influenza, which is slightly decreased from recent weeks. Influenza type A accounts for 98% of influenza viruses detected. Of the influenza type A viruses characterized, 91% are H3N2 and the 16%, H1N1 (2009); the H1N1 (2009) proportion is increased from 6% in week 52. In the United States (U.S.), several influenza activity indicators have also decreased nationally, though the proportion of outpatient visits due to ILI remains slightly above the epidemic threshold. Rates of death reported due to pneumonia and influenza from a sentinel surveillance system that includes data from 122 cities across the country are below the epidemic threshold. About 26% of clinical specimens tested in week 2 of 2011 were positive for influenza, a slight increase from the 20% reported previously. The proportion of the detections accounted for by influenza type B decreased to 15% of the samples tested, from 26% of influenza isolates in previous weeks. Of the influenza type A viruses that have been sub-typed, 74% are H3N2 and the other 26% are H1N1 (2009) which is an increase in the proportion of H1N1 (2009) in comparison to earlier weeks. As in previous weeks, all of the influenza A and 92% of the influenza type B viruses characterized antigenically are of the same lineages as contained in the current trivalent seasonal influenza vaccine. In Mexico the percentage of samples positive for influenza was 30% with around 60% influenza type B and 30% H3N2. Based on the proportion of samples testing positive, influenza activity in Mexico appears to be decreasing.
Europe: Influenza transmission continues to increase in Europe, particularly in the West. In the United Kingdom, where intensive care units have been stressed by the numbers of severe influenza cases, activity has peaked and the number of severe cases is now declining. Many countries of Western Continental Europe, are now reporting significant numbers of severe and fatal cases of influenza though intensive care units (ICU) have not yet been stressed to the degree seen in the UK. In Denmark, the proportion of patients in ICU due to influenza infection rose from 4% two weeks ago to 9.5% in the past week. In France, 290 influenza cases have required intensive care management. Notably, 41% of ICU cases in France were found to not have pre-existing underlying risk conditions, a higher proportion than seen in the UK, and 58% are between the ages of 15 and 64 years. Similar to observations in the U.K., only 8% of severe cases in France had been vaccinated against influenza. Of the samples tested from sentinel sites across Europe 43% were positive for influenza, unchanged for the last two weeks. The dominant strain in Europe is currently H1N1 (2009), with a few countries reporting co-circulation of influenza type B. Norway is a notable exception with predominance of influenza type B. Of influenza viruses that have been subtyped, 72% were type A and 28% type B. Of the type A viruses subtyped, 95% were H1N1 (2009) and 5% influenza A (H3N2). H1N1 (2009) appears to be over-represented in severe cases compared to frequency with which it is detected in the community. For example, in France of the ICU cases for which subtype data are available, 90% were found to have H1N1 (2009) or unsubtyped influenza A, 1% influenza A (H3N2) and only 8% influenza type B. A similar disparity between mild and severe cases was earlier noted in the U.K. As in North America, viruses that were antigenetically characterized in Europe were nearly all of the same lineages as those in the current trivalent seasonal influenza vaccine, with only a small number of influenza B viruses of the Yamagata lineages detected.
North Africa and the Middle East: Influenza transmission in North Africa and the Middle East appears to have peaked overall, though data from Tunisia, Morocco and Pakistan suggest an increasing trend. H1N1 (2009) is the predominant virus in the area, with the exception of Algeria which has reported a predominance influenza type B in recent weeks. However, the proportion of influenza type B in Algeria is declining with increasing detections of H1N1 (2009) . Transmission in Egypt peaked in late December 2010 or early January, along with the rest of the region, and the weekly reporting of fatal cases is declining.
Northern Asia: Influenza transmission in the temperate areas of Asia, including Mongolia, northern China, the Republic of Korea, and Japan has recently peaked and is now declining. There were some differences noted in the predominant viruses detected in countries of the area. Mongolia and northern China reported a predominance influenza A (H3N2), which appears to have peaked around the third week of December at which time increasing numbers of H1N1 (2009) viruses began to be detected. The Republic of Korea, in contrast, experienced peak of transmission coinciding with the peak in northern China, but over 90% of influenza viruses reported there were H1N1 (2009). Transmission in Japan began with a predominance of H3N2, with a peak that coincided with the rest of temperate Asia, but more recently H1N1 (2009) has become the predominant virus detected there though overall levels of detection are declining.
