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18 Feb 2013

Influenza Update N° 179 - 15 February 2013 (WHO, edited)

[Source: World Health Organization, full PDF document: (LINK). Edited.]

Influenza Update N° 179 - 15 February 2013



  • Influenza activity in North America, though high with A(H3N2) virus predominant, started decreasing. In the United States of America, the number of pneumonia and influenza-related hospitalizations among adults aged 65+ years continued to increase.
  • In Europe influenza activity continued to increase in the majority of countries, with A(H1N1)pdm09 virus predominant. Most countries reported medium-intensity transmission, wide geographic spread and increasing trends.
  • Influenza activity throughout the temperate region of Asia is ongoing.
  • In the Caribbean, Central America and tropical South America, influenza activity remained at low levels.
  • Most countries in Africa experienced decreasing influenza activity.
  • Influenza in the southern hemisphere remained at inter-seasonal levels.

Note: Global epidemiology and surveillance updates are periodically collected from data reported by National authorities or organizations responsible for reporting this data. For further information on specific influenza virus activity in the world and scientific literature for practitioners and other professionals in the field, please visit the links provided at the end of this document.



Countries in the temperate zone of the northern hemisphere

North America

Influenza activity in North America during week 5 (January 27 - February 2) of the 2012-13 influenza season was stable or decreased in most areas in the region, but widespread transmission was still reported by both Canada and the U.S.

In Canada, the number of laboratory-confirmed cases nationally, was at the same level with the previous week, and many regions continued to report widespread and localized activity. The national influenza-like illness (ILI) consultation rate increased from 36.6/1000 patient visits in the fourth week of January to 53.7 during the first week of February, which is above the expected level of this season.

On the other hand, the Influenza-positive percentage of clinical specimens from ILI patients that tested was stable from the previous week (22.4%) while the percentage for tests positive for respiratory syncytial virus as increased sharply. The number of outbreaks reported in hospitals, longterm care facilities, and schools, was 99, which is decreasing from the peak of the second week of January. In contrast, the number of outbreaks in schools increased compared to previous reports. In addition, 3010 influenza-associated hospitalizations have been reported through the Aggregate Surveillance System (a subset of all influenza admissions in Canada) compared to a total of 1777 reported for the entire previous season. More than half (57.9%) of the cases with available age were aged ≥65 years and 13.3% were children aged 0-4 years. Since the beginning of the 2012-13 season, 203 deaths have also been reported, 83.3% (169/203) of which were aged ≥65 years. Among 542 influenza-associated pediatric admissions under the age of 16 years reported through the Immunization Monitoring Program Active (IMPACT) network since the beginning of the season, 44.3% (240/542) were under the age of 24 months.

Of the 1511 influenza viruses detected in the first week of February, 94.6% were identified as influenza A and 5.4% were identified as influenza B. Of the influenza A viruses with subtype information, 80.5% were A(H3N2) and 19.5% A(H1N1)pdm09. The proportion of influenza B was slightly higher than the cumulative proportion throughout this season. Among 542 influenza-associated pediatric hospitalizations reported since the start of the season, 95.0% (515/542) have been associated with influenza A and 5.0% (27/542) with influenza B. Most of influenza A viruses (85.4%) were not subtyped; of those with subtype information, 86.7% (65/75) were A(H3N2) and 13.3% (10/75) were A(H1N1)pdm09. In comparison, the percentage of A(H1N1)pdm09 reported among hospitalized adults was 5.3% (5/95).

Since the start of the season, the National Microbiology Laboratory has antigenically characterized 452 influenza viruses (297 A(H3N2), 58 A(H1N1)pdm09 , and 72 influenza B). Of these, all influenza A(H3N2) and A(H1N1)pdm09 viruses were antigenically similar to the vaccine virus A/Victoria/361/2011 and A/California/07/09 respectively. Among the influenza B viruses, 58 were antigenically similar to the vaccine virus B/Wisconsin/01/2010 (Yamagata lineage) and 14 were similar to B/Brisbane/60/2008 (Victoria lineage; component of the 2011-2012 seasonal influenza vaccine). Thus far in the 2012-13 season, 396 influenza viruses have been tested for sensitivity to the neuraminidase-inhibitors, oseltamivir and zanamivir, and none were found to be resistant.

In the United States of America (USA), influenza activity decreased in most part of the country but still remained high in the first week of February. Nationally activity appears to have peaked in the second week of January. Nationally, the proportion of ILI outpatient consultations decreased for six consecutive weeks to 3.6% from a peak of 6.1% in the last week of 2012. The proportion of clinical ILI specimens testing positive for influenza also declined to 23.3% in the first week of February, from 37.6% at the end of 2012. In the first week of February, 38 states reported widespread geographic influenza activity, compared to 47 in the previous report. Those states reporting regional activity tended to be in the southeast region. The proportion of all deaths attributed to pneumonia and influenza (P&I) reported through the 122 Cities Mortality Reporting System slightly decreased to 9.0% in the first week of February, from the peak of 9.8% in the previous report, which is still well above the epidemic threshold of 7.4%. In the previous 10 years period, the highest reported peak of P&I mortality was 9.1% in the 2007-8 influenza season. In contrast, 59 influenza-related paediatric deaths have been reported so far this season, compared to 34 for the entire season in 2011-12, 122 in 2010-11, and 282 during the winter season of the 2009-10 influenza pandemic.

