A Comprehensive Molecular and Epidemiological Characterization of Influenza Viruses Circulating 2016–2020 in North Macedonia

Influenza viruses know no boundaries, representing an example of rapid virus evolution combined with pressure exerted by the host’s immune system. Seasonal influenza causes 4–50 million symptomatic cases in the EU/EEA each year, with a global death toll reaching 650,000 deaths. That being the case, in 2014 North Macedonia introduced the sentinel surveillance in addition to the existing influenza surveillance in order to obtain more precise data on the burden of disease, circulating viruses and to implement timely preventive measures. The aims of this study were to give a comprehensive virological and epidemiological overview of four influenza seasons (2016–2020), assess the frequency and distribution of influenza circulating in North Macedonia and to carry out molecular and phylogenetic analyses of the hemagglutinin (HA) and neuraminidase (NA) genes of influenza A(H1N1)pdm09, A(H3N2) from ILI and SARI patients. Our results showed that out of 1,632 tested samples, 46.4% were influenza positive, with influenza A(H1N1)pdm09 accounting for the majority of cases (44%), followed by influenza B (32%) and A(H3N2) (17%). By comparing the sentinel surveillance system to the routine surveillance system, we showed that the newly applied system works efficiently and gives great results in the selection of cases. Statistically significant differences (p = < 0.0000001) were observed when comparing the number of reported ILI cases among patients aged 0–4, 5–14, 15–29, and 30–64 years to the reference age group. The phylogenetic analysis of the HA sequences unveiled the resemblance of mutations circulating seasonally worldwide, with a vast majority of circulating viruses belonging to subclade 6B.1A. The PROVEAN analysis showed that the D187A substitution in the receptor binding site (RBS) of the A(H1N1)pdm09 HA has a deleterious effect on the its function. The A(H3N2) viruses fell into the 3C.2a and 3C.3a throughout the analyzed seasons. Molecular characterization revealed that various substitutions in the A(H3N2) viruses gradually replaced the parental variant in subsequent seasons before becoming the dominant variant. With the introduction of sentinel surveillance, accompanied by the advances made in whole-genome sequencing and vaccine therapeutics, public health officials can now modify their approach in disease management and intervene effectively and in a timely manner to prevent major morbidity and mortality from influenza.

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Enhancement of the Influenza Surveillance System in the Russian Federation: the Main Results of the Sentinel Surveillance for Influenza and other Acute Respiratory Viral Infections

Epidemiology and Vaccinal Prevention ◽

10.31631/2073-3046-2017-16-1-7-15 ◽

2017 ◽

Vol 16 (1) ◽

pp. 7-15 ◽

Cited By ~ 2

Author(s):

A. A. Sominina ◽

E. A. Smorodintseva ◽

K. A. Stolyarov ◽

A. A. Mel'nikova

Keyword(s):

Respiratory Infection ◽

Surveillance System ◽

Acute Respiratory Infection ◽

Influenza A ◽

Hospitalized Patients ◽

Sentinel Surveillance ◽

Influenza Surveillance ◽

Influenza B Virus ◽

Influenza B ◽

Influenza A H1n1

Existing influenza surveillance system is constantly improved to obtain comprehensive information for understanding of continuously changing situation with the influenza, which is a consequence of the highest variability of the pathogen, its ability to reassortment and the imminence of emergence a new shift-variants of the virus that could cause the next pandemic events. For this purpose, since the 2010 - 2011 epidemic season, in addition to the traditional surveillance system (TS) a new well standardized sentinel surveillance system (SS) for rapid clinical and epidemiological data obtaining was introduced in Russia. A total 7812 hospitalized patients with severe acute respiratory infection (SARI) and 9854 outpatients with influenza-like illness and acute respiratory infection (ILI/ARI) were investigated during the 6-year period in SS. Percent of SARI among all hospitalized patients ranged from 1.7 to 3.1%; about 5.3 - 7.5% SARI patients were placed in the Intensive Care Unit. Etiological monitoring using PCR showed influenza spread trends in SS similar to those registered in the TS: a clear predominance of influenza A (H1N1) pdm09 among SARI and ILI/ARI in 2010 - 2011 and 2015 - 2016 epidemic seasons, influenza A (H3N2) in the epidemic seasons 2011 - 2012 and 2014 - 2015, the co-circulation of these pathogens in 2012 - 2013, 2013 - 2014 seasons in Russia. SARI caused by influenza B virus were detected less frequently than influenza A but increased influenza B activity was registered in the epidemic of 2014 -2015, when Yamagata lineage changed suddenly for the Victorian one. The average frequency of influenza diagnosis among SARI between the seasons varied in the range 12.5 - 27.1%, at the peak of the epidemic it reached 44.8 - 73.5% and was the highest during the season with active circulation of influenza A (H1N1) pdm09 virus. The rate of influenza diagnosis among ILI/ARI has always been lower than that among SARI. Studies have also shown the importance of rhinovirus, RS-virus and parainfluenza infections in SARI development. The frequency of registration of coronaviruses, metapneumovirus and bocavirus infection was very low in SARI and ILI/ARI. It was found that in all studied seasons most of SARI patients with influenza have not been vaccinated. Among ILI/ARI outpatients with influenza, the frequency of vaccinated individuals for the entire period of the study was estimated as 10.1%, which was 4.2 times higher than that in SARI, where only 2.4% of patients were vaccinated. In addition, it was found that for all six seasons the SARI patients with influenza were treated with antivirals drugs 2 times less often compared to outpatients. Analysis of data on concomitant diseases and conditions in SARI patients with influenza confirmed the leading role of pregnancy as a risk factor for hospitalization in all influenza epidemics, irrespective of their etiology. In addition, diabetes and cardiovascular disease were recognized as risk factors for influenza associated SARI development.