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Countries in the tropical zone
The most active area of transmission in tropical regions of the world is in Asia. In tropical areas of the Americas, reports indicate very little transmission of influenza viruses. Previously reported active circulation of H3N2 in Paraguay, continues to decline. The few viruses detected in the area otherwise are a mixture of influenza type B and (H1N1) 2009. In sub-Saharan Africa, available data indicate little activity in most countries. Madagascar, however, still has moderate levels of circulation of mainly H3N2 virus. In Asia, Singapore and Hong Kong report crossing their epidemic threshold of respiratory disease activity with the largest fraction of samples tested being H1N1 (2009) and an increase in H1N1 (2009) positive samples is also reported from Southern China. Data from South East Asia indicate only small numbers of influenza virus detections and no increases in respiratory disease activity.
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Countries in the temperate zone of the southern hemisphere
The countries of the southern temperate regions of the world have had very little influenza transmission since the end of their winter season. Australia, however, continues to report out-of-season transmission of small numbers of H3N2 and influenza type B viruses.
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Virological surveillance
During weeks 1 to 2, influenza activity in parts of the northern hemisphere continued to increase, in particular in Europe with A(H1N1) and B viruses predominating. In Asia influenza A(H1N1) and A(H3N2) predominated and in the North Americas A(H3N2) and B viruses co-circulated.
FluNet reports
During weeks 1 to 2 (02/01/2011 to 15/01/2011), National Influenza Centres (NICs) from 74 countries, areas or territories reported data to FluNet*. A total of 21,104 specimens were reported as positive for influenza viruses, 16,586 (78.6%) were typed as influenza A, 4,518 (21.4%) as influenza B. Of the sub-typed influenza A viruses reported, 74.5% were influenza H1N1(2009) and 25.4% were influenza A(H3N2). The proportion of H1N1(2009) compared to A(H3N2) reported to FluNet increased markedly compared to December.
Influenza virus detection by type/subtype in countries, areas or territories:
- Influenza H1N1 (2009): Albania, Algeria, Australia, Austria, Belarus, Belgium, Bosnia and Herzegovina, Bulgaria, Cameroon, Canada, Chile, China, Croatia, Czech Republic, Denmark, Egypt, Finland, France, France French Guiana, France Guadeloupe, Georgia, Germany, Greece, Hungary, Iceland, India, Iran (Islamic Republic of), Italy, Japan, Kenya, Latvia, Lithuania, Luxembourg, Malta, Mauritius, Mexico, Mongolia, Morocco, Netherlands, Norway, Oman, Pakistan, Poland, Portugal, Republic of Korea, Russian Federation, Serbia, Singapore, Slovenia, South Africa, Spain, Sri Lanka, Sweden, Switzerland, Togo, Tunisia, Turkey, Ukraine, United Kingdom of Great Britain and Northern Ireland, United States of America.
- Influenza A(H3N2): Albania, Australia, Canada, Chile, China, Costa Rica, Cuba, Denmark, Dominican Republic, France, France Guadeloupe, France Martinique, Germany, Ghana, Greece, Honduras, Iceland, Italy, Japan, Kenya, Lao People's Democratic Republic, Madagascar, Mexico, Mongolia, Morocco, Norway, Poland, Republic of Korea, Romania, Russian Federation, Serbia, Singapore, South Africa, Spain, Sweden, Turkey, United Kingdom of Great Britain and Northern Ireland, United States of America.
- Influenza B: Albania, Algeria, Armenia, Australia, Austria, Belarus, Belgium, Bosnia and Herzegovina, Cambodia, Cameroon, Canada, China, Costa Rica, Denmark, Dominican Republic, El Salvador, Finland, France, Georgia, Germany, Ghana, Greece, Honduras, Hungary, Iceland, India, Iran (Islamic Republic of), Italy, Japan, Kenya, Kyrgyzstan, Lao People's Democratic Republic, Latvia, Lithuania, Luxembourg, Madagascar, Malta, Mexico, Morocco, Netherlands, Norway, Oman, Poland, Portugal, Romania, Russian Federation, Singapore, Slovenia, South Africa, Spain, Sri Lanka, Sweden, Switzerland, Tunisia, Turkey, Ukraine, United Kingdom of Great Britain and Northern Ireland and United States of America.
- No influenza activity reported: Afghanistan, Angola, Azerbaijan, Central African Republic, Ethiopia and Slovakia.
* Some NICs report to FluNet retrospectively resulting in updates of previous data in FluNet and subsequent charts and maps which based on FluNet 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 25 January 2011, 15:00 UTC