[Source: World Health Organization, full page: (LINK). Edited.]
Influenza update - 02 March 2012, Update number 154
- Influenza activity in the temperate regions of the northern hemisphere is low but increasing in North America and most of Europe. A few countries of southern Europe appear to have now peaked along with the countries of northern Africa and the Middle East.
- Countries in the tropical zone reported low levels of influenza activity.
- Influenza activity in the temperate countries of the southern hemisphere is at inter-seasonal levels.
- The most commonly detected virus type or subtype throughout the northern hemisphere temperate zone has been influenza A(H3N2). Mexico is the exception, where influenza A(H1N1)pdm09 is the predominant subtype circulating and China and the surrounding countries where influenza type B is predominant. Influenza type B has been increasing in recent weeks in Canada as well.
- Oseltamivir resistance has not increased notably over levels reported in previous seasons.
- While most of the viruses characterized early this season were antigenically related viruses in the current trivalent vaccine, the vaccine strain selection committee in a meeting held on 20 - 24 February noted that there is evidence of increasing antigenic and genetic drift in circulating influenza A(H3N2) recently and that the proportion of type B viruses that are from the Yamagata lineage of type B has been increasing relative to the Victoria lineage. The committee therefore recommended a change in the composition of the next northern hemisphere vaccine formulation to include an A/Victoria/361/2011 (H3N2)-like virus and a B/Wisconsin/1/2010-like virus of the Yamagata lineage, and continuing the inclusion of an A/California/7/2009 (H1N1)pdm09-like virus.
- Recommended composition of influenza virus vaccines for use in the 2012-2013 northern hemisphere influenza season- full report pdf, 69kb
Countries in the temperate zone of the northern hemisphere
Influenza activity in the northern hemisphere increased overall but has already peaked in a few countries of Europe, Northern Africa, and the Middle East. Whereas in some regions it appears to be a mild season, particularly in the United States of America (US) and the United Kingdom (UK) in other countries activity has reached levels similar to previous years.
In Canada, overall influenza activity remained low in most regions of the country, but continued to increase. Six regions reported localized influenza activity and 22 regions reported sporadic influenza activity, a slight increase from the last report, but with no widespread activity reported. Consultation rates for influenza-like illness (ILI) increased in parallel to an increase in the proportion of samples positive for influenza from 6.4% to 10.5%.
The level of ILI activity reported nationally is near the national average for previous seasons. This season, 35% of pediatric hospitalizations have been in children aged <2 years and 49% of adult hospitalizations occurred in patients aged >65 years. Forty-three percent of pediatric admissions have been associated with influenza type B.
Overall, influenza type A viruses make up 56% of influenza viruses detected since the start of this season and 44% are influenza B. The proportion due to influenza type B has been increasing over the last few weeks.
Of the influenza A viruses that have been subtyped, 81% have been influenza A(H3N2) and 19% A(H1N1)pdm09.
Distribution of virus types and subtypes varied across age groups. In patients aged <5 years, influenza type B has been found in 58% of cases compared to only 25% of cases in adults aged >65 years. Similarly, 41% of all influenza A(H1N1)pdm09 cases were in children age <5 years and only 13% in adults aged >65 years.
Of the 354 influenza viruses that have been characterized since the start of the season, all 165 influenza A viruses were antigenically related to the viruses contained in the current northern hemisphere trivalent influenza vaccine. However, only 54% of influenza B viruses are antigenically related to the vaccine strain B/Brisbane/60/2008; the remaining 46% of influenza B viruses belong to the Yamagata lineage. This proportion of B viruses that are from the Yamagata lineage has been increasing during the season. Since the start of the season, all viruses tested for antiviral resistance were susceptible to oseltamivir (279 tested) and zanamivir (238 tested).
In the United States of America, influenza activity increased but remained relatively low. Nationally, ILI consultations have still not crossed the national seasonal threshold, however, the percentage of samples testing positive for influenza increased to 14%, a further increase from previous weeks and above the seasonal threshold for this parameter. Only one state, Missouri, reported high level ILI activity.
The proportion of deaths due to pneumonia and influenza reported in the 122 cities sentinel surveillance system has been below the seasonal baseline for the last 3 weeks.
One influenza-associated paediatric death was reported bringing the total for the season to 3.
Among 347 hospitalized cases from a sentinel facility surveillance network, 293 (84.4%) were influenza A and 45 (13.0%) were influenza B. Among those with influenza A subtype information, 87 were H3N2 and 26 were 2009 H1N1. The most commonly reported underlying medical conditions among adults were chronic lung diseases, metabolic disorders and obesity. The most common underlying medical conditions in children were chronic lung diseases, asthma, neurologic disorders and obesity. However, 47.5% of hospitalized children had no identified underlying medical conditions.