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Laboratory based Influenza sentinel surveillance in Pakistan At NIC, Islamabad 2007-2017

European Journal of Public Health ◽

10.1093/eurpub/ckz186.499 ◽

2019 ◽

Vol 29 (Supplement_4) ◽

Author(s):

F Bashir ◽

K Fawad Khan ◽

S Zafar Qureshi ◽

F Khaudaidad ◽

R Sonia

Keyword(s):

Influenza A ◽

Vaccine Development ◽

Sudden Onset ◽

Sentinel Surveillance ◽

Influenza Viruses ◽

H3n2 Virus ◽

Influenza Surveillance ◽

Influenza B ◽

Pandemic Preparedness ◽

Influenza A H1n1

Abstract Background A country-wide lab-based surveillance system for ILI and Severe Acute Respiratory Illness (SARI) with weekly sampling and reporting was established in 2008.This system was necessary for early detection of emerging novel influenza subtypes and timely response for influenza prevention and control. Objectives To assess the trends of Influenza-like-Illness(ILI) and to monitor the predominant circulating strains of influenza viruses through Lab based sentinel surveillance. Methods A cross-sectional study was conducted based on ten years (2007-2017) influenza surveillance data obtained from National Influenza Central Laboratory Pakistan (NICLP) from January to March 2018.Study was done from the data records and samples of suspected ILI patients and SARI patients received from all seven sentinel sites. An ILI case was defined as sudden onset of fever of ≥ 38 C° and cough, with onset within last 10 days, while patients with sudden onset of fever (>38 °C), cough/sore throat requiring hospital admission within 7 days were termed as SARI. Samples were tested at NICLP for confirmation of virus, typing and subtyping by RT-PCR. Results A total of 15885 samples were analyzed during ten years period, out of which 3475(21.9%) were found positive for influenza virus. Among positive samples 26(0.75%) were Influenza-A (H1N1), 550(38%) were A/H3N1,550(15.9%) were A/H3N1,1587(45.7%) were A/H1N1 pdm09and 1312(37.8%) were influenza B. Males were predominant(54%).Influenza Maximum cases were reported from age group 01->12 years(66%).Virus circulation was detected throughout the year along with few cases of seasonal A/H1N1 virus during late winter(January February) and spring(March). Influenza A/H3N2 virus circulation was mainly observed during summer months (August-October). Conclusions The findings of this study emphasize the need for continuous and comprehensive influenza surveillance to predict seasonal trends for vaccine development and to further fortify pandemic preparedness. Key messages The need for continuous and comprehensive influenza surveillance. Public health importance by pandemic preparedness.

