17 February 2012
Update number 153
- Countries in the temperate zone of the northern hemisphere
- Countries in the tropical zone
- Countries in the temperate zone of the southern hemisphere
- From the peer-reviewed literature
- Virological surveillance
• Influenza activity in the temperate regions of the northern hemisphere remains low overall. It has continued to increase in the United States and Canada, though overall activity is low. Some countries of western Europe, North Africa, and northern China appear to have reached peak transmission but activity continues to increase in eastern Europe. The levels of both mild and severe disease have been relatively low compared to previous years in most areas reporting.
•Countries in the tropical zone reported low levels of influenza activity with the exception of a few countries in the Americas and parts of southern Asia.
• The most commonly detected virus type or subtype throughout the northern hemisphere temperate zone has been influenza A(H3N2) with the exception of Mexico, where influenza A(H1N1)pdm09 is the predominant subtype circulating, and China and the surrounding countries which are reporting a predominance of influenza type B.
• Notable differences have been reported in the distribution of viruses in severe cases and between age groups. In Canada, influenza A(H1N1)pdm09 accounted for 27% of all influenza A viruses that have been subtyped in <5 year olds but only 5% of subtyped influenza A viruses in cases over the age of 65 years. In Europe, influenza A(H1N1)pdm09 was disproportionately found in cases admitted to hospital for severe acute respiratory infection compared to outpatient cases of influenza-like illness (13-20% vs. ~1.5% respectively).
• Nearly all influenza A viruses detected were antigenically related to the viruses contained in the current northern hemisphere trivalent vaccine.
• Oseltamivir resistance continues to be observed at very low levels and has not increased notably over levels reported in previous seasons.
Although persistent upward trends have been reported across the temperate region, it appears that some countries are reaching their peak, including a few countries of western Europe and all of North Africa. The season looks mild by all indicators.
In Canada, overall influenza activity increased in the third week of January but remained low in some areas of the country. The national consultation rate for influenza-like illness (ILI) in Canada decreased slightly but the proportion of samples testing positive increased to 3.4%. Seven regions reported localized influenza activity and 16 regions reported sporadic influenza activity. Eight outbreaks of influenza were reported, three in hospitals and five in long term care facilities, an increase from previous weeks. The national consultation rate for ILI and general level of influenza activity is mid to low range compared to this time period in previous years. Eighteen influenza-associated hospitalizations were reported this week (three pediatric and 15 adult). Since the start of the season, 38% of all pediatric influenza hospitalizations have occurred in children under the age of two years, while 45% of all adult hospitalizations have occurred in patients aged >65 years. In that time frame, 79% of laboratory confirmed cases were influenza type A and 21% type B; of the influenza A viruses that were subtyped, 90% have been influenza A(H3N2). Notably, the distribution of virus types and subtypes has not been uniform across all age groups. Fifty-three percent of all laboratory confirmed influenza A(H1N1)pdm09 cases, and 36% of all laboratory confirmed influenza B cases, have been in patients aged <5 years. Influenza A(H1N1)pdm09 accounted for 33% of all influenza A viruses that have been subtyped in <5 years old but only 3% of subtyped influenza A viruses in cases over the age of 65 years. All influenza A viruses characterized this season in Canada are antigenically related to the viruses contained in the current northern hemisphere trivalent influenza vaccine; however, only 21 of 35 (60%) influenza B viruses are antigenically related to the vaccine strain contained in the current vaccine. The other 14 influenza B viruses were antigenically related to the reference virus B/Wisconsin/01/2010-like, which belongs to the Yamagata lineage. All 79 influenza A viruses tested for antiviral resistance were susceptible to oseltamivir and zanamivir.
Nationally in the United States of America of America (USA), ILI consultations were low (1.4%) and remained below the national baseline level (2.4%). The percentage of samples positive for influenza increased to 4.9% but was as high as 14% in one region. ILI activity was reported to be low or minimal in all states. The proportion of deaths due to pneumonia and influenza reported in the 122 cities sentinel surveillance system has reached the epidemic threshold for the first time since the start of the season after being predominantly below the seasonal baseline for several weeks. Since October 2011, 166 laboratory-confirmed influenza hospitalizations were reported. Among these cases, 120 (72.3%) were influenza A, 38 (22.9%) were influenza B, and 2 (1.2%) were influenza A and B co-infections; 6 (3.6%) had no virus type information. Among the 52 of hospitalized cases with influenza A subtype information, 48 (92.3%) were A(H3N2) and four (7.7%) were A(H1N1)pdm09. The most commonly reported underlying medical conditions among adults hospitalized with influenza infection were chronic lung diseases, asthma and obesity. The most common underlying medical conditions in children hospitalized with influenza infection were neurologic disorders and obesity. More than a third of hospitalized children had no identified underlying medical condition. In the USA, the circulating virus is almost exclusively influenza A(H3N2), except in 6 States (Arkansas, Louisiana, New Mexico, Oklahoma and Texas) where A(H1N1)pdm09 has been predominant in the past 3 weeks. Ninety-nine percent of influenza A(H3N2) and 97% of A(H1N1)pdm09 viruses characterized were antigenically related to viruses contained in the current seasonal trivalent influenza vaccine. Fourteen of the 28 influenza B viruses tested 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 viruses tested since 01 October 2011 have been susceptible to the neuraminidase inhibitor antiviral medications oseltamivir and zanamivir.
