[Source: World Health Organization, full PDF document: (LINK). Edited.]
Influenza Update N° 181 - 15 March 2013
- Influenza activity in North America continued to decrease overall, though activity remained high in some areas.
- The proportion of influenza B has increased in the United States of America (USA), but influenza A(H3N2) still remained the most commonly detected virus.
- The season in the USA has been more severe than any since 2003-4 as reflected in numbers of pneumonia and influenza deaths but the impact has been greatest in individuals over the age of 65 years.
- Activity in Mexico has also decreased over the past several weeks since peaking in mid to late January.
- Influenza activity remained high across Europe but an increasing number of countries reported declining transmission.
- The proportion of types and subtypes of viruses circulating was not uniform across the continent.
- Influenza B has been more commonly detected than A in some countries while, mainly in Eastern parts of Europe very little circulation of influenza B has been detected.
- Excess mortality in most countries has been moderate and most deaths occurred among people aged 65 and older.
- Influenza activity throughout the temperate region of Asia decreased overall except in Mongolia and the Republic of Korea where activity persists.
- Low levels of influenza activity were reported across the tropical regions of the world and activity in countries of the southern hemisphere remained at inter-seasonal levels.
- A couple of viruses with resistance to neuraminidase inhibitors have been detected in countries doing testing.
Countries in the temperate zone of the northern hemisphere
Overall influenza activity in North America continued to decrease during the first week of March since peaking in early January in Canada and the USA and approximately two weeks later in Mexico.
Despite this decrease, influenza activity still remained high in some areas of the region.
In Canada, the number of laboratory-confirmed cases increased slightly while still showing an overall decreasing trend from the previous weeks and a clear decrease from the peak in early January.
The national influenza-like illness (ILI) consultation rate continued to decrease from 31.8/1000 patient visits in the last week of February to 20.5/1000 in the first week of March.
The percentage of positive influenza tests during the first week of March increased slightly from the previous week, from 12% to 15%, but still remained at relatively low levels compared to previous levels.
The number of outbreaks reported in hospitals, long-term care facilities, and schools, was 31, a continued decrease from the peak of more than 120 during the second week of January.
During the first week in March, 112 influenza-associated hospitalizations were reported through the Aggregate Surveillance System (a subset of all influenza admissions in Canada). Those aged ≥65 years continued to make up a majority of hospitalizations (46%). Eleven influenza-related deaths were reported during the first week in March, of which eight were adults aged ≥65 years.
Since the beginning of the 2012-13 season, 254 deaths have been reported, 83% (211/254) of which were ≥65 years of age. Twenty-six new laboratory-confirmed influenza-associated paediatric admissions under the age of 16 years were reported through the Immunization Monitoring Program Active (IMPACT) network making a total of 654 reported since the beginning of the 2012-13 influenza season.
Of the 793 influenza viruses detected in the first week of March in Canada, 64.1% were identified as influenza A; of the influenza A viruses with subtype information, 54% were A(H3N2), 46% were A(H1N1)pdm09. The proportion of positive influenza detections attributed to influenza B has notably increased over the past several weeks from 2.1% of weekly detections in the middle of January, to 35.9% in the first week of March. However, cumulative seasonal proportions still remain largely higher for influenza A (94.8%) compared to influenza B (5.2%).
Regionally, this increased proportion of influenza B detected appeared to be concentrated in more western provinces with the proportion decreasing further east. Among paediatric hospitalizations reported during the first week of March, 50% (13/26) were associated with influenza B, compared to 20% (3/15) among adults aged ≥ 16 years.
Cumulatively, since the beginning of the 2012-13 season among paediatric (aged ≤16 years) and adult (aged ≥ 16 years) hospitalizations, 9.9% (65/654) and 2.7% (37/1385) respectively were identified as influenza B.
Since the start of the season, the National Microbiology Laboratory has antigenically characterized 703 influenza viruses (463 A(H3N2), 93 A(H1N1)pdm09, and 147 influenza B). Of these, all influenza A and influenza B viruses were antigenically similar to the 2012-13 northern hemisphere vaccine viruses, with the exception of 32/147 influenza B viruses which were similar to the B/Brisbane/60/2008 (Victoria lineage) virus; a component of the 2011-2012 seasonal influenza vaccine.
In the 2012-13 season, none of the influenza viruses that have been tested against oseltamivir and zanamivir have shown resistance.
In the USA, influenza activity continued to decrease in most areas since peaking in late December to early January but still remained elevated during the first week of March. Nationally, the proportion of ILI outpatient consultations decreased from 2.8% in the previous report to 2.3% during the first week of March, but still remained slightly above the national baseline of 2.2%. The proportion of clinical ILI specimens testing positive for influenza increased slightly from 16.8% in the last report to 17.2% during the first week of March, but remained lower than the peak of 38% in the last week of 2012.