A total of 8293 laboratory-confirmed influenza-related hospitalizations have been reported since the beginning of the season (cumulative rate of 29.8/100 000 population). While still increasing, it is already higher than the previous three seasons (8.6, 21.4, and 29.0/100000 population for the 2011-12, 2010-11, 2009-10 seasons respectively). Among age groups, the rate for individuals hospitalized for influenza over age 65 years is drastically higher, reaching 134.8/100 000 compared to year-end cumulative totals of 25.3, 64.0, and 30.5/100 000 for the years 2009-10, 2010-11, and 2011-12 respectively. In contrast, the rate for other age groups remained at expected levels from previous years.In contrast to Canada, a higher proportion of influenza B virus has been observed in the US. Of 51 129 influenza viruses tested since the beginning of the season, 80.2% were influenza A and 19.8% were influenza B, compared to Canada, where 94.6% of the all confirmed influenza were type A viruses.

Since the beginning of the 2012-13 influenza season, the Centers for Disease Control and Prevention (CDC) has antigenically characterized 972 influenza viruses. All 66 A(H1N1)pdm09 viruses tested were characterized as A/California/7/2009-like and 99.7% (606/608) of the A(H3N2) influenza viruses tested were A/Victoria/361/2011-like, both of which are components of the 2012-13 Northern Hemisphere trivalent influenza vaccine. Of the 298 influenza B viruses tested, 70.8% (211/298) were characterized as B/Wisconsin/1/2010-like of the Yamagata lineage, a component of this season’s trivalent influenza vaccine, and 29.2% (87/298) were of the Victoria lineage.

Since the beginning of the season, none of the tested 1001 influenza A(H3N2) and 298 influenza B viruses were resistant to the neuraminidase-inhibitors, oseltamivir and zanamivir. One oseltamivir-resistant A(H1N1)pdm09 virus was reported during the third week in January out of 184 tested in this season.




Influenza activity in Europe continued to increase in the majority of countries of the region, though some decreases have also been reported. Similar to the last report, most countries reported mediumintensity transmission, wide geographic spread, and 22 countries reported increasing trends in influenza activity. Belgium, Germany, Luxembourg and Sweden reported high-intensity transmission while 19 countries reported medium intensity and 4 (including Poland and the United Kingdom) reported low intensity. Some countries (such as Norway, Poland and the United Kingdom) seem to have experienced peaks in clinical activity earlier in the season, although some countries experienced a resurgence of ILI rates (Denmark, Greece, Ireland and Luxembourg).

Circulation of influenza viruses remained diverse across the region and similar to the previous two weeks. Influenza A, mainly A(H1N1)pdm09, was reported as the dominant virus in countries in northern, eastern and central Europe, including Austria, the Czech Republic, Germany, and the Russian Federation, while influenza B was reported as the dominant virus in some countries in the southern and western parts of the Region, including France, Ireland, Italy, Spain and United Kingdom .

Between these areas, co-circulation of A(H1N1)pdm09, A(H3N2) and influenza B was reported. The proportion of influenza-positive sentinel specimens increased to reach the highest level so far this season, 52% (1541/2949), increasing from about 40% over the preceding 3 consecutive weeks. The proportion of hospitalized cases of severe acute respiratory infection (SARI) that were positive for influenza also continued to increase, and was mainly due to influenza A(H1N1)pdm09 viruses. It seems that although there is an increase in the proportion testing positive at sentinel hospitals, there has not been increase in number of cases hospitalized. In data from European sentinel sites, the proportions of viruses found in ILI and ARI specimens is similar to that found in hospitalized cases. Of cases with reported age, most were in the 0-4 age group. Pooled numbers of all-cause deaths reported by 13 countries participating in the European Mortality Monitoring project have also been close to historical median values. However, 2 of the 16 reporting countries, Denmark and the United Kingdom (Scotland & England) saw increased mortality in people 65 years of age and above.

During the 2012-2013 influenza season, 70% (21 097/30 119) of influenza virus types observed were influenza A and 30% (9022/30 119) influenza B. Of the influenza A viruses that were subtyped, 71% (9355/13 107) were A(H1N1)pdm09 and 29% (3752/13107) were A(H3N2). The proportion of A(H1N1)pdm09 viruses has increased over the past two weeks. Since the start of the 2012-13 season, the majority of viruses characterized are antigenically similar to the viruses included in the current Northern Hemisphere seasonal influenza vaccine.

