Evaluation of indirect fluorescent antibody assays compared to rapid influenza diagnostic tests for the detection of pandemic influenza A (H1N1) pdm09

Performance of indirect fluorescent antibody (IFA) assays and rapid influenza diagnostic tests (RIDT) during the 2009 H1N1 pandemic was evaluated, along with the relative effects of age and illness severity on test accuracy. Clinicians and laboratories submitted specimens on patients with respiratory illness to public health from April to mid October 2009 for polymerase chain reaction (PCR) testing as part of pandemic H1N1 surveillance efforts in Orange County, CA; IFA and RIDT were performed in clinical settings. Sensitivity and specificity for detection of the 2009 pandemic H1N1 strain, now officially named influenza A(H1N1)pdm09, were calculated for 638 specimens. Overall, approximately 30% of IFA tests and RIDTs tested by PCR were falsely negative (sensitivity 71% and 69%, respectively). Sensitivity of RIDT ranged from 45% to 84% depending on severity and age of patients. In hospitalized children, sensitivity of IFA (75%) was similar to RIDT (84%). Specificity of tests performed on hospitalized children was 94% for IFA and 80% for RIDT. Overall sensitivity of RIDT in this study was comparable to previously published studies on pandemic H1N1 influenza and sensitivity of IFA was similar to what has been reported in children for seasonal influenza. Both diagnostic tests produced a high number of false negatives and should not be used to rule out influenza infection.     

一种新型H5N1禽流感病毒血凝素抗原快速检测试剂的建立

利用5株广谱特异性抗H5亚型血凝素单克隆抗体和酶联免疫渗滤技术成功地建立了一种适于现场检测H5亚型禽流感病毒血凝素蛋白的抗原快速检测试剂H5-HA(Ag)Dot-ELISA。该试剂对41株代表当前亚洲地区流行的各种遗传变异亚系H5N1禽流感病毒检测均为阳性,对多数毒株的分析灵敏度优于0.1个血凝滴度(HA titer),其中部分优于0.01个血凝滴度;比较该试剂与早期开发的同类ELISA试剂,发现前者对后者未能检出的H5N1新变异株检测均为阳性;利用该试剂和商品化Directigen Flu A(BD)试剂检测两株H5N1病毒株,提示前者灵敏度高于后者;该试剂对一株H5N1病毒的检测灵敏度与标准RT-PCR相当;该试剂对24株非H5亚型病毒检测均为阴性,显示出良好特异性。以上结果提示,此研究建立的H5N1病毒抗原快速检测试剂在H5禽流感现场检测上具有较好的应用前景。     

Estimating the Influenza Vaccine Effectiveness against Medically Attended Influenza in Clinical Settings: A Hospital-Based Case-Control Study with a Rapid Diagnostic Test in Japan

Background

Influenza vaccine effectiveness (VE) studies are usually conducted by specialized agencies and require time and resources. The objective of this study was to estimate the influenza VE against medically attended influenza using a test-negative case-control design with rapid influenza diagnostic tests (RIDT) in a clinical setting.

Methods

A prospective study was conducted at a community hospital in Nagasaki, western Japan during the 2010/11 influenza season. All outpatients aged 15 years and older with influenza-like illnesses (ILI) who had undergone RIDT were enrolled. A test-negative case-control design was applied to estimate the VEs: the cases were ILI patients with positive RIDT results and the controls were ILI patients with negative RIDT results. Information on patient characteristics, including vaccination histories, was collected using questionnaires and medical records.

Results

Between December 2010 and April 2011, 526 ILI patients were tested with RIDT, and 476 were eligible for the analysis. The overall VE estimate against medically attended influenza was 47.6%, after adjusting for the patients'' age groups, presence of chronic conditions, month of visit, and smoking and alcohol use. The seasonal influenza vaccine reduced the risk of medically attended influenza by 60.9% for patients less than 50 years of age, but a significant reduction was not observed for patients 50 years of age and older. A sensitivity analysis provided similar figures.

Conclusion

The test-negative case-control study using RIDT provided moderate influenza VE consistent with other reports. Utilizing the commonly used RIDT to estimate VE provides rapid assessment of VE; however, it may require validation with more specific endpoint.     

Temporal Patterns of Influenza A and B in Tropical and Temperate Countries: What Are the Lessons for Influenza Vaccination?

