• Influenza activity in the temperate regions of the northern hemisphere remains at baseline inter-seasonal levels.
• Countries in the tropical zone mostly report low influenza activity but with some transmission reported in countries of the Americas, western Africa, and southern Asia.
• The influenza season is ongoing in South Africa though it appears to have recently peaked. Some detailed preliminary information is now available for severe cases in South Africa (see below). In Australia – the season appears to have started with notable increases in influenza-like illness (ILI) consultations and confirmed cases. The most common virus detected nationally in Australia is influenza A(H1N1)2009, though this is not consistent in every state
Countries in the temperate zone of the northern hemisphere
The influenza season in the northern hemisphere temperate areas has ended. Nearly all of the countries of North America, Europe, northern Africa and north Asia reported low or no influenza activity.
Countries in the tropical zone
Influenza activity in countries of tropical zone was low overall with a few areas of active transmission, most notably in West Africa and South Asia. In countries of the Caribbean Epidemiology Centre (CAREC) countries, there was a small increase in the percent of severe acute respiratory infection (SARI) admissions. No SARI deaths were reported but approximately 3% of total hospitalizations were for SARI in children between 6 to 48 months of age. In the Dominican Republic, the percentage of samples testing positive for respiratory viruses decreased slightly; influenza A(H1N1)2009 has been the primary virus in circulation there since late March. No influenza virus activity was reported in Central American countries. In Colombia, there was low level co-circulation of influenza A(H1N1)2009 and A(H3N2). In Brazil, there are reports of the increased number of influenza cases especially in the south, southeast, and midwest of the country. Influenza viruses including A(H1N1)2009 were identified in circulation.
In sub-Saharan Africa, influenza type B virus continues to be predominant strain in both western and eastern Africa. Active transmission of predominantly influenza B appears to be ongoing in Ghana mixed with smaller numbers of influenza A(H1N1)2009. Much smaller numbers of cases of influenza B reported in Nigeria and Cameroon. Transmission in Kenya and Uganda has dropped to low levels and the previously noted influenza A(H3N2) transmission in Rwanda has also diminished to very low levels.
The overall influenza activity in the tropical Asia remained low with some notable localized areas of transmission. Low numbers of influenza A(H1N1)2009 viruses were reported from India associated with unconfirmed media reports of cases occurring in the southern part of the country. In Singapore ILI made up only 2% of polyclinic attendances for acute respiratory illness, which is considered low, however 51% of ILI cases tested positive for influenza virus in the last four weeks. Eighty-three percent of influenza viruses from ILI cases were influenza A(H3N2); influenza A(H1N1)2009 and influenza B accounted for 11% and 6% of positive cases respectively.
Countries in the temperate zone of the southern hemisphere
Only low influenza activity was reported in the temperate regions of South America. In Chile, ILI activity was less than the previous week, no deaths from influenza were reported, and influenza virus detects were in low proportion compared to other respiratory viruses. In Argentina, about 2% of respiratory specimens tested were positive for influenza, mostly influenza A(H1N1)2009, but rates of ILI are low. In Uruguay percent of SARI deaths among all deaths remained stable and less than five percent. However, the percent of SARI admissions to intensive care among all ICU admissions has been trending upwards over the last month.
Influenza transmission in South Africa appears to have peaked and is in early decline, though still quite active. Transmission in the country has been primarily associated with influenza A(H1N1)2009, which has accounted for more than 83% of influenza viruses in ILI cases. Influenza A(H3N2) and type B have accounted for 7.5% and 3.8% of viruses respectively in that group. Notably, influenza type B has made up a larger proportion of cases with severe infections admitted to hospital (17% of all influenza viruses from SARI cases). Based on a preliminary analysis of case data by the National Institute for Communicable Diseases, the case-fatality ratio among influenza positive patients admitted to hospital in 2011 is less than in the 2010 season when influenza B was the most common circulating strain (3% vs. 9% respectively (p=0.06)). The age distribution of severe cases has been similar this season as compared to 2010; 37% of cases have been between the ages of 2 and 4 years (37% in 2010). Another 30% of severe cases occurred in the age group from 25 to 44 years and only 19% of cases were over the age of 45 years.Of the four influenza positive patients enrolled into the SARI sentinel surveillance programme that have died so far in 2011, three were positive for influenza A(H1N1)2009 and one was positive for influenza B.
Australia and New Zealand and South Pacific
ILI consultations have continued to rise nationally in Australia along with notifications of laboratory confirmed influenza, most notably in South Australia, Queensland and New South Wales. The distribution of virus types has varied somewhat between states. Nationally, influenza A(H1N1)2009 has accounted for the majority of virus detections; however, 85% of viruses detected in the state of South Australia have been influenza B. These accounted for the majority of influenza B virus reported from the country as a whole. There numbers of confirmed cases of influenza reported through the National Notifiable Diseases Surveillance System is much higher than in the same period of 2010 (5,640 to date in 2011 vs. 1,088 for the same period of 2010). In New Zealand, the rate of national ILI consultations has not crossed the baseline levels although some of the districts were well above the national average. For this week, influenza B virus was the predominant strain followed by influenza A(H3N2) virus.
From the peer-reviewed literature
A recently published study assessed the frequency and distribution of risk factors globally among influenza A(H1N1)2009 patients reported during the pandemic. Risk factors were evaluated at three levels of severity: hospitalization, intensive care admission, and death. The study found that while the highest per capita risk of hospitalization was among patients (Risk Factors for Severe Outcomes following 2009 Influenza A(H1N1) Infection: A Global Pooled Analysis. PLoS Med 8(7): e1001053. doi:10.1371/journal.pmed.1001053))
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 Network) 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.