Integrated respiratory surveillance by the Global Influenza Surveillance and Response System (GISRS): frequently asked questions

1. What is the role of integrated sentinel surveillance for early detection of unusual events?

Routine surveillance data are essential in establishing baseline levels and thresholds for which to compare future activity. Epidemiology and laboratory expertise from routine surveillance are important resources and capacity for investigating and supporting outbreak response efforts.  

2. What is the advice for countries considering a switch from an ILI/SARI case definition to a broader ARI case definition that does not include fever? 

If switching from ILI/SARI to ARI or extended SARI, surveillance teams should ensure that sentinel sites capture information on symptoms for each enrolled patient, especially the presence or absence of fever. This information should be reported by sentinel sites along with the specimen, for a post-hoc categorization into ILI/SARI to ensure comparability with historical ILI/SARI data. Doing so would allow surveillance data that is now capturing ARI to be compared with historic data that captured ILI/SARI. 

3. How was the minimum weekly target of testing 50 specimens per case definition (for example, 50 samples from ILI patients and 50 samples from SARI patients) at the national level determined? 

The minimum target of testing 50 specimens per week per case definition—such as ILI or SARI—was derived through sample size calculations informed by historical patterns or respiratory virus circulation. This target was established to ensure sufficient statistical confidence in monitoring trends of respiratory virus activity (using percent positivity) and to enable timely detection of the onset of respiratory virus activity in each population of interest. The integrated surveillance guidance includes an annex with more details about how this number was derived. The guidance also recognizes that there needs to be a balance between enough information to provide situational awareness according to national surveillance objectives and the available resources. 

4. Is the minimum target of testing 50 specimens per case definition throughout the year the number of samples to be tested per week from each sentinel site?

No, this target is at national level - a total from all sentinel sites in the country. The guidance recommends that countries aim to test 50 specimens each week at the national level from patients with ILI, and 50 specimens per week at the national level from patients with SARI, to meet surveillance objectives at the national level. However, countries may want to monitor respiratory illness and respiratory virus circulation at a sub-national level, for example, and may aim to collect 50 specimens per week from each population in a geographic area that requires adequate representation in the surveillance system. 

5. What if countries cannot meet this year-round target of 50 specimens for testing per week at the national level at certain times of the year? 

Patterns of respiratory illness fluctuate during a year in most countries and there may be times in which there are not enough patients presenting to sentinel sites with respiratory illness to meet the testing goals. During those "low volume" weeks, countries should continue to test as much as possible among the ILI/ARI or SARI patients they do see.  

6. What if the patient volume at sentinel sites nationally is less than what is recommended in the guidance? 

Sentinel systems that evaluate fewer than 1000 patients each week for each case definition across all sentinel sites would likely not identify 50 patients with ILI or SARI in most weeks of the year. This means there could be less confidence in the percent positivity estimates that are used to monitor trends in acute respiratory illnesses. However, fewer patients and fewer samples to be tested each week may still provide enough information to understand whether influenza is circulating and give some indications of trends in detections over time and in which populations. What is important is to maintain consistent, timely and complete surveillance to obtain quality data, even when the number of specimens does not meet the minimum requirement.  

7. Why are the estimates of patients to be seen and samples to be tested per week at the national level not based on population size? 

Estimating the size of the sentinel system based on the population size may not result in a sustainable system in larger countries. In general, good quality data from a few sentinel sites is preferred over poor-quality data from many sites. Smaller countries may have challenges but are encouraged to reach these targets. 

8. Is the estimated number of patients to be evaluated for meeting the case definitions each week the number of patients seen for any illnesses or patients presenting with respiratory infection only? 

The estimate of 1000 patients per week at the national level considers patients presenting and evaluated for any reason. The estimate considers the proportion of ILI cases out of all outpatients, for example, or SARI cases out of all hospital admissions. At a general hospital that has multiple wards, this would include all hospital admissions into those wards that participate in surveillance.