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Epidemiology of influenza: findings from surveillance system of 40 hospital in Thailand, 2010-2019

European Journal of Public Health ◽

10.1093/eurpub/ckaa165.1098 ◽

2020 ◽

Vol 30 (Supplement_5) ◽

Author(s):

T Eamchotchawalit ◽

P Piyaraj ◽

P Narongdej ◽

S Charoensakulchai ◽

C Chanthowong

Keyword(s):

Real Time ◽

Surveillance System ◽

Influenza A ◽

Seasonal Influenza ◽

Influenza Viruses ◽

Cold Chain ◽

Influenza Surveillance ◽

Influenza B ◽

Private Hospitals ◽

Influenza A H1n1

Abstract Background Although recent efforts from some Asian countries to describe burden of influenza disease and seasonality, these data are missing for the vast majority, including the private section of Thailand. A near real-time laboratory-based influenza surveillance system, in a network of 40 hospitals was implemented aiming to determine influenza strains circulating in the private hospitals of Thailand and know characteristics, trend and burden of influenza viruses. Methods We obtained the data by monitoring patients with influenza-like illness (ILI) at a network of 40 private hospitals across Thailand. Throat-swab specimens in viral transport media were collected and transported within 24 h of collection using a cold-chain system. The respiratory samples were tested by rapid influenza diagnostic tests and real-time reverse transcription polymerase chain reaction. Results From January 2010 to November 2019, a total of 1,300,594 subjects were tested and 320,499 cases of influenza were identified. Of those positive cases, 116,317(36.3%) were influenza type B, 185,512(57.9%) were influenza A unspecified subtype, 8,833(2.7%) were influenza A(H1N1)pdm2009 and 6,371(1.9%) were seasonal influenza A(H3N2). Positive rate were 50.5 and 49.5 in female and male. Positivity rate was 41.4% in persons 15-49 years followed by 29.1% in 15-14 years, 17.6% in under five children and 11.7% in > 49 years. In 2018-2019 season, the highest positivity rate observed in February and March (39.3%) followed by April (34.2%) and January (32.3%) while the lowest positivity rate was in May (18.1%). Conclusions In Thailand, seasonal Influenza A(H3N2), Influenza A(H1N1)pdm2009 and Influenza B viruses were circulating during 2010-2019. In last season, positivity rate and number of cases peaked in February and March. Key messages Influenza is one of public health problems in Thailand. The need to introduce influenza vaccine and antivirus is important to prevent and treat the disease in future.

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Influenza Project in Myanmar

Journal of Disaster Research ◽

10.20965/jdr.2014.p0842 ◽

2014 ◽

Vol 9 (5) ◽

pp. 842-847

Author(s):

Reiko Saito ◽

◽

Yadanar Kyaw ◽

Yi Yi Myint ◽

Clyde Dapat ◽

...

Keyword(s):

Influenza A ◽

Drug Susceptibility ◽

Influenza Viruses ◽

World Health ◽

Influenza Surveillance ◽

Influenza B ◽

Laboratory Capacity ◽

Influenza A H1n1 ◽

Resistant Strains ◽

Health Organization

The epidemiological study of influenza in Southeast Asia is limited. We surveyed influenza in Myanmar from 2007 to 2013. Nasopharyngeal swabs were collected from patients in the two cities of Yangon and Nay Pyi Taw. Samples were screened using rapid influenza diagnostic kits and identified by virus isolation. Isolates were characterized by cyclingprobe-based real-time PCR, drug susceptibility assay, and sequencing. Samples collected numbered 5,173, from which 1,686 influenza viruses were isolated during the seven-year study period. Of these, 187 strains were of seasonal influenza A(H1N1), 274 of influenza A(H1N1)pdm09, 791 of influenza A(H3N2), and 434 of influenza B. Interestingly, two zanamivir and amantadine-resistant strains each were detected in 2007 and 2008. These rare dual-resistant strains had a Q136K mutation in the NA protein and S31N substitution in the M2 protein. Our collaboration raised the influenza surveillance laboratory capacity in Myanmar and led Yangon’s National Health Laboratory – one of the nation’s leading research institutes – to being designated a National Influenza Center by the World Health Organization.

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What will the next influenza season bring about: seasonal influenza or the new A(H1N1)v? An analysis of German influenza surveillance data

Eurosurveillance ◽

10.2807/ese.14.32.19303-en ◽

2009 ◽

Vol 14 (32) ◽

Author(s):

H Uphoff ◽

S Geis ◽

A Grüber ◽

A M Hauri

Keyword(s):

Influenza A ◽

Seasonal Influenza ◽

Influenza Season ◽

Influenza Viruses ◽

Moderate Intensity ◽

Influenza Surveillance ◽

Influenza B ◽

Low Intensity ◽

Influenza A H1n1 ◽

Using Data

For the next influenza season (winter 2009-10) the relative contributions to virus circulation and influenza-associated morbidity of the seasonal influenza viruses A(H3N2), A(H1N1) and B, and the new influenza A(H1N1)v are still unknown. We estimated the chances of seasonal influenza to circulate during the upcoming season using data of the German influenza sentinel scheme from 1992 to 2009. We calculated type and subtype-specific indices for past exposure and the corresponding morbidity indices for each season. For the upcoming season 2009-10 our model suggests that it is unlikely that influenza A(H3N2) will circulate with more than a low intensity, seasonal A(H1N1) with more than a low to moderate intensity, and influenza B with more than a low to median intensity. The probability of a competitive circulation of seasonal influenza A with the new A(H1N1)v is low, increasing the chance for the latter to dominate the next influenza season in Germany.

