Seasonal influenza

Overview

Seasonal influenza, commonly known as the flu, is an acute respiratory infection caused by influenza viruses. The disease can cause mild to severe illness, sometimes leading to death. There are three main types of influenza viruses – Type A, B and C. Influenza A (H1N1-2009), Influenza A (H3N2), and Influenza B are the predominant strains circulating globally and in Singapore. Influenza C is associated with mild sporadic illness and occurs less frequently.

Disease epidemiology

Influenza A and B viruses circulate year-round in Singapore, with bimodal peaks typically observed in May to August and December to March. These peaks correspond approximately with influenza seasons in temperate regions of the Southern and Northern hemispheres.

Pathogen(s)

Influenza Viruses (Types A, B, and C)

Transmission

Influenza primarily transmits through inhalation or direct deposition of infectious respiratory particles (IRPs) such as when an infected person coughs or sneezes. It can also spread via contact with contaminated surfaces such as when IRPs settle on surfaces or transfer through contact (e.g. handshake) before reaching the mouth, nose or eyes.

Incubation period: Typically 2 days, ranging from 1 to 4 days.

Infectious period: From 1 day before onset of symptoms until 5 to 7 days after onset in adults. Children and immunocompromised persons with weakened immune systems may be infectious for longer periods of time.

Clinical features

The signs and symptoms of seasonal influenza range from mild respiratory symptoms to severe illnesses such as pneumonia.

Common symptoms include:

• Fever, chills

• Headache

• Cough

• Sore throat

• Muscle aches

• General malaise and fatigue

• Gastrointestinal symptoms, such as nausea, vomiting, or diarrhoea, more commonly in children

Severe complications include:

• Secondary bacterial pneumonia (most often Staphylococcus aureus, Streptococcus pneumoniae, or Haemophilus influenzae)

• Myocarditis

• Encephalitis

• Sepsis

• Multi-organ failure

In severe cases, influenza can result in pneumonia, hospitalisation or even death, especially in populations at higher risk of developing severe disease.

Risk factors

Individuals at greater risk of severe disease or complications when infected include:

• Persons aged 65 years and above

• Children aged 6 months to under 5 years

• Adults and children with chronic pulmonary or cardiovascular disorders

• Adults and children who have been hospitalised or require regular medical follow-up due to chronic metabolic diseases, renal, neurologic, hepatic, or haematologic disorders

• Adults and children with immunosuppression

• Children and adolescents aged 6 months to 18 years who are receiving long-term aspirin therapy and therefore might be at risk for developing Reye syndrome after influenza infection

• Women at all stages of pregnancy

Diagnosis

Detection of influenza virus in respiratory samples through rapid diagnostic tests (antigen-based), rapid molecular assays, polymerase chain reaction (PCR), sequencing or virus isolation.

Treatment and management

Seasonal influenza is usually mild and self-limiting. Supportive care and symptomatic treatment, such as treatment with antipyretics, plenty of fluids, and adequate bed rest, are usually sufficient in the management of acute symptoms of influenza.

Antiviral medications may be prescribed to reduce the severity and duration of symptoms, as well as promote recovery.

Precaution, prevention and control

Isolation measures

In healthcare settings, standard and droplet precautions should be implemented. Patients should wear a surgical mask and be isolated when possible. Good hand hygiene and respiratory etiquette should be reinforced for both healthcare workers and the general public.

Contact management

Post-exposure prophylaxis with antivirals may be offered to high-risk contacts of confirmed influenza cases.

Vaccination

Individuals at a higher risk of developing severe influenza infections, such as seniors and other vulnerable persons with underlying medical conditions, are recommended to consult their attending physician and receive their influenza vaccinations as per the prevailing national vaccination recommendations.

Notification

Seasonal influenza is not a legally notifiable disease in Singapore.

Note: While single cases are not notifiable, medical practitioners should report events of public health significance (e.g. cluster of infectious diseases in the community). Please refer to the Infectious Disease Notification for more information.

Resources

Please refer to the Weekly Infectious Diseases Bulletin for the latest influenza statistics in Singapore.

For more information on seasonal influenza, please refer to the World Health Organization (WHO) website. Click here for the latest WHO update on the influenza situation.

For general travel advisory, please refer to Health Advice for Travellers.