IntroductionDetermining the optimal time to vaccinate is important for influenza vaccination programmes. Here, we assessed the temporal characteristics of influenza epidemics in the Northern and Southern hemispheres and in the tropics, and discuss their implications for vaccination programmes.MethodsThis was a retrospective analysis of surveillance data between 2000 and 2014 from the Global Influenza B Study database. The seasonal peak of influenza was defined as the week with the most reported cases (overall, A, and B) in the season. The duration of seasonal activity was assessed using the maximum proportion of influenza cases during three consecutive months and the minimum number of months with ≥80% of cases in the season. We also assessed whether co-circulation of A and B virus types affected the duration of influenza epidemics.Results212 influenza seasons and 571,907 cases were included from 30 countries. In tropical countries, the seasonal influenza activity lasted longer and the peaks of influenza A and B coincided less frequently than in temperate countries. Temporal characteristics of influenza epidemics were heterogeneous in the tropics, with distinct seasonal epidemics observed only in some countries. Seasons with co-circulation of influenza A and B were longer than influenza A seasons, especially in the tropics.DiscussionOur findings show that influenza seasonality is less well defined in the tropics than in temperate regions. This has important implications for vaccination programmes in these countries. High-quality influenza surveillance systems are needed in the tropics to enable decisions about when to vaccinate.     

Fluorescent Immunochromatography for Rapid and Sensitive Typing of Seasonal Influenza Viruses

Lateral flow tests also known as Immunochromatography (IC) is an antigen-detection method conducted on a nitrocellulose membrane that can be completed in less than 20 min. IC has been used as an important rapid test for clinical diagnosis and surveillance of influenza viruses, but the IC sensitivity is relatively low (approximately 60%) and the limit of detection (LOD) is as low as 10³ pfu per reaction. Recently, we reported an improved IC assay using antibodies conjugated with fluorescent beads (fluorescent immunochromatography; FLIC) for subtyping H5 influenza viruses (FLIC-H5). Although the FLIC strip must be scanned using a fluorescent reader, the sensitivity (LOD) is significantly improved over that of conventional IC methods. In addition, the antibodies which are specific against the subtypes of influenza viruses cannot be available for the detection of other subtypes when the major antigenicity will be changed. In this study, we established the use of FLIC to type seasonal influenza A and B viruses (FLIC-AB). This method has improved sensitivity to 100-fold higher than that of conventional IC methods when we used several strains of influenza viruses. In addition, FLIC-AB demonstrated the ability to detect influenza type A and influenza type B viruses from clinical samples with high sensitivity and specificity (Type A: sensitivity 98.7% (74/75), specificity 100% (54/54), Type B: sensitivity 100% (90/90), specificity 98.2% (54/55) in nasal swab samples) in comparison to the results of qRT-PCR. And furthermore, FLIC-AB performs better in the detection of early stage infection (under 13h) than other conventional IC methods. Our results provide new strategies to prevent the early-stage transmission of influenza viruses in humans during both seasonal outbreaks and pandemics.     

Point of Care Strategy for Rapid Diagnosis of Novel A/H1N1 Influenza Virus

Background

Within months of the emergence of the novel A/H1N1 pandemic influenza virus (nA/H1N1v), systematic screening for the surveillance of the pandemic was abandoned in France and in some other countries. At the end of June 2009, we implemented, for the public hospitals of Marseille, a Point Of Care (POC) strategy for rapid diagnosis of the novel A/H1N1 influenza virus, in order to maintain local surveillance and to evaluate locally the kinetics of the pandemic.

Methodology/Principal Findings

Two POC laboratories, located in strategic places, were organized to receive and test samples 24 h/24. POC strategy consisted of receiving and processing naso-pharyngeal specimens in preparation for the rapid influenza diagnostic test (RIDT) and real-time RT-PCR assay (rtRT-PCR). This strategy had the theoretical capacity of processing up to 36 samples per 24 h. When the flow of samples was too high, the rtRT-PCR test was abandoned in the POC laboratories and transferred to the core virology laboratory. Confirmatory diagnosis was performed in the core virology laboratory twice a day using two distinct rtRT-PCR techniques that detect either influenza A virus or nA/N1N1v. Over a period of three months, 1974 samples were received in the POC laboratories, of which 111 were positive for nA/H1N1v. Specificity and sensitivity of RIDT were 100%, and 57.7% respectively. Positive results obtained using RIDT were transmitted to clinical practitioners in less than 2 hours. POC processed rtRT-PCR results were available within 7 hours, and rtRT-PCR confirmation within 24 hours.

