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Review of the 2012?2013 winter influenza season, northern hemisphere (WHO, May 31 2013)

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  • Review of the 2012?2013 winter influenza season, northern hemisphere (WHO, May 31 2013)

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


    Weekly epidemiological record / Relev? ?pid?miologique hebdomadaire

    31 may 2013, 88th year / 31 mai 2013, 88e ann?e, No. 22, 2013, 88, 225?232 - http://www.who.int/wer


    Review of the 2012?2013 winter influenza season, northern hemisphere

    This report summarizes the chronology, epidemiology and virology of the 2012?2013 winter influenza season in the temperate regions of the northern hemisphere. The review includes data collected from October 2012 until the end of April 2013.

    The data presented have been derived primarily from reports published by national ministries of health or other official bodies reporting on their behalf, or information reported to WHO through FluNet(1) or FluID.(2)
    Data sources and references are listed throughout the document.

    Transmission characteristics, illness and mortality data are presented by geographical region: North America, Europe, North Africa, Middle East and North Asia.

    Human influenza transmission usually occurs in the winter months in the northern hemisphere temperate zone but the exact timing and duration of the influenza season varies by country and year. The influenza season can begin as early as October but typically peaks around January.

    For the purpose of this report, the peak influenza activity is the week with the highest percentage of respiratory specimens testing positive for influenza viruses (Figure 1). Distributions of virus subtypes by transmission zone and/or countries are shown in Map 1.


    North America Transmission

    In North America, the start of the 2012?2013 influenza season was the earliest since 2003, except for the 2009 A(H1N1)pdm09 pandemic period. (...) In Canada and the United States of America (USA), influenza detections began to increase in early November and peaked in mid-January. The USA, which defines a threshold value for the percentage of outpatient visits that are related to influenza-like illness (ILI) to signal the start of the season, crossed the seasonal threshold in mid-November.

    In contrast, in the 2011?2012 influenza season, which was later than usual, the number of viral detections did not increase notably until early January and peak transmission occurred in mid-March.

    In Mexico, which lies in the tropics and subtropics, transmission became apparent even earlier than in the 2 temperate countries and peaked later at the end of January in the 2012?2013 winter.

    The distribution of influenza virus types and subtypes circulating in the 3 countries differed slightly. In Mexico, influenza B was the most commonly detected virus until the end of December accounting for >75% of viruses detected. (...)

    However, by early January influenza A(H3N2) had become much more prevalent. Influenza A(H1N1)pdm09 was not reported by Mexico at any point during the 2012?2013 season.

    In contrast, the USA and Canada detected relatively small numbers of influenza B early in the season. Throughout most of the season, influenza A(H3N2) accounted for >70% of the viruses detected in both countries until very late in the season. In both, influenza B became more common than influenza A towards the end of February, after peak transmission, when overall numbers of detections were relatively low.

    This persistence of influenza B coincided with a slight prolongation of ILI detections in the USA, a slight increase in paediatric influenza-associated deaths and a rise in the rate of outpatient visits for ILI in Canada ? these factors resulted in a slight prolongation of the transmission season.

    Relatively few influenza A(H1N1)pdm09 viruses were detected throughout the season, accounting for <5% of influenza A viruses with subtype information in the USA and about 12% in Canada.


    Illness and mortality

    More severe disease was seen in the USA and Canada in the 2012?2013 influenza season compared with recent years, particularly among the elderly. The percentage of ILI outpatient visits at sentinel hospitals was in the range seen in previous seasons and illness was rated as moderately severe by the US Centers for Disease Control and Prevention.

    The percentage of all deaths that were attributed to pneumonia and influenza reported through the 122 Cities Mortality Reporting System peaked at 9.8% in mid-January, the highest peak since the 2003?2004 season, and remained above the epidemic threshold for 12 weeks.(3)

    The reported number of paediatric deaths in 2012?2013 was approximately 3 times higher than in the 2011?2012 season (131 versus 34 paediatric deaths, respectively) but similar to the 2010?2011 season (123 deaths) and less than in the 2009?2010 pandemic season (282 deaths).

    However, population based laboratory-confirmed hospitalization rates among people ≥65 years old were more than 6 times higher in the 2012?2013 season compared with the 2011?2012 season (190.5/100 000 population versus 30.3/100 000 population) and nearly 3 times higher than in 2010?2011 (64.8/100 000 population).

    Similarly, in Canada, ILI consultation rates peaked above the 95% confidence interval of the average of previous 15 years (excluding the pandemic year).