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A multiplex one-step real-time RT-PCR assay for influenza surveillance

Eurosurveillance ◽

10.2807/ese.16.07.19798-en ◽

2011 ◽

Vol 16 (7) ◽

Author(s):

I Huber ◽

H Campe ◽

D Sebah ◽

C Hartberger ◽

R Konrad ◽

...

Keyword(s):

Real Time ◽

High Throughput ◽

Influenza A ◽

Multiplex Assay ◽

Influenza Viruses ◽

Influenza Surveillance ◽

Influenza B ◽

Rt Pcr ◽

Influenza A H1n1 ◽

One Step

For surveillance purposes real-time PCR assays for influenza viruses had to be adapted to the pandemic influenza A(H1N1)2009 strain. We combined published primers and probes for influenza A, influenza B and an internal amplification control with a detection system for influenza A(H1N1)2009 to set up a rapid, reliable, simple and cost-effective high-throughput multiplex one-step real-time RT-PCR. The workflow also includes automated sample preparation for high-throughput screening. The lower limit of detection of the multiplex assay was 3.5x102 RNA copies per PCR reaction. The diagnostic sensitivity of the multiplex assay was 87.7%, but increased to 99.4% for influenza-positive samples yielding Ct values of less than 34 cycles in the respective diagnostic assay. High specificity was confirmed by sequencing and correct detection of 15 reference samples from two quality assurance studies. The multiplex PCR was introduced for surveillance of samples from a network of general practitioners and paediatricians in Bavaria, Germany during the influenza pandemic of 2009. Comparison with surveillance data from reported cases proved the reliability of the multiplex assay for influenza surveillance programmes.

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Morbidity patterns associated with seasonal influenza A/H1N1in Swaziland

Online Journal of Public Health Informatics ◽

10.5210/ojphi.v10i1.8939 ◽

2018 ◽

Vol 10 (1) ◽

Author(s):

Vusie Lokotfwako ◽

Nhlanhla Nhlabatsi ◽

Phinda Khumalo ◽

Siphiwe Shongwe ◽

Bongani Tsabedze ◽

...

Keyword(s):