Conclusions/Significance

The POC strategy is of benefit, in all cases (with or without rtRT-PCR assay), because it provides continuous reception/processing of samples and reduction of the time to provide consolidated results to the clinical practitioners. We believe that implementation of the POC strategy for the largest number of suspect cases may improve the quality of patient care and our knowledge of the epidemiology of the pandemic.     

Oseltamivir Treatment for Children with Influenza-Like Illness in China: A Cost-Effectiveness Analysis

BackgroundInfluenza is a common viral respiratory infection that causes epidemics and pandemics in the human population. Oseltamivir is a neuraminidase inhibitor—a new class of antiviral therapy for influenza. Although its efficacy and safety have been established, there is uncertainty regarding whether influenza-like illness (ILI) in children is best managed by oseltamivir at the onset of illness, and its cost-effectiveness in children has not been studied in China.ObjectiveTo evaluate the cost-effectiveness of post rapid influenza diagnostic test (RIDT) treatment with oseltamivir and empiric treatment with oseltamivir comparing with no antiviral therapy against influenza for children with ILI.MethodsWe developed a decision-analytic model based on previously published evidence to simulate and evaluate 1-year potential clinical and economic outcomes associated with three managing strategies for children presenting with symptoms of influenza. Model inputs were derived from literature and expert opinion of clinical practice and research in China. Outcome measures included costs and quality-adjusted life year (QALY). All the interventions were compared with incremental cost-effectiveness ratios (ICER).ResultsIn base case analysis, empiric treatment with oseltamivir consistently produced the greatest gains in QALY. When compared with no antiviral therapy, the empiric treatment with oseltamivir strategy is very cost effective with an ICER of RMB 4,438. When compared with the post RIDT treatment with oseltamivir, the empiric treatment with oseltamivir strategy is dominant. Probabilistic sensitivity analysis projected that there is a 100% probability that empiric oseltamivir treatment would be considered as a very cost-effective strategy compared to the no antiviral therapy, according to the WHO recommendations for cost-effectiveness thresholds. The same was concluded with 99% probability for empiric oseltamivir treatment being a very cost-effective strategy compared to the post RIDT treatment with oseltamivir.ConclusionIn the Chinese setting of current health system, our modelling based simulation analysis suggests that empiric treatment with oseltamivir to be a cost-saving and very cost-effective strategy in managing children with ILI.     

Influenza activity in Czechoslovakia and the USSR, 1980-1985

Between 1980 and 1985, Czechoslovakia had experienced 4 and the USSR 3 major influenza outbreaks. Of the 3 epidemic outbreaks in the USSR, 2 were associated with influenza B virus (in the 1980/81 and 1983/84 seasons) and 1 with influenza A virus of the H3N2 subtype. In the USSR, influenza A (H1N1) virus never predominated as a cause of epidemic during the 5 years period. In Czechoslovakia, 2 epidemics (in the 1980/81 and 1983/84 seasons) were due to influenza A (H1N1) virus. The epidemic in the 1981/82 season had two waves of unequal heights and a mixed type B and subtype A (H3N2) etiology; a two-wave epidemic associated with isolates of influenza A (H1N1) and influenza B viruses was also recorded in the 1983/84 season. The influenza A (H3N2) epidemic in 1983 was of explosive character. All influenza viruses circulating in the two countries between 1980 and 1985 were of the same antigenic profile, but were isolated from the epidemics that occurred in different influenza seasons. The virological surveillance revealed strains of virus closely related to drift variants detected from outbreaks in 1977-1979 and the new variants A/Chile 1/83, A/Philippines 2/82, A/Caen 1/84 and B/USSR 100/83.     

Novel Virus Influenza A (H1N1sw) in South-Eastern France,April-August 2009

Background

In April 2009, the first cases of pandemic (H1N1)-2009 influenza [H1N1sw] virus were detected in France. Virological surveillance was undertaken in reference laboratories of the seven French Defence Zones.