    The highest number of outbreaks reported in Canada was 130 in the second week of January, more than twice as many as were recorded during the peak of transmission in either of the previous 2 years. The number of laboratory-confirmed influenza-related paediatric hospitalizations was higher in the 2012?2013 season than in 2011?2012 (820 versus 571, respectively).

    The number of laboratory-confirmed adult influenza hospitalizations reported to the Aggregate Surveillance System in 2012?2013 was 2.5 times that of the previous season (4639 versus 1866, respectively) and the number of deaths exceeded the previous year by almost 3 times (295 versus 104, respectively).

    As in the USA, adults ≥65 years of age were the worst affected making up 53% of adult hospitalizations and 82% of adult deaths.(4)

    Data from the USA indicate that among hospitalized adults for influenza, the most commonly reported underlying medical conditions were cardiovascular disease, metabolic disorders, obesity, and chronic lung disease.

    The most commonly reported underlying conditions among paediatric hospitalized patients were asthma, neurologic disorders, and immune suppression.


    Europe, North Africa and the Middle East Transmission

    In Europe, the influenza season began and peaked 2 weeks later than in North America with eastern Europe experiencing a slightly later start to the season than western Europe.

    The season was also unusually long, associated with a late rise in influenza B cases in many countries.

    Notable increases in influenza virus detections was observed in mid-December 2012 in western Europe and reached a peak in mid to late February 2013.

    In eastern Europe transmission began in late December to early January and peaked in late February to early March. By the end of April 2013, influenza activity in most countries had returned to inter-seasonal levels (Figure 1).(5, 6)

    In contrast to North America, the majority of influenza viruses characterized in Europe were influenza A(H1N1)pdm09. However, there were marked regional differences with influenza type B being more prevalent in some countries at different parts of the season.

    There was much less A(H3N2) observed during the season than either of the other 2 viruses. Throughout the season, >94 000 samples were collected and tested in Europe, of which 63% were influenza A and 37% were influenza B.

    Of the influenza A viruses with subtype information, 66% were A(H1N1)pdm09 and 34% A(H3N2). Influenza A(H3N2) accounted for 14% of viruses detected for all of Europe and but was less common in the east than the west.

    As in North America, the relative proportions of viruses detected shifted through the season, with influenza type B becoming much more common later in the season. In the third week of January, influenza A viruses accounted for 76% of all viruses detected but by late February, influenza B had become more common than influenza A. In the third week of April influenza B accounted for more than 60% of all influenza viruses.

    Transmission patterns in North Africa had a similar onset, peak and distribution of virus types and subtypes to Europe, with the notable exception of Egypt.

    In Egypt, increases in virus detections were reported in late October, almost entirely A(H3N2); peak transmission occurred in late December. Only very small numbers of A(H1N1)pdm09 viruses were detected in Egypt and only very late in the season.

    Influenza A(H1N1)pdm09 was also the most commonly detected virus In the Middle East and neighbouring countries in Asia throughout the season with relatively little A(H3N2) or type B.

    Only Jordan experienced a secondary peak of type B late in the season. The timing of peak transmission was variable, extending from late December in Iran to late February in Turkey.


    Illness and mortality

    Pooled all-cause mortality data from 13 countries and regions reported to the European Mortality Monitoring Project reflected a cumulative winter excess number of deaths higher than in the previous 3 seasons. This was most notable among persons ≥65 years old.(7) In England, where Public Health England reports mortality estimates throughout the season, estimated all-cause and respiratory deaths were similar to the last 3 non-pandemic years.


    Northern Asia Transmission

    Notable increases in detections of influenza viruses in the northern temperate regions of Asia began in mid-December 2012 for most countries in the region and had largely peaked by late January.

    The Republic of Korea was the one exception with a slightly later onset and a peak in late February. The entire area reported more influenza A(H3N2) than A(H1N1)pdm09, with some minor regional differences.

    In northern China, early transmission was almost entirely associated with influenza A(H3N2) but was largely replaced by A(H1N1) late in the season.

    In Mongolia and the Republic of Korea, both viruses circulated at roughly the same time, while in Japan, almost no A(H1N1)pdm09 viruses were detected during the entire season. Very few influenza type B viruses were reported by any of the countries in the region.(8)


    Illness and mortality

    Overall, ILI activity in reporting countries for the 2012?2013 season was similar to previous years with the exception of the 2009?2010 pandemic year.