The proportion of all deaths attributed to pneumonia and influenza (P&I) reported through the 122 Cities Mortality Reporting System continued to decrease from the peak of 9.8% in the fourth week of January, to 7.7% in the first week of March. This is still slightly above the epidemic threshold of 7.5%.
This season’s P&I peak represents the second highest level seen in the previous 10 years, with only the 2003-04 influenza season achieving a higher peak (10.4%). In addition, 87 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 10 721 laboratory-confirmed influenza-related hospitalizations have been reported since the beginning of the season (cumulative rate of 38.5/100 000 population). This is notably higher than the previous three seasons (8.6, 21.4, and 29.0/100 000 population for the 2011-12, 2010-11, 2009-10 seasons respectively). The rate for individuals hospitalized for influenza over age 65 years was markedly higher than for other age groups, representing 51% of all reported cases. The rate for other age groups remained at levels similar to previous years.
Since the beginning of the current influenza season, 76% of influenza viruses have been influenza A and 24% influenza B. Of the influenza A viruses with subtype information, 96% have been influenza A(H3N2). However, in recent weeks the proportion of influenza B viruses detected has increased.
During the first week of March, 64% (689/1074) of all influenza positive specimens reported were influenza B. This seasonal trend in the USA is in contrast to Canada, where 96.5% of the confirmed specimens were influenza A. Since the beginning of the 2012-13 influenza season, the Centers for Disease Control and Prevention has antigenically characterized 1472 influenza viruses (105 influenza A(H1N1)pdm09, 937 influenza A(H3N2), and 430 influenza B viruses). Of these, all influenza A and influenza B viruses were antigenically similar to the 2012-13 northern hemisphere vaccine viruses, with the exception of 122/430 influenza B viruses which belonged to the B/Victoria/02/87 –like lineage.
In addition, 0.4% (4/937) of A(H3N2) and 1% (1/105) of A(H1N1)pdm09 viruses tested showed reduced titres with antiserum produced against the A/Victoria/361/2011 and A/California/7/2009 viruses respectively. Since the beginning of the season, none of the influenza A(H3N2) and influenza B viruses tested were resistant to the neuraminidase-inhibitors, oseltamivir and zanamivir.
Two oseltamivir-resistant A(H1N1)pdm09 virus have been reported out of 358 tested in this season.
Mexico recently reported updated data which indicates a decreasing trend of influenza activity over the last few weeks with a majority of influenza A(H3N2); mirroring the USA. Influenza activity in Mexico appears to have peaked approximately two weeks after the USA.
Influenza activity remained elevated across Europe but an increasing number of countries reported declining transmission, particularly in the western part of the continent. Consultation rates of ILI and acute respiratory infections peaked around the last week of January in western Europe, but all countries that use seasonal thresholds reported rates above their national threshold levels.
The proportion of influenza positive sentinel specimens continued to decrease from previous reports. For all of Europe, 49% of respiratory specimens tested (1079/2217) were positive for influenza in the last week of February compared to nearly 60% at the peak in the last week of January. In the countries with surveillance for severe acute respiratory infections (SARI), the weekly number of cases has also peaked, though not in all countries in the eastern part of the region.
The proportion of respiratory specimens testing positive for influenza in the Russian Federation increased in the last week of February and do not yet appear to have peaked.
Excess mortality seen in pooled numbers of all-cause mortality from 15 countries participating in the European Mortality Monitoring Project was near the baseline for this time of year and has been lower overall for the entire season compared to the previous two years. In most countries winter excess mortality is considered moderate in comparison to previous years. However in Denmark, the highest increase and longest sustained excess mortality have been observed; this season influenza activity in Denmark has been dominated by A(H3N2) circulation.
Overall, A(H1N1)pdm09 remained the most commonly detected influenza virus in Europe, however, this pattern has not been uniform across the continent. Of more than 64 000 influenza viruses characterized since the beginning of the season in Europe, 66% were type A while 34% were type B.
Of the influenza A viruses with subtype information, 72% (19 944/27 810) were A(H1N1)pdm09 and 28% (7866/27 810) were A(H3N2). However, France, Ireland, Italy, Spain adn The United Kingdom detected a higher proportion of type B viruses. In contrast, very little circulation of influenza B was observed in Eastern Europe.
Of the 1560 antigenic characterizations of influenza A viruses reported for sentinel and non-sentinel specimens since the beginning of the season, all influenza 1079 A(H3N2) viruses that have been antigenically characterized have been A/Victoria/361/2011(H3N2)-like, the strain contained in the current seasonal trivalent vaccine. Of the 1136 antigenic characterizations of influenza B viruses reported, 50% (566/1136) have been characterized as B/Estonia/55669/2011-like (B/Yamagata/16/88-lineage) and 24% (272/1136) as B/Wisconsin/1/2010-like (B/Yamagata/16/88-lineage).