Methodology/Principal Findings

We report results of virological analyses performed in the Public Hospitals of Marseille during the first months of the outbreak. (i) Nasal swabs were tested using rapid influenza diagnostic test (RIDT) and two RT-PCR assays. Epidemiological characteristics of the 99 first suspected cases were analyzed, including detection of influenza virus and 18 other respiratory viruses. During three months, a total of 1,815 patients were tested (including 236 patients infected H1N1sw virus) and distribution in age groups and results of RIDT were analyzed. (ii) 600 sera received before April 2009 and randomly selected from in-patients were tested by a standard hemagglutination inhibition assay for antibody to the novel H1N1sw virus. (iii) One early (May 2009) and one late (July 2009) viral isolates were characterized by sequencing the complete hemagglutinine and neuraminidase genes. (iiii) Epidemiological characteristics of a cluster of cases that occurred in July 2009 in a summer camp were analyzed.

Conclusions/Significance

This study presents new virological and epidemiological data regarding infection by the pandemic A/H1N1 virus in Europe. Distribution in age groups was found to be similar to that previously reported for seasonal H1N1. The first seroprevalence data made available for a European population suggest a previous exposure of individuals over 40 years old to influenza viruses antigenically related to the pandemic (H1N1)-2009 virus. Genomic analysis indicates that strains harbouring a new amino-acid pattern in the neuraminidase gene appeared secondarily and tended to supplant the first strains. Finally, in contrast with previous reports, our data support the use of RIDT for the detection of infection in children, especially in the context of the investigation of grouped cases.     

Epidemiological Characteristics of Influenza A and B in Macau, 2010–2018

Influenza is one of the major respiratory diseases in humans. Macau is a tourist city with high density of population and special population mobility. The study on the epidemiological characteristics of influenza in Macau should bring great value for preventing influenza in tourist cities like Macau in the world. In this study, we collected a total of 104,874 samples with influenza-like illness (ILI) in Macau from 2010 to 2018. Chi-square test and binary multivariable logistic regression were used to investigate the epidemiological characteristics of influenza A and B in Macau. Among these ILI samples, the overall positive rate is 17.17% for influenza A and 6.97% for influenza B. The epidemics of influenza in three years (i.e., 2012, 2017 and 2018) differ from the remaining years (i.e., normal years). In a normal year, influenza A occurs year-round whereas influenza B is seasonal. Our research shows significant differences in influenza infections between different age groups in normal years. Interestingly, our analysis shows no significant difference between locals and tourists in influenza A and B infection in a normal year, whereas the odds of influenza A in tourists were significantly higher than those in locals in July 2017 and the odds of influenza B in tourists were significantly higher than those in locals in January–February 2012 and January–February 2018. This is possibly attributed by the policy of free vaccination to everyone in Macau. These findings should be valuable for preventing influenza in not only Macau but also the world.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12250-021-00388-6.     

Mortality Attributable to Influenza in England and Wales Prior to,during and after the 2009 Pandemic

Very different influenza seasons have been observed from 2008/09–2011/12 in England and Wales, with the reported burden varying overall and by age group. The objective of this study was to estimate the impact of influenza on all-cause and cause-specific mortality during this period. Age-specific generalised linear regression models fitted with an identity link were developed, modelling weekly influenza activity through multiplying clinical influenza-like illness consultation rates with proportion of samples positive for influenza A or B. To adjust for confounding factors, a similar activity indicator was calculated for Respiratory Syncytial Virus. Extreme temperature and seasonal trend were controlled for. Following a severe influenza season in 2008/09 in 65+yr olds (estimated excess of 13,058 influenza A all-cause deaths), attributed all-cause mortality was not significant during the 2009 pandemic in this age group and comparatively low levels of influenza A mortality were seen in post-pandemic seasons. The age shift of the burden of seasonal influenza from the elderly to young adults during the pandemic continued into 2010/11; a comparatively larger impact was seen with the same circulating A(H1N1)pdm09 strain, with the burden of influenza A all-cause excess mortality in 15–64 yr olds the largest reported during 2008/09–2011/12 (436 deaths in 15–44 yr olds and 1,274 in 45–64 yr olds). On average, 76% of seasonal influenza A all-age attributable deaths had a cardiovascular or respiratory cause recorded (average of 5,849 influenza A deaths per season), with nearly a quarter reported for other causes (average of 1,770 influenza A deaths per season), highlighting the importance of all-cause as well as cause-specific estimates. No significant influenza B attributable mortality was detected by season, cause or age group. This analysis forms part of the preparatory work to establish a routine mortality monitoring system ahead of introduction of the UK universal childhood seasonal influenza vaccination programme in 2013/14.     