In contrast to Canada and the USA, in Mexico the majority of all laboratory confirmed cases of influenza since late December 2011 were influenza A(H1N1)pdm09. Localized outbreaks of A(H1N1)pdm09 have also been detected in parts of the country, mostly in the southern States. The Ministry of Health of Mexico has reported that the situation there is similar to previous influenza seasons and that there is no evidence that the virus has changed in its behavior.
In Europe, influenza activity appeared to be levelling off in Spain and Italy, suggesting the season may now be peaking in a few countries in the west of Europe. In eastern Europe, Russia, Romania and Bulgaria have seen a trends of increasing influenza activity over recent weeks, though overall activity was still relatively low. All-cause mortality in the 20 western European countries that are partners in the European Mortality Monitoring Project remains low compared to previous years at this time of the season. In the 5th week of 2012, 37% of samples from sentinel outpatient clinics tested positive for influenza viruses in Europe, a continued increase from recent weeks. Of these, 96% were influenza type A and 4% were influenza B; 98% of influenza A specimens subtyped were A(H3N2). Hospitalizations due to severe acute respiratory infection (SARI) continue to be relatively stable with most cases being in the group aged 0-4 years. Nineteen percent of specimens from hospitalized cases of severe acute respiratory infections (SARI) tested positive for influenza, all type A. The distribution of virus types and subtypes in SARI cases has been reported to be different from that in ILI cases. In 159 SARI cases with subtype information from western Europe, 80% were associated with A(H3N2) infection, 13% with A(H1N1)pdm09 and 7% with type B viruses. From eastern Europe, 150 respiratory specimens were collected from SARI patients, of which 25 were subtyped: 20 (80%) as A(H3N2) and 5 (20%) as A(H1N1)pdm09. Twelve countries have characterized 103 influenza viruses antigenically. All of the influenza A viruses characterized have been antigenically related to the viruses in the current trivalent vaccine; four of eight type B viruses characterized were of the Victoria lineage included in the vaccine and the other four were of the Yamagata lineage. None of the 15 A(H1N1)pdm09, 46 A(H3N2) and 7 type B viruses tested for neuraminidase inhibitor susceptibility have been resistant since the start of the season.
Northern Africa and eastern Mediterranean
The northern Africa and eastern Mediterranean regions have begun to report a decreasing trend in numbers of positive influenza specimens though virus detection remained widespread. As in Europe, influenza A(H3N2) was the predominant subtype detected, accounting for nearly all of the viruses that have been subtyped.
Temperate countries of Asia
In northern China, both the percentage of outpatient visits that were due to ILI and the proportion of specimens testing positive for influenza (13%) increased since the last report. In contrast to other reporting regions, influenza type B virus is the predominant type across China. In the first week of 2012, 89% of all viruses subtyped in northern China were influenza type B. The Republic of Korea and Japan have reported a persistent increase in numbers of influenza positive specimens in recent weeks, predominantly A(H3N2). Influenza is spreading nationally across the Republic of Korea.
Tropical countries of the Americas
Circulation of influenza A(H1N1)pdm09 and A(H3N2) has been reported in Costa Rica Colombia, and Ecuador. Influenza A(H1N1)pdm09 has been the most common virus detected in Colombia and A(H3N2) slightly more common in Costa Rica and Ecuador.
In sub-Saharan Africa, only sporadic detections were reported.
Overall, the influenza activity in tropical Asia remained low. Influenza B circulation in India and South China has continued to decrease. In South China the percentage of hospital visits for ILI has decreased to 3.4%, lower compared to recent weeks and at the same time last year. Influenza type B detections increased in Lao People's Democratic Republic, Bhutan and Singapore.
In temperate countries of the southern hemisphere, influenza activity is at inter-seasonal levels. The previously noted persistent inter-seasonal transmission in Chile, Paraguay and Australia has diminished and virus detections are now sporadic in these countries.
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.