Friday, June 15, 2012

WER: Reviewing The 2011-2012 Northern Hemisphere Flu Season





Distribution of 2011-2012  Flu Strains – Source W.E.R.


# 6389


This week the World Health Organization’s Weekly Epidemiological Record (WER) is devoted to a review of the flu season just completed in the Northern Hemisphere. As you’ll see - while it was unusually mild in the United States - the experiences in other regions of the world varied.



I’ve excerpted portions of the report below, but you’ll probably want to read the entire article at this link:



15 June 2012, vol. 87, 24 (pp 233–240)

232 Review of the 2011–2012 winter influenza season, northern hemisphere


Review of the 2011–2012 winter influenza season, northern hemisphere

This report summarizes the chronology, epidemiology and virology of the winter influenza season in the temperate regions of the northern hemisphere. The review covers influenza activity data collected from October 2011 until the end of April 2012. The data presented have been derived primarily from reports published by national ministries of health or other official bodies reporting on their behalf, or reported to WHO through FluNet and FluID.



The report describes the mild flu season in the United States:


Illness and mortality

In the USA, influenza activity was considerably less in tense than in previous years; clinical consultations for ILI, reported hospitalizations, pneumonia and influenza mortality, and reported influenza-associated paediatric deaths were all lower than in recent years. The percentage of outpatient visits to sentinel physicians for ILI reached the national baseline of 2.4% but never exceeded it, a pattern which has not been observed in at least the last 15 years.


Laboratory confirmed influenza associated hospitalizations reported through the Emerging Infections Program, covering 80 counties in 10 of
the 50 states of the USA were lower than the previous year (8.6 per 100 000 population as of 30 April versus 21 per 100 000 population in 2010–2011) and mortality attributed to pneumonia and influenza (P&I) in the 122 Cities Mortality Reporting System slightly exceeded the epidemic threshold (1.645 standard deviations above the weekly mean) only once this season and was below the weekly historical 5-year average for much of the


The flu season in Europe was less consistent, with some countries seeing low activity, while others saw an average flu season.


Overall, influenza severity indicators were not consistent across Europe. In western Europe, numbers of ILI cases seen in primary care settings were more variable than usual with some countries experiencing relatively few cases scarcely reaching the epidemic baseline (e.g. the United Kingdom and Ireland) while others had more typical seasons (e.g. France and Spain).


The number of severe acute respiratory infections reported by 7  participating countries that were positive for influenza was slightly lower than last season (1282 as of 11 May 2012 versus 1548 at the end of the 2010–2011 season). The European Mortality Monitoring Project (EUROMOMO), which pools all-cause mortality data from 15 countries of Europe, reported excess mortality among persons ≥65 years of age in some countries,peaking in February 2012.

This was most notable in countries that experienced more community transmission, around the same time as the peak in influenza transmission. Mortality in the 15 to 64 year old age group was notably lower compared to the 2010–2011 season when A(H1N1)pdm09 was the predominant virus circulating in the area.


Meanwhile, Asia saw a very typical flu season, with the exception of Japan, which reported unusually heavy flu activity.


Illness and mortality

Reporting rates for ILI visits in northern China, Mongolia and Republic of Korea were all similar to those in previous seasons; however, Japan experienced the highest number of influenza-confirmed cases since 2002, except for the 2009 pandemic period. In Mongolia, the proportion of hospitalizations for pneumonia and the reported number of pneumonia deaths were lower
than during the 2010–2011 season.


As is often seen, the influenza strains varied in different regions around the world. While H3N2 virus was the most common strain reported, in Mexico it was the A(H1N1)pdm09 strain that dominated.


The season was predominantly associated with A(H3N2) in Europe and North Africa, though influenza B did increase slightly late in the season. Temperate countries of Asia had both influenza B peaks and A(H3N2) peaks, with influenza B appearing first in China and Mongolia followed by A(H3N2) and the reverse sequence in the Republic of Korea and Japan.



As far as antiviral resistance is concerned, while there were some instances reported, the numbers remain very low.


The great majority of the viruses tested this season were sensitive to oseltamivir. However, the late-season appearance of a number of cases with oseltamivir   resistant A(H1N1)pdm09 viruses in Texas, most of which had no direct or indirect exposure to the drug, raises some concern. A cluster of 29 oseltamivir resistant viruses was reported in New South Wales, Australia in the 2011 southern hemisphere winter season but did not result in onward persistence of the resistant virus.


And finally, as you’ve probably already heard, a new flu vaccine formulation will be introduced this fall that will include two new flu strains (see WHO: Northern Hemisphere 2012-2013 Flu Vaccine Composition).


The rationale for this change (the first in 3 years) is provided in this report:


antigenic testing

The seasonal trivalent vaccine for 2011–2012 contained the same 3 viruses as the 2010–2011 northern hemisphere vaccine: A/California/7/2009 (H1N1)-like virus, A/Perth/16/2009 (H3N2)-like virus and B/Brisbane/60/2008-like virus (B Victoria lineage). Early in the season, nearly all of the influenza A viruses detected globally were antigenically similar to the vaccine viruses.

However, increasing antigenic diversity was noted in A(H3N2) viruses in the latter part of the season. These viruses had reduced titre cross-reactivity with antiserum produced against the/Perth/16/2009 virus but higher titres against A/Victoria/361/2011-like reference viruses. In Europe this was associated with lower vaccine effectiveness than in previous seasons in well controlled field observational studies.

Because of antigenic heterogeneity within influenza A(H3N2) viruses irculating during this influenza season, the updated trivalent influenza vaccine for the northern hemisphere will contain an A/Victoria/361/2011-like virus.


Influenza B viruses of both the B/Victoria and the  B/Yamagata lineages circulated during this influenza
season in nearly equal proportions in some areas. The increasing proportion of viruses of the  /Yamagata  lineage prompted a change in the next season  vaccine composition to include a Yamagata virus  (B/Wisconsin/1/2010-like virus).



Now that the flu season in the Northern Hemisphere is essentially over (there are still a couple areas of activity, including Hong Kong and Bermuda), all eyes turn to the southern hemisphere where their flu season is just about to get started.


This year’s flu shot for the southern hemisphere is the same as was used in Europe and the Americas last fall, and so we shall be interested to see if the trend in lower vaccine effectiveness reported in Europe persists south of the equator.


As far as what this last flu season tells us about the next flu season to come?  Well, the old adage is that if you’ve seen one flu season, you’ve seen one flu season.


Influenza viruses are notoriously unpredictable, and flu seasons that span the globe, even more so.  We won’t know what kind of flu season we are going to have until we’ve had it.


A good enough reason to get that flu shot every year, and to practice good flu hygiene (covering coughs, washing hands, staying home when sick) all year round.