Clinical Characteristics Are Similar across Type A and B Influenza Virus Infections

Background

Studies that aimed at comparing the clinical presentation of influenza patients across virus types and subtypes/lineages found divergent results, but this was never investigated using data collected over several years in a countrywide, primary care practitioners-based influenza surveillance system.

Methods

The IBVD (Influenza B in Vircases Database) study collected information on signs and symptoms at disease onset from laboratory-confirmed influenza patients of any age who consulted a sentinel practitioner in France. We compared the clinical presentation of influenza patients across age groups (0–4, 5–14, 15–64 and 65+ years), virus types (A, B) and subtypes/lineages (A(H3N2), pandemic A(H1N1), B Victoria, B Yamagata).

Results

Overall, 14,423 influenza cases (23.9% of which were influenza B) were included between 2003–2004 and 2012–2013. Influenza A and B accounted for over 50% of total influenza cases during eight and two seasons, respectively. There were minor differences in the distribution of signs and symptoms across influenza virus types and subtypes/lineages. Compared to patients aged 0–4 years, those aged 5–14 years were more likely to have been infected with type B viruses (OR 2.15, 95% CI 1.87–2.47) while those aged 15–64 years were less likely (OR 0.83, 95% CI 0.73–0.96). Males and influenza patients diagnosed during the epidemic period were less likely to be infected with type B viruses.

Conclusions

Despite differences in age distribution, the clinical illness produced by the different influenza virus types and subtypes is indistinguishable among patients that consult a general practitioner for acute respiratory infections.     

Economic Impact of a New Rapid PCR Assay for Detecting Influenza Virus in an Emergency Department and Hospitalized Patients

Seasonal influenza causes significant morbidity and mortality and has a substantial economic impact on the healthcare system. The main objective of this study was to compare the cost per patient for a rapid commercial PCR assay (Xpert^® Flu) with an in-house real-time PCR test for detecting influenza virus. Community patients with influenza like-illness attending the Emergency Department (ED) as well as hospitalized patients in the Hospital Clínic of Barcelona were included. Costs were evaluated from the perspective of the hospital considering the use of resources directly related to influenza testing and treatment. For the purpose of this study, 366 and 691 patients were tested in 2013 and 2014, respectively. The Xpert^® Flu test reduced the mean waiting time for patients in the ED by 9.1 hours and decreased the mean isolation time of hospitalized patients by 23.7 hours. This was associated with a 103€ (or about $113) reduction in the cost per patient tested in the ED and 64€ ($70) per hospitalized patient. Sensitivity analyses showed that Xpert^® Flu is likely to be cost-saving in hospitals with different contexts and prices.     

Effectiveness of Trivalent Inactivated Influenza Vaccine in Children Estimated by a Test-Negative Case-Control Design Study Based on Influenza Rapid Diagnostic Test Results

We assessed vaccine effectiveness (VE) against medically attended, laboratory-confirmed influenza in children 6 months to 15 years of age in 22 hospitals in Japan during the 2013–14 season. Our study was conducted according to a test-negative case-control design based on influenza rapid diagnostic test (IRDT) results. Outpatients who came to our clinics with a fever of 38°C or over and had undergone an IRDT were enrolled in this study. Patients with positive IRDT results were recorded as cases, and patients with negative results were recorded as controls. Between November 2013 and March 2014, a total of 4727 pediatric patients (6 months to 15 years of age) were enrolled: 876 were positive for influenza A, 66 for A(H1N1)pdm09 and in the other 810 the subtype was unknown; 1405 were positive for influenza B; and 2445 were negative for influenza. Overall VE was 46% (95% confidence interval [CI], 39–52). Adjusted VE against influenza A, influenza A(H1N1)pdm09, and influenza B was 63% (95% CI, 56–69), 77% (95% CI, 59–87), and 26% (95% CI, 14–36), respectively. Influenza vaccine was not effective against either influenza A or influenza B in infants 6 to 11 months of age. Two doses of influenza vaccine provided better protection against influenza A infection than a single dose did. VE against hospitalization influenza A infection was 76%. Influenza vaccine was effective against influenza A, especially against influenza A(H1N1)pdm09, but was much less effective against influenza B.     

