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. 2019 Mar;13(2):148-157.
doi: 10.1111/irv.12620. Epub 2019 Jan 18.

Influenza epidemics observed in primary care from 1984 to 2017 in France: A decrease in epidemic size over time

Cécile Souty  1 , Philippe Amoros  1 , Alessandra Falchi  2 , Lisandru Capai  1   2 , Isabelle Bonmarin  3 , Sylvie van der Werf  4   5   6   7 , Shirley Masse  1   2 , Clément Turbelin  1 , Louise Rossignol  1 , Ana-Maria Vilcu  1 , Daniel Lévy-Bruhl  3 , Bruno Lina  8   9 , Laëtitia Minodier  2 , Yves Dorléans  1 , Caroline Guerrisi  1 , Thomas Hanslik  1   10   11 , Thierry Blanchon  1
Affiliations

Influenza epidemics observed in primary care from 1984 to 2017 in France: A decrease in epidemic size over time

Cécile Souty et al. Influenza Other Respir Viruses. 2019 Mar.

Abstract

Background: Epidemiological analysis of past influenza epidemics remains essential to understand the evolution of the disease and optimize control and prevention strategies. Here, we aimed to use data collected by a primary care surveillance system over the last three decades to study trends in influenza epidemics and describe epidemic profiles according to circulating influenza viruses.

Methods: Influenza-like illness (ILI) weekly incidences were estimated using cases reported by general practitioners participating in the French Sentinelles network, between 1984 and 2017. Influenza epidemics were detected by applying a periodic regression to this time series. Epidemic (co-)dominant influenza virus (sub)types were determined using French virology data.

Results: During the study period, 297 607 ILI cases were reported allowing the detection of 33 influenza epidemics. On average, seasonal epidemics lasted 9 weeks and affected 4.1% of the population (95% CI 3.5; 4.7). Mean age of cases was 29 years. Epidemic size decreased over time by -66 cases per 100 000 population per season on average (95% CI -132; -0.2, P value = 0.049) and epidemic height decreased by -15 cases per 100 000 (95% CI -28; -2, P value = 0.022). Epidemic duration appeared stable over time. Epidemics were mostly dominated by A(H3N2) (n = 17, 52%), associated with larger epidemic size, higher epidemic peak and older age of cases.

Conclusions: The declining trend in influenza epidemic size and height over the last 33 years might be related to several factors like increased vaccine coverage, hygiene improvements or changing in influenza viruses. However, further researches are needed to assess the impact of potential contributing factors to adapt influenza plans.

Keywords: epidemics; influenza; influenza-like illness; primary care; surveillance.

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Figures

Figure 1
Figure 1
Incidence of influenza‐like illness estimated per 100 000 population between 1984 and 2017; (A) weekly continuous series; (B) layered epidemic (the clearer the line, the older the epidemic); (C) cumulative ILI incidence rate by epidemic and 95% confidence intervals, Sentinelles network, France
Figure 2
Figure 2
Incidence of influenza‐like illness estimated per 100 000 population by epidemic for four age groups and overall, with linear regression lines, from 1984/85 to 2016/17, Sentinelles network, France
Figure 3
Figure 3
Characteristics of influenza epidemics grouped by viral dominant or co‐dominant (sub)types; (A) epidemic starting period; (B) cumulative incidence rate during epidemics; (C) incidence rate at epidemic peak; from 1984/85 to 2016/17, Sentinelles network, France
Figure 4
Figure 4
Age distribution of influenza epidemics grouped by viral dominant or co‐dominant (sub)types; (A) mean cumulative incidence rates; (B) mean relative illness ratio (RIR); from 1984/85 to 2016/17, Sentinelles network, France. Numbers of epidemics included for each series was reported in the legend
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Comment in

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