Influenza Virus ◽

Influenza A ◽

Seasonal Influenza ◽

Index Case ◽

Influenza Viruses ◽

Influenza Surveillance ◽

Influenza B ◽

Epidemic Disease ◽

Influenza A H1n1 ◽

Private Laboratory

ObjectiveTo establish morbidity patterns of influenza A/H1N1 in Swaziland from 10th July to 15th August 2017.IntroductionInfluenza infection is caused by the influenza virus, a single-stranded RNA virus belonging to the Orthomyxoviridae family. Influenza viruses are classified as types A, B and C. Influenza A and B viruses can cause epidemic disease in humans and type C viruses usually cause a mild, cold-like illness. The influenza virus spreads rapidly around the world in seasonal epidemics, resulting in significant morbidity and mortality. On the 10th of July 2017, a case of confirmed Influenza A/H1N1 was reported through the immediate disease notification system from a private hospital in the Hhohho region. A 49 year old female was diagnosed of Influenza A/H1N1 after presenting with flu-like symptoms. Contacts of the index case were followed and further positive cases were identified.MethodsUpon identification of the index case, the rapid response teams conducted further investigations. Two nasal swaps from each sample were taken and sent to a private laboratory in South Africa for the detection of the virus RNA using RT-PCR to assess for the presence Influenza A, B and Influenza A/H1N1. Further laboratory results were sourced from a private laboratory to monitor trends of influenza. Data was captured and analyzed in STATA version 12 from STATA cooperation. Descriptive statistics were carried out using means and standard deviations. The Pearson Chi square test and student t test were used to test for any possible association between influenza A/H1N1 and the explanatory variables (age and sex).ResultsSurveillance data captured between 10th July 2017 and 15th August 2017 indicated that a total of 87 patients had their samples taken for laboratory confirmation. There were 45 females and 42 males and the mean age was 27 years (SD= 17). At least 25 of the 87 patients tested positive for influenza A while only 1 tested positive for influenza B. The prevalence of influenza A/H1N1 was 16%. The prevalence of influenza A/H1N1 among males was 19% compared to 13% in females; however the difference was not statistically significant (p=0.469). There was no association noted between age and influenza A/H1N1 (p=427). Upon further sub-typing results indicated that the circulating strain was influenza A/H1N1 pdm 09 strain which is a seasonal influenza. The epidemic task forces held weekly and ad-hoc meetings to provide feedback to principals and health messaging to the general population to allay anxiety.ConclusionsThough WHO has classified the influenza A/H1N1 strain pdm 0029 as a seasonal influenza, surveillance remains important for early detection and management. There is therefore an urgent need to set up sentinel sites to monitor and understand the circulating influenza strains. Health promotion remains crucial to dispel anxiety as the general public still link any influenza to the 2009 pandemic influenza. Finally the Ministry of Health should consider introducing influenza vaccines into the routine immunization schedule especially for children.References1. Global Epidemiological Surveillance Standards for Influenza. 2014 [cited 2015 15 April]; Available from: http://www.who.int/influenza/resources/documents/influenza_surveillance_manual/en/.2. Human cases of influenza at the human-animal interface, 2013. Wkly Epidemiol Rec, 2014.89(28): p. 309-20.3. WHO Global Influenza Surveillance Network. Manual for the laboratory diagnosis and virological surveillance of influenza. 2011 [cited 2015 April27]; Available from: http://www.who.int/influenza/gisrs_laboratory/manual_diagnosis_surveillance_influenza/en/.

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Report on influenza viruses received and tested by the Melbourne WHO Collaborating Centre for Reference and Research on Influenza in 2019

Communicable Diseases Intelligence ◽

10.33321/cdi.2021.45.43 ◽

2021 ◽

Vol 45 ◽

Author(s):

Heidi Peck ◽

Jean Moselen ◽

Sook Kwan Brown ◽

Megan Triantafilou ◽

Hilda Lau ◽

...

Keyword(s):

Influenza A ◽

Seasonal Influenza ◽

Significant Proportion ◽

Influenza Viruses ◽

World Health ◽

Influenza Surveillance ◽

Influenza B ◽

Surveillance And Response ◽

Influenza A H1n1 ◽

Potential Use

As part of its role in the World Health Organization’s (WHO) Global Influenza Surveillance and Response System (GISRS), the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne received a record total of 9,266 human influenza positive samples during 2019. Viruses were analysed for their antigenic, genetic and antiviral susceptibility properties. Selected viruses were propagated in qualified cells or embryonated hen’s eggs for potential use in seasonal influenza virus vaccines. In 2019, influenza A(H3N2) viruses predominated over influenza A(H1N1)pdm09 and B viruses, accounting for a total of 51% of all viruses analysed. The majority of A(H1N1)pdm09, A(H3N2) and influenza B viruses analysed at the Centre were found to be antigenically similar to the respective WHO recommended vaccine strains for the Southern Hemisphere in 2019. However, phylogenetic analysis indicated that a significant proportion of circulating A(H3N2) viruses had undergone genetic drift relative to the WHO recommended vaccine strain for 2019. Of 5,301 samples tested for susceptibility to the neuraminidase inhibitors oseltamivir and zanamivir, four A(H1N1)pdm09 viruses showed highly reduced inhibition with oseltamivir, one A(H1N1)pdm09 virus showed highly reduced inhibition with zanamivir and three B/Victoria viruses showed highly reduced inhibition with zanamivir.

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DETECTION OF INFLUENZA VIRUSES AMONG HOSPITALIZED CASES SUFFERING FROM SEVERE ACUTE RESPIRATORY ILLNESS (SARI) IN SANA’A CITY, YEMEN

Universal Journal of Pharmaceutical Research ◽

10.22270/ujpr.v4i4.296 ◽

2019 ◽

Author(s):

Dina Abdulljabbar Abdullah Al-Ademi ◽

Abdulilah Hussein Al-Harazi ◽

Hassan A. Al-Shamahy ◽

Bushra Mohammed Jaadan

Keyword(s):