Partial and Full PCR-Based Reverse Genetics Strategy for Influenza Viruses

Since 1999, plasmid-based reverse genetics (RG) systems have revolutionized the way influenza viruses are studied. However, it is not unusual to encounter cloning difficulties for one or more influenza genes while attempting to recover virus de novo. To overcome some of these shortcomings we sought to develop partial or full plasmid-free RG systems. The influenza gene of choice is assembled into a RG competent unit by virtue of overlapping PCR reactions containing a cDNA copy of the viral gene segment under the control of RNA polymerase I promoter (pol1) and termination (t1) signals – herein referred to as Flu PCR amplicons. Transfection of tissue culture cells with either HA or NA Flu PCR amplicons and 7 plasmids encoding the remaining influenza RG units, resulted in efficient virus rescue. Likewise, transfections including both HA and NA Flu PCR amplicons and 6 RG plasmids also resulted in efficient virus rescue. In addition, influenza viruses were recovered from a full set of Flu PCR amplicons without the use of plasmids.     

Correlation between National Influenza Surveillance Data and Google Trends in South Korea

Background

In South Korea, there is currently no syndromic surveillance system using internet search data, including Google Flu Trends. The purpose of this study was to investigate the correlation between national influenza surveillance data and Google Trends in South Korea.

Methods

Our study was based on a publicly available search engine database, Google Trends, using 12 influenza-related queries, from September 9, 2007 to September 8, 2012. National surveillance data were obtained from the Korea Centers for Disease Control and Prevention (KCDC) influenza-like illness (ILI) and virologic surveillance system. Pearson''s correlation coefficients were calculated to compare the national surveillance and the Google Trends data for the overall period and for 5 influenza seasons.

Results

The correlation coefficient between the KCDC ILI and virologic surveillance data was 0.72 (p<0.05). The highest correlation was between the Google Trends query of H1N1 and the ILI data, with a correlation coefficient of 0.53 (p<0.05), for the overall study period. When compared with the KCDC virologic data, the Google Trends query of bird flu had the highest correlation with a correlation coefficient of 0.93 (p<0.05) in the 2010-11 season. The following queries showed a statistically significant correlation coefficient compared with ILI data for three consecutive seasons: Tamiflu (r = 0.59, 0.86, 0.90, p<0.05), new flu (r = 0.64, 0.43, 0.70, p<0.05) and flu (r = 0.68, 0.43, 0.77, p<0.05).

Conclusions

In our study, we found that the Google Trends for certain queries using the survey on influenza correlated with national surveillance data in South Korea. The results of this study showed that Google Trends in the Korean language can be used as complementary data for influenza surveillance but was insufficient for the use of predictive models, such as Google Flu Trends.     

Estimating Influenza Disease Burden from Population-Based Surveillance Data in the United States

Annual estimates of the influenza disease burden provide information to evaluate programs and allocate resources. We used a multiplier method with routine population-based surveillance data on influenza hospitalization in the United States to correct for under-reporting and estimate the burden of influenza for seasons after the 2009 pandemic. Five sites of the Influenza Hospitalization Surveillance Network (FluSurv-NET) collected data on the frequency and sensitivity of influenza testing during two seasons to estimate under-detection. Population-based rates of influenza-associated hospitalization and Intensive Care Unit admission from 2010–2013 were extrapolated to the U.S. population from FluSurv-NET and corrected for under-detection. Influenza deaths were calculated using a ratio of deaths to hospitalizations. We estimated that influenza-related hospitalizations were under-detected during 2010-11 by a factor of 2.1 (95%CI 1.7–2.9) for age < 18 years, 3.1 (2.4–4.5) for ages 18-64 years, and 5.2 (95%CI 3.8–8.3) for age 65+. Results were similar in 2011-12. Extrapolated estimates for 3 seasons from 2010–2013 included: 114,192–624,435 hospitalizations, 18,491–95,390 ICU admissions, and 4,915–27,174 deaths per year; 54–70% of hospitalizations and 71–85% of deaths occurred among adults aged 65+. Influenza causes a substantial disease burden in the U.S. that varies by age and season. Periodic estimation of multipliers across multiple sites and age groups improves our understanding of influenza detection in sentinel surveillance systems. Adjusting surveillance data using a multiplier method is a relatively simple means to estimate the impact of influenza and the subsequent value of interventions to prevent influenza.