Influenza and influenza-like illness

1. Key messages

On average, around 600,000 people are affected by influenza-like illness (ILI) each year in Belgium, i.e., about 5% of the total population. About 50-60% of these cases are actual influenza cases.
While ILI is in most cases a benign illness, about 2% to 3% of influenza cases require hospitalization. Of the hospitalized cases, 13% develop severe complications, including 6% who die in the course of hospital stay; usually more than 80% of these deaths occur in people of 65 years and over.
The Belgian flu epidemic of the 2018-2019 season lasted 8 weeks, which was average compared to previous seasons. An estimated 506,000 Belgians consulted their GP for flu-like symptoms. The severity indicators indicated that this flu epidemic was relatively no more serious than in previous seasons.

2. Background

Influenza-like illness (ILI) is a medical diagnosis of possible influenza or other illness causing a set of symptoms. These symptoms commonly include fever, shivering, chills, malaise, dry cough, loss of appetite, body aches, and nausea, typically in connection with a sudden onset of illness. In most cases ILI is a benign illness, but for elderly people, pregnant women and people with chronic diseases, the complications of flu can be dangerous.

ILI is caused by a variety of respiratory pathogens, including, amongst others, influenza virus, parainfluenza virus, adenovirus, RSV, and Mycoplasma pneumoniae. The influenza A(H1N1) and A(H3N2) viruses as well as the influenza B virus are the cause of seasonal influenza epidemics in Belgium and circulate every year to different extents. Vaccination remains the best way of preventing transmission, complications and hospitalization of flu.

Sciensano coordinates sentinel networks of general practitioners and hospitals to ensure the permanent surveillance of ILI and influenza activity, intensity and severity of epidemics and impact on the population. The epidemic threshold is the minimum number of GP consultations for flu-like symptoms per 100,000 inhabitants per week needed to officially speak of an epidemic. This number is calculated by the European Centre for Disease prevention and Control (ECDC), and varied between 135 and 157 GP visits/100,000 inhabitants over the last 5 years. Sciensano also hosts the National Reference Centre (NRC) for influenza virus.

At the international level, influenza surveillance and pandemic preparedness is coordinated by ECDC and the World Health Organization.

3. ILI season 2018-19

Incidence

The Belgian flu epidemic of the 2018-2019 season lasted 8 weeks, which was average compared to previous seasons. The epidemiological threshold was exceeded from week 4/2019 (January 21 to January 27, 2019) to week 11/2019 (March 11 until March 17, 2019). The epidemic was of medium activity.

Number of GP visits for flu-like symptoms per 100,000 inhabitants, Belgium, 2014-19
Source: Sciensano Sentinel GP Network [1]

From week 40/2018 to week 18/2019, respiratory samples from patients diagnosed with ILI were collected by the sentinel network of general practitioners and analyzed at the National Influenza Center. Of these samples, 53% were positive for influenza viruses.

Based on surveillance through the sentinel network of general practitioners, it was estimated that about 506,000 Belgians consulted their GP for flu-like symptoms and, if they were all systematically tested, about 307,000 of them would actually be infected with the flu virus.

All age groups were affected at the approximately same weeks during the epidemic, with peak incidences in weeks 7/2019 and 8/2019.

Number of GP visits for flu-like symptoms per 100,000 inhabitants, by age, Belgium, 2018-19
Source: Sciensano Sentinel GP Network [1]

Severity

Although there were many hospital admissions for a severe acute respiratory tract infection (SARI) during this intense season, the severity indicators of the confirmed flu cases (estimated through the network of 6 sentinel hospitals) indicated that this flu epidemic was relatively no more serious than in previous seasons.

The average length of stay for a serious flu infection during this season was 8.8 days (varying in age from 3.6 days in the age of 0-4 to 13.3 days in persons aged 85 years and older). This is comparable to the other seasons.

About 2% to 3% of people with influenza were hospitalized. Among those, 13% had serious complications, which were fatal in 6% of hospitalized patients; these indicators are however similar to those observed in the previous seasons. The vast majority of patients who suffered from severe complications had pre-existing comorbidity (91%).

Deaths were only observed in people aged 54 and over and 90% of these deaths were in patients with one or more comorbidities. The median age of death of patients without comorbidities was 87 years (range: 71-90 years) and the median age of death of patients with comorbidities was 79 years (range: 54-97 years).

International comparison

The observations in Belgium were in line with those in the rest of Europe.

At European level, influenza activity started in week 49/2018, peaked in week 7/2019 and returned to baseline levels in week 17/2019. Influenza viruses started circulating at high levels in week 52/2018 until week 12/2019.

Intensive care unit (ICU) cases were mainly aged 65 years and older, but in certain countries there was a substantial number of hospitalizations in persons aged 40–64 years.

Influenza intensity reported during the 2018-19 season, Europe
Source: European Centre for Disease Prevention and Control [2]

4. Comparison of the last ILI seasons

Incidence

Each year, between 300,000 and 900,000 people visit their GP because of influenza-like illness in Belgium. Among these, the annual number of influenza infections is estimated between 116,000 and 472,000.

Number of GP visits for influenza-like illness by age, Belgium, 2008-19
Source: Sciensano Sentinel GP Network [1]

Severity

Since 2012, between 11% and 15% of patient hospitalized with a confirmed influenza infection have developed severe complications, including 4% to 9% who have died in the course of hospital stay.

Severity indicators in patients with confirmed influenza infection, Belgium, 2011-19
Source: Sciensano network of sentinel hospitals [1]

5. Read more

View the metadata for this indicator

Influenza monitoring

Results of the surveillance of influenza and influenza-like illness in Belgium

Laboratory surveillance of influenza in Belgium

Definitions

Influenza-like illness (ILI)
ILI can be caused by a variety of microbial agents other than influenza viruses. Influenza infection causes a clinical syndrome not easily distinguished from other respiratory infections. ILI is defined by the World Health Organization as an acute respiratory infection with: measured fever of ≥ 38 C°; and cough; with onset within the last 10 days.
Severe acute respiratory tract infection (SARI)
SARI is defined by the World Health Organization as an acute respiratory infection with: history of fever or measured fever of ≥ 38 C°; and cough; with onset within the last 10 days; and requires hospitalization. The SARI definition aims to capture both the influenza-related pneumonias and influenza-related exacerbations of chronic illnesses such as asthma or heart disease.

References

  1. Bossuyt N, Bustos Sierra N, Thomas I, Barbezange C. Surveillance of influenza-like illness in season 2018-2019. Brussels: Sciensano; 2020. Available from https://epidemio.wiv-isp.be/ID/diseases/Pages/Influenza.aspx.
  2. Infographic: Influenza in Europe, Season 2018-2019. Stockholm: ECDC; 2019. Available from https://www.ecdc.europa.eu/en/publications-data/infographic-influenza-europe-season-2018-2019