Since the start of the 2012-2013 season, only 1 oseltamivir-resistant A(H1N1) virus, of 278 screened, has been reported in Europe. All other viruses tested have been susceptible to the neuraminidase inhibitors.



Northern Africa

Across the northern Africa the number of positive specimens reported has continued to decrease with minor fluctuations over the past few weeks. However, some individual countries have shown increases in the number of influenza positive specimens reported in the last few weeks such as Algeria and Tunisia. Both having co-circulating virus influenza A (H1 and H3) viruses as well as influenza B with slight H1N1(pmd09) dominance.



Western Asia transmission zone

In western Asia there was an increase in influenza positivity in the last week, especially in Turkey and Georgia. However, some individual countries have reported have reported decreasing trends in the such as Egypt and Jordan. Virus subtype predominance across the region varied with more northern countries such as Georgia and Turkey showing strong A(H1N1)pdm09 dominance while other areas such as Egypt and Jordan reported more A(H3N2) and influenza B respectively. While these respective trends may be consistent with those of previous weeks for countries like Egypt and Turkey, Bahrain and Jordan have seen much more fluctuation between A(H1N1) and influenza B dominance.



Central Asia

Influenza activity throughout the Central Asian region has increased slightly over the past several weeks in countries such as Kazakhstan, Kyrgyzstan, and Uzbekistan. Kazakhstan reported predominance of A(H3N2), Uzbekistan reported co-dominance of influenza B and A(H3), and Kyrgyzstan reported co-dominance of influenza B and A(H1N1)pdm09.


Temperate Asia

Influenza activity throughout the temperate region of Asia is ongoing.

In China, during the last week in January, the percentage of visits due to ILI in northern China remained stable at 3.5% which is in the range seen in the previous 6 years. In Mongolia during the same period, ILI activity continued to show an increasing trend with an increase in detection of influenza activity .

In both south and north China, the percentage of A(H1N1)pdm09 circulating continued to increases and exceeded the percentage of A(H3N2) and influenza B. During the last week in January, 23.2% (543/2342) of samples tested for influenza were positive, of which 98.5% (535/543) was influenza A.

The percent positive tests in both north and south china during the last week in January were both slightly higher, 31.8% and 14.5% respectively, than in previous weeks.

Among influenza viruses antigenically characterized by the Chinese National Influenza Center in during the last week of January, all influenza A(H1N1)pdm09 and A(H3N2) were related to A/California/7/2009-like and A/Victoria/361/2011(H3N2)-like respectively. Likewise, all influenza B/Victoria and B/Yamagata viruses were related to B/Brisbane/60/2008-like and B/Wisconsin/01/2010-like respectively. None of the influenza specimens tested were resistant to the neuraminidase inhibitors, oseltamivir and zanamivir. In China, Hong Kong SAR specifically, similar trends were seen, however one sporadic isolate of A(H1N1)pdm09 was found to be resistant to oseltamivir.

Influenza activity continued to increase in the Republic of Korea, with detections of A(H3N2) and A(H1N1)pdm09. Influenza activity spiked in Japan during the second week in January, but has since gradually decreased, with influenza A(H3N2) continuing to be the dominant sub-type.



Countries in the tropical zone

Tropical countries of the Americas

In both Central America and the Caribbean, influenza activity was similar or decreased compared to previous weeks and continued to decrease from their peaks in late summer. In general, most ILI and ARI cases are reported to be non-influenza illnesses, with RSV and rhinovirus the most commonly reported infections. In those countries reporting influenza transmission, co-circulation of influenza A(H1N1)pdm09, A(H3N2) and B is occurring at low levels.


Central African Belt

Most countries in the Central African Belt experienced decreasing detections of influenza. Influenza A(H1N1)pdm09 that had been circulating previously in the Democratic Republic of the Congo and Ghana has decreased to inter-seasonal levels. Cameroon and Madagascar continued to report low levels of influenza B, while Ghana and Tanzania reported slight increase of Influenza B.



Tropical Asia

Influenza activity in most countries of South East Asia has remained similar to previous weeks, with slight decreasing and low-level circulation in Cambodia, India, Sri Lanka, Thailand and Viet Nam. The Islamic Republic of Iran saw a moderate decrease in influenza activity since the last report.

India saw A(H1N1)pdm09 and influenza B predominance while Sri Lanka continued to see cocirculation of all three predominant subtypes. In contrast, more south eastern countries such as Cambodia, Thailand, and Viet Nam showed much higher influenza B and A(H3N2)Influenza codominance.

In Thailand particularly, this is in contrast to approximately 12-15 weeks prior, where A(H1N1)pdm09 was instead the co-circulator with influenza B.

Influenza activity in Singapore remained below seasonal thresholds and continues to report cocirculation of all three seasonal virus types virus.



Countries in the temperate zone of the southern hemisphere

Influenza activity in all temperate countries of the southern hemisphere is now at inter-seasonal levels.



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 and Response System) 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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