03 February 2012
Update number 152
- 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 though notable local increases in activity have been reported in North America, the western part of Europe, and northern China.
• Countries in the tropical zone reported low levels of influenza activity with the exceptions of southern China, Colombia, and Ecuador.
• Influenza activity in the temperate countries of the southern hemisphere is at inter-seasonal levels. The low level inter-seasonal transmission of A(H3N2) previously noted in Chile and Australia appears to be diminishing and becoming more sporadic.
• The most commonly detected virus type or subtype throughout the northern hemisphere temperate zone has been influenza A(H3N2) with the exception of China, which is reporting a predominance of influenza type B, and Mexico, where influenza A(H1N1)pdm09 is the predominant subtype circulating. In addition to Mexico, some southern states of the United States of America and Colombia in northern South America have also reported a predominance of A(H1N1)pdm09 in recent weeks.
• Nearly all influenza A viruses characterized are antigenically related to the viruses contained in the current northern hemisphere trivalent vaccine. About half of the small number of influenza type B viruses characterized are of the Yamagata lineage, which is not contained in the current vaccine.
• Oseltamivir resistance has been observed at very low levels and has not increased notably over levels reported in previous seasons.
Persistent upward trends in influenza activity have been reported across the northern temperate region numbers of both mild and severe cases are low so far.
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 is slowly increasing, particularly in the west, and is currently dominated by influenza A(H3N2). In countries in the west of the region, low influenza activity was reported by 23 of the 27 reporting countries while Bulgaria, Iceland, Italy and Spain reported medium activity. Eight countries reported local or regional spread and nine countries reported increasing clinical activity trends relative to the previous week. Notable increasing trends in virus detections have been observed to be persisting over several weeks in Spain, Italy, Turkey, Belgium, Sweden and Norway and across Europe, 29% of samples tested from sentinel outpatient clinics were positive for an influenza virus, a continued increase from recent weeks. Bulgaria, Iceland, Italy and Spain reported medium intensity of influenza activity. Italy also reported a moderate impact on health care services. All-cause mortality remains low compared to previous years at this time of the season. To the east, in the 11 countries reporting on severe acute respiratory infections (SARI) the number of hospitalizations remained largely unchanged compared to the previous week, except in Kazakhstan, which reported increases in SARI hospitalization mainly in children aged 0–4 years. Ninety-five percent of influenza viruses detected were type A and of the influenza A viruses subtyped, 99% were A(H3N2). Fifty-six influenza viruses were characterized antigenically: 46 were A(H3N2) and 2 were A(H1N1)pdm09; all were antigenically related to the viruses found in the current Northern Hemisphere trivalent vaccine. Of the eight influenza type B viruses characterized, four were B/Brisbane/60/2008-like, the B/Victoria lineage contained in the vaccine, and four were of the Yamagata lineage). All 35 viruses (including 27 A(H3N2) viruses, four A(H1)pdm09 and four type B) tested for sensitivity to neuraminidase inhibitors were susceptible.
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) was reported in Costa Rica, Colombia and Ecuador. Influenza A(H1N1)pdm09 has been the most common virus detected in Colombia and Ecuador and A(H3N2) slightly more common in Costa Rica. Notable increasing trends in virus detections were observed to be persisting over several weeks in Ecuador and Colombia.
In sub-saharan Africa, only sporadic detections or low level transmission were reported. Sporadic circulation of influenza A(H3N2) and type B has been reported in Togo while Cameroon reported cases of A(H3N2). Kenya has reported increasing case numbers of influenza type B in the past 2 weeks.
Overall, the influenza activity in tropical Asia remained low. Since September, India has continued to report low level influenza B circulation. Southern China is reporting increasing cases of influenza type B and the percentage of hospital visits for ILI has increased to 3.7%, higher than in recent weeks and higher than at the same time last year. The percentage of specimens testing positive for influenza was 34% and of those, 95% were influenza type B. Influenza type B and A(H3N2) continued to be detected in low numbers across other parts the region particularly in Lao People's Democratic Republic, Viet Nam, and Indonesia.
In temperate countries of the southern hemisphere, influenza activity is at inter-seasonal levels. The previously noted persistent inter-seasonal transmission of influenza A(H3N2) in Chile and Australia appears to be diminishing. Paraguay has reported an increase in the number of samples testing positive for influenza, predominantly type A(H3N2).
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.