Distinct influenza surveillance networks and their agreement in recording regional influenza circulation: Experience from Southeast Michigan

Abstract

Introduction: In Southeast Michigan, active surveillance studies monitor influenza activity in hospitals, ambulatory clinics, and community households. Across five respiratory seasons, we assessed the contribution of data from each of the three networks towards improving our overall understanding of regional influenza circulation.

Methods: All three networks used case definitions for acute respiratory illness (ARI) and molecularly tested for influenza from research-collected respiratory specimens. Age- and network-stratified epidemic curves were created for influenza A and B. We compared stratified epidemic curves visually and by centering at seasonal midpoints.

Results: Across all seasons (from 2014/2015 through 2018/2019), epidemic curves from each of the three networks were comparable in terms of both timing and magnitude. Small discrepancies in epidemics recorded by each network support previous conclusions about broader characteristics of particular influenza seasons.

Conclusion: Influenza surveillance systems based in hospital, ambulatory clinic, and community household settings appear to provide largely similar information regarding regional epidemic activity. Together, multiple levels of influenza surveillance provide a detailed view of regional influenza epidemics, but a single surveillance system-regardless of population subgroup monitored-appears to be sufficient in providing vital information regarding community influenza epidemics.

Keywords: influenza; sentinel surveillance; surveillance; syndromic surveillance.

FIGURE 1

Location of households, ambulatory clinics, and the hospital system enrolled or previously enrolled in three prospective influenza surveillance studies across Southeast Michigan census tracts, 2014/2015 through 2018/2019

FIGURE 2

Detection of influenza A and B and their respective subtypes and lineages as a proportion of all cases, across five surveillance seasons

FIGURE 3

Overall epidemic curves for influenza A and B recorded by three surveillance networks of Southeast Michigan across five surveillance seasons. The left column (A) represents influenza A cases (aggregate of H1N1, H3N2, undetermined subtypes) and the right column (B) represents influenza B cases (aggregate of Victoria, Yamagata, undetermined lineages). The grey bars reflect the epidemic curve of all influenza cases from all networks reported that season. The colored lines reflect influenza A and B epidemics of each of the three networks. Y‐axis units represent the network‐standardized weekly number of cases reported, as a proportion of all cases reported in the network that season. Horizontal, colored lines are equivalent within rows and represent the period during which the middle 50% of all influenza cases (influenza A and B combined) were reported to a given network that season

FIGURE 4

Epidemic curves centered at overall community midpoint in Southeast Michigan, stratified by surveillance network and age group across five surveillance seasons. The left column (A) represents epidemic curves of all influenza A and B, stratified by network. The right column (B) represents epidemic curves of all influenza A and B, stratified by age group of individual. Y‐axis units represent the network (or age group) standardized weekly number of cases reported, as a proportion of all cases reported in the network (or age group) that season. The seasonal midpoint is defined as the calendar week at which 50% of a season's total influenza cases (A and B, across all three networks) had been reported. The week is noted in the facet title for each row