Mortality Rate ◽

Influenza A ◽

Respiratory Illness ◽

Hospitalized Patients ◽

Influenza Viruses ◽

Influenza Surveillance ◽

Influenza B ◽

Acute Respiratory Illness ◽

Influenza A H1n1 ◽

Severe Acute Respiratory Illness

Influenza is a major cause of morbidity and mortality around the world. So national influenza surveillance have been important for understanding the epidemiology of influenza over time. The aims of this study were to determine the prevalence rate of influenza viruses among hospitalized patients with severe acute respiratory illness (SARI), identify circulating types and subtypes of influenza viruses among them, and determine the risk factors associated with SARI. A total of 320 hospitalized patients suffering from SARI at Al Joumhouri University hospital in Sana’a city were enrolled; and their age was ranged from < 1 year to ≥ 56 years. Both nasopharyngeal and oro-pharyngeal swabs were collected from each patient and tested by using rRT-PCR technique for the detection of influenza A, influenza B and subtypes of influenza A viruses (A/H1N1(2009) and A/H3N2). The crude prevalent rate of influenza viruses among SARI patients was 10.9%;the female rate was 12.4%, and the male rate was 9.9%. The rate of Flu A in the total SARI cases was 5.9% and for Flu B was 5%. In addition 3.8% of SARI patients were suffering from influenza A/H3N2, 2.2% from influenza A/H1N1(2009) infections; and the mortality rate for influenza infections was 17.1%. Also, a high mortality rate was occurred in influenza infections in age groups 36-45 years and 6-15 years. Also, there was a significant association between flu infection; and 46-55 years group (OR=2.8), Winter time (OR=17.5), cardiac diseases (OR=9.1), and diabetic mellitus (OR=3.7). In conclusion: both influenza A and B were represented as a causative agents of SARI, and Influenza A/H3N2 was present subtype followed by A/H1N1(2009). The frequency of influenza viruses ascertain among SARI patients in Yemen highlights the need for health authorities to develop strategies to reduce morbidity among at-risk population in the course of vaccine recommendation.

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Influenza epidemiology and characterization of influenza viruses in patients seeking treatment for acute fever in Cambodia

Epidemiology and Infection ◽

10.1017/s095026880999063x ◽

2009 ◽

Vol 138 (2) ◽

pp. 199-209 ◽

Cited By ~ 29

Author(s):

P. J. BLAIR ◽

T. F. WIERZBA ◽

S. TOUCH ◽

S. VONTHANAK ◽

X. XU ◽

...

Keyword(s):

Influenza Virus ◽

Influenza A ◽

Influenza Viruses ◽

Influenza Surveillance ◽

Influenza B ◽

Viral Aetiology ◽

Influenza A H1n1 ◽

Avian Influenza Virus H5n1 ◽

Viral Influenza ◽

Diagnostic Samples

SUMMARYThe epidemiology, symptomology, and viral aetiology of endemic influenza remain largely uncharacterized in Cambodia. In December 2006, we established passive hospital-based surveillance to identify the causes of acute undifferentiated fever in patients seeking healthcare. Fever was defined as tympanic membrane temperature >38°C. From December 2006 to December 2008, 4233 patients were screened for influenza virus by real-time reverse-transcriptase polymerase chain reaction (rRT–PCR). Of these patients, 1151 (27·2%) were positive for influenza. Cough (68·8%vs. 50·5%,P<0·0001) and sore throat (55·0%vs. 41·9%,P<0·0001) were more often associated with laboratory-confirmed influenza-infected patients compared to influenza-negative enrollees. A clear influenza season was evident between July and December with a peak during the rainy season. Influenza A and B viruses were identified in 768 (66·3%) and 388 (33·7%) of the influenza-positive population (n=1153), respectively. In December 2008, passive surveillance identified infection of the avian influenza virus H5N1 in a 19-year-old farmer from Kandal province who subsequently recovered. From a subset of diagnostic samples submitted in 2007, 15 A(H1N1), seven A(H3N2) and seven B viruses were isolated. The predominant subtype tested was influenza A(H1N1), with the majority antigenically related to the A/Solomon Island/03/2006 vaccine strain. The influenza A(H3N2) isolates and influenza B viruses analysed were closely related to A/Brisbane/10/2007 or B/Ohio/01/2005 (B/Victoria/2/87-lineage) vaccine strains, respectively. Phylogenetic analysis of the HA1 region of the HA gene of influenza A(H1N1) viruses demonstrated that the Cambodian isolates belonged to clade 2C along with representative H1N1 viruses circulating in SE Asia at the time. These viruses remained sensitive to oseltamivir. In total, our data suggest that viral influenza infections contribute to nearly one-fifth of acute febrile illnesses and demonstrate the importance of influenza surveillance in Cambodia.

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