In contrast to Canada, influenza type B accounts for only 9% of overall virus detections in the USA. The majority (81%) of influenza A viruses with subtype information are A(H3N2). Of the 397 influenza viruses characterized antigenically, 96.6% of influenza A(H3N2) and 95.2% of A(H1N1)pdm09 viruses were related to viruses contained in the current seasonal trivalent influenza vaccine.
Some antigenic drift away from the vaccine viruses has been detected recently in A(H3N2) viruses. Twenty-two of the 48 (45.8%) influenza B viruses characterized belong to the Victoria lineage of viruses and were characterized as B/Brisbane/60/2008-like, the influenza B component of the 2011-2012 northern hemisphere influenza vaccine. All 491 viruses tested since October 1, 2011 have been susceptible to the neuraminidase inhibitor antiviral medications oseltamivir and zanamivir.
In Mexico, since the beginning 2012 there were a total of 5,544 cases of influenza, of which 90.9% were influenza A(H1N1)pdm09. Among these cases, there were 180 deaths of which a similar proportion (92.2%) were associated with influenza A(H1N1)pdm09. By late February, the proportion of samples testing positive for influenza had decreased to 35% from a peak of over 50% in early January.
Europe an Central Asia
In Europe, influenza activity was reported to be increasing in 27 countries but the trend is variable across the continent. Influenza activity appears to have peaked for some of the countries including Italy and Bulgaria but continues to increase in the northern and eastern countries of Europe and Central Asia.
Turkey, however, peaked much earlier in parallel to countries of North Africa and the Middle East, and has been declining for several weeks.
Overall, 46% of specimens collected at sentinel outpatient clinics in the last week of February tested positive for influenza viruses, slightly more than the previous week. Rates of ILI are similar in magnitude to previous seasons in countries where the data are available and the number of all-cause deaths reported by the European Mortality Monitoring Project is below average for this time of year.
Influenza A(H3N2) continues to be the dominant subtype. Across Europe, 91% of laboratory confirmed ILI cases in sentinel sites have had influenza type A and 9% type B; 99% of influenza A viruses with subtype information have been A(H3N2).
Among cases of severe acute respiratory infections (SARI), in six countries of western Europe 87.8% of 611 cases reported since the beginning of the influenza season were associated with influenza A(H3N2) infection, 7% with A(H1N1)pdm09, and 5.2% with type B.
In 11 countries reporting 172 SARI cases from eastern Europe, 66.9% were associated with influenza A(H3N2), 26.1% with A(H1N1)pdm09, and 2.3% with type B.
Of 218 viruses characterized antigenically, all of the 198 type A viruses have been related to the viruses found in the current trivalent seasonal influenza vaccine; 12 of 20 influenza B viruses were of the Yamagata lineage, which is not in the vaccine. None of the nearly 200 viruses tested for oseltamivir or zanamivir sensitivity have been resistant.
Northern Africa and eastern Mediterranean
Influenza activity in the northern Africa and eastern Mediterranean regions peaked at the end of 2011 and beginning of 2012. Countries continued to report a decreasing trend in numbers of positive influenza specimens. As in Europe, influenza A(H3N2) has been the predominant subtype detected, accounting for nearly all of the viruses that have been subtyped.
Temperate countries of Asia
ILI rates in national sentinel hospitals in north China and in Mongolia were slightly lower than in the previous week and have been declining since peaking in early February. Generally rates of ILI this season have been similar to the 2010/11 winter in both countries. Influenza type B virus is still the predominant circulating type in both countries, but there has been a decrease in the number of positive cases reported overall.
In contrast, influenza activity in the Republic of Korea has reported a continued increase in numbers of influenza positive specimens in recent weeks, primarily associated with influenza A(H3N2) but with an increasing proportion of influenza type B.
ILI activity in the Republic of Korea is also similar in magnitude to previous years. In Japan the number of ILI cases appears to have peaked at late January, primarily associated with A(H3N2), and is currently declining.
Countries in the tropical zone
Tropical countries of the Americas
In this region, transmission was recently reported in Colombia (primarily A(H1N1)pdm09) and Ecuador (nearly even A(H3N2) and A(H1N1)pdm09) though data have not been updated in the last few weeks.
In sub-Saharan Africa, only low level or sporadic detections of influenza virus has been reported.
In this region influenza activity remained low. In India, influenza activity continues to decrease and there has been an increase in the proportion of influenza A. In Sri Lanka, influenza transmission is transitioning to influenza B. In Viet Nam and the Lao People’s Democratic Republic, influenza B is still the predominant type. Overall influenza circulation has increased in south China and influenza B is still the predominant strain.
Countries in the temperate zone of the southern hemisphere
In temperate countries of the southern hemisphere, influenza activity is 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.