    In northern China, the percentage of emergency visits due to ILI at sentinel hospitals established in 2005 peaked at just over 5%, which is a mid-range peak over the past 5 years except for the 2009 pandemic year.

    ILI rates reported in Mongolia, the Republic of Korea and Japan were also mid-range when compared with each country?s previous year?s data. In Mongolia, the percentage of all hospitalizations and the numbers of deaths due to pneumonia were similar to the 2011?2012 influenza season.


    Antigenic testing

    For the 2012?2013 season, the recommendations for the influenza trivalent vaccine included A/California/7/2009 (H1N1)pdm09-like, A/Victoria/361/2011 (H3N2)-like and B/Wisconsin/1/2010-like (from the B/Yamagata lineage) viruses. Throughout the season, 99% of both the A(H1N1)pdm09 and A(H3N2) viruses characterized have been A/California/7/2009 (H1N1)pdm09-like and A/Victoria/361/2011 (H3N2)-like, suggesting a good match with the current seasonal vaccine.

    However, the situation was somewhat more complicated with the B viruses. The large majority of influenza B viruses detected globally were of the Yamagata lineage and nearly all of those were antigenically similar to the B/Wisconsin/1/2010-like virus contained in the trivalent seasonal vaccine.

    A significant number of Victoria lineage viruses were also reported accounting for up to 30% of the B viruses in some countries.


    Antiviral sensitivity testing

    Less than 1% of the nearly 3000 viruses tested from across the northern hemisphere were found to have reduced sensitivity to neuraminidase inhibitors, oseltamivir and zanamivir. All were resistant to adamantanes.


    Summary

    The 2012?2013 influenza season started earlier in North America than other parts of the northern hemisphere temperate zone; however, transmission across the zone was largely finished by the end of April. The most common virus associated with transmission in this season varied markedly from place to place and throughout the course of the season.

    Influenza A(H3N2) was the most common in North America for most of the season; A(H1N1)pdm09 in Europe, North Africa and the Middle East; and A(H3N2) mixed with A(H1N1)pdm09 or A(H3N2) alone in much of temperate Asia. Influenza type B was seen more commonly than either of the influenza A subtypes by the end of the season in North America and Europe but not in other parts of the temperate northern hemisphere.

    The late appearance of influenza type B was associated with a slight prolongation of the season in the areas where it appeared.

    The season was more severe than usual in North America, particularly for those aged over 65 years, but appeared similar to previous seasons in the rest of the temperate zone.

    Notably, towards the end of the season in the USA when influenza B became more common, there was a slight transient increase in the number of paediatric deaths.

    The antigenic similarity of viruses tested this season to those contained in the trivalent seasonal vaccine suggest that the vaccine was generally a good match in the 2012?2013 season.

    While there were significant numbers of Victoria lineage viruses circulating, overall they made up a relative small number of the total influenza viruses detected globally.

    Reduced sensitivity to neuraminidase inhibitors was observed at only very low levels and does not appear to be increasing; however, as in previous years nearly all viruses tested were resistant to adamantanes. 
    ________

    (1) See WHO Influenza FluNet. (Available at http://www.who.int/influenza/gisrs_laboratory/flunet/en/, accessed May 2013.)

    (2) See WHO Influenza FluID. Available at http://www.who.int/influenza/surveillance_monitoring/fluid/en/, accessed May 2013.)

    (3) See CDC FLUVIEW weekly surveillance reports. (Available at http://www.cdc.gov/flu/weekly/weeklyarchives2012-2013/weekly41.htm and http://www.cdc.gov/flu/weekly/weeklyarchives2012-2013/weekly12.htm, accessed May 2013.)

    (4) Public Health Agency of Canada FluWatch. (Available at http://www.phac-aspc.gc.ca/fluwatch/12-13/index-eng.php, accessed May 2013.)

    (5) See WHO European Region EuroFlu Bulletin. (Available at http://www.euroflu.org/, accessed May 2013.)

    (6) See European Centre for Disease Prevention and Control Weekly Influenza Surveillance Overview. (Available at http://ecdc.europa.eu/en/healthtopics/seasonal_influenza/epidemiological_data/pages/weekly_influenza_surveillance_overview.aspx, accessed May 2013.)

    (7) See European Mortality Monitoring Project. (Available at http://www.euromomo.eu/, accessed 10 May 2013.)

    (8) See WHO Western Pacific Regional Office Influenza Situation Update. (Available at http://www.wpro.who.int/emerging_diseases/Influenza/en/index.html, accessed May 2013.)


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