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

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.

Factors associated with influenza-like-illness: a crowdsourced cohort study from 2012/13 to 2017/18

“…International studies have provided estimates in the same ballpark (2.8 to 10.9% in the US [33], 10 to 25% in Canada [34]), however the comparison can be difficult as influenza impact depends on the specific season but also on the surveillance system and the case definition. In France, an average 3.4% of the population was estimated to have an ILI requiring a medical consultation from general practitioners surveillance data [20], a lower value compared to our GN estimate that can be explained by the limited fraction of illnesses leading to health-seeking behavior (56.7% of GN ILI- episodes, 32.6% with the ILI ECDC definition) [35]. …”

“…Regular information was defined as having filled at least one profile survey and three weekly symptoms surveys for a given season. At least one symptoms survey should have been completed before, another during and one after the influenza epidemic period as stated by the French national surveillance network in primary care, called Sentinelles network [20]. A similar inclusion criterion was used in a previous work conducted at the European level [16].…”

Baseline characteristics and clinical symptoms related to respiratory viruses identified among patients presenting with influenza-like illness in primary care

“…Explanatory variables studied were those identifiable by the physician during the consultation: age (eight groups: <5, 5e14, 15e29, 30e44, 45e64 and !65 years), influenza vaccination for current season (received at least 14 days before onset of symptoms), period of onset of symptoms (before, during or after the influenza epidemic as published by the Sentinelles network for France [8]dreported in Fig. 1), cough, rhinorrhoea, dyspnoea, sore throat, conjunctivitis, headache, malaise, vomiting and diarrhoea.…”

“…In France, in primary care, since 1984, the Sentinelles network has been conducting surveillance of patients with influenza-like illness, with a specific definition for the purpose of influenza surveillance (called thereafter Sentinelles influenza-like illness (S-ILI)) [7,8]. Since 2014, this surveillance has been linked to virological analyses to detect and characterize circulating respiratory viruses [9].…”

Excess cases of Influenza like illnesses in France synchronous with COVID19 invasion

“…The Sentinelles network monitors Influenza Like Illnesses (ILI) and Acute Respiratory Infections (ARI) in general practice in France (1). According to routine surveillance, the 2019-2020 influenza epidemic reached its peak in mid-February in France (http://www.sentiweb.fr ) and it was expected that ILI incidence would decrease over time, as it did over all past seasons.…”