Since week 40/2012, a total of 730 viruses from 11 countries have been tested for antiviral susceptibility to the neuraminidase inhibitors oseltamivir and zanamivir.
Five A(H1N1)pdm09 viruses were found to have the neuraminidase H275Y amino acid substitution, causing resistance to oseltamivir. two viruses from the United Kingdom were detected in outpatient patients, not exposed to oseltamivir through treatment. Three viruses from the Netherlands (2) and Switzerland (1) were detected in hospitalized immunocompromised patients exposed to oseltamivir through treatment. The 194 influenza A(H3N2) tested showed susceptibility to both drugs. Of the 234 influenza B viruses tested, 233 showed susceptibility to both drugs while one virus with reduced inhibition with oseltamivir, was detected in the United Kingdom in a patient without exposure to antiviral treatment.
Northern Africa and the eastern Mediterranean region
Across the northern Africa region the number of positive influenza specimens reported has increased over the past several weeks with individual countries such as Algeria and Tunisia reporting increased co-circulation of influenza A(H1N1)pdm09 and influenza B.
In the eastern Mediterranean region, influenza activity has continued to decrease from its peak in the first week of February. A(H1N1)pdm09 has remained the dominant virus of this season through this region. During the first week of March, Israel reported relatively high but decreasing activity.
Influenza activity continued to decrease in much of the temperate region of Asia.
In northern China and Japan ILI activity and the percentage of ILI specimens testing positive for influenza has declined for approximately four weeks. In contrast, activity in Mongolia has remained persistently high and has not yet appeared to decrease, but is still within seasonal tolerance levels. Influenza activity in the Republic of Korea has continued to increase slightly and may not yet have peaked.
As described in previous reports, influenza A(H3N2) has been the most commonly detected virus in most of northern Asia this season, however, northern China continued to report an increasing relative proportion of influenza A(H1N1)pdm09 in recent weeks. In northern China during the first week of March, all 75 influenza viruses detected were influenza A. Of those, 70.7% (53/75) were A(H1N1)pdm09 and 28.0%(21/75) were A(H3N2). Among influenza viruses antigenically characterized by the Chinese National Influenza Center since the beginning of the 2012-13 season, 99.2% (130/131) of influenza A(H1N1)pdm09 and 100% (519) of A(H3N2) were related to A/California/7/2009-like and A/Victoria/361/2011(H3N2)-like respectively. For influenza B viruses, 96.5% (139/144) of influenza B/Victoria viruses and 100% (20) of influenza B/Yamagata viruses were related to B/Brisbane/60/2008-like and B/Wisconsin/01/2010-like respectively.
None of the three influenza A(H1N1)pdm09 and four influenza B viruses tested for antiviral resistance during the first week of March were resistant to the neuraminidase inhibitors, oseltamivir and zanamivir.
Countries in the tropical zone
Tropical countries of the Americas/Central America and the Caribbean
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 acute respiratory cases were reported to be non-influenza illnesses, with respiratory syncytial virus and rhinovirus as the most commonly reported infections.
Individually, in Cuba and Nicaragua, the most commonly detected viruses were A(H1N1) and B respectively.
In Tropical South America, acute respiratory disease activity remained low and within expected levels.
Brazil, Ecuador, and Peru reported small numbers of influenza A(H3N2) and B.
Central African tropical region
Most countries in the Central African tropical area experienced low detections of influenza, while in Cameroon and Madagascar, slight but continuous circulation of influenza B was reported.
Influenza transmission in southern Asia was at low grade levels with not much activity. In Thailand, there is a co-circulation of seasonal influenza A(H3N2) and type B. In the first week of March, most countries in the region reported either very low numbers or no positive specimens for influenza from ILI surveillance.
India reported low grade transmission of A(H1N1)pdm09 and even fewer influenza B compared to previous weeks. Pakistan reported decreasing levels of influenza activity with only influenza B in circulation while Sri Lanka reported an increase in number of specimens positive for influenza compared to previous week with all three types/subtypes in nearly equal proportions.
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.
Link to web pages
- Epidemiological Influenza updates: http://www.who.int/influenza/surveillance_monitoring/updates/latest_update_GIP_surveillance
- Epidemiological Influenza updates archives 2012: http://www.who.int/influenza/surveillance_monitoring/updates/GIP_surveillance_2012_archives
- Virological surveillance updates : http://www.who.int/influenza/gisrs_laboratory/updates/summaryreport
- Virological surveillance updates archives : http://www.who.int/influenza/gisrs_laboratory/updates/en/index.html