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NCID > For Healthcare Professionals > Diseases and Conditions > Influenza

Influenza

Influenza

Causative Agent

Influenza virus type A (subtypes H1N1, H2N2, H3N2), B and C. Influenza type C infection causes a febrile mild upper respiratory illness and does not occur in epidemics.

Incubation Period

24 - 72 hours

Infectious Period

One day before symptoms develop and up to 5-7 days after symptom onset.

Transmission

Mainly via respiratory droplets and direct contact with nasal or throat secretions.

Epidemiology

Influenza A and B virus infections occur all year round in Singapore with small peaks in the middle and the end/beginning of the year.

In April 2009, an outbreak of influenza caused by a novel influenza A virus (H1N1- 2009), was reported in Mexico. It subsequently spread to the rest of the world. Singapore reported its first case in a returning student from New York, US, on 26 May 2009. On 11 June, the World Health Organisation (WHO) declared the first influenza pandemic of the 21st century. By September 2009, it was estimated that at least 270,000 persons had been infected in Singapore. A total of 18 H1N1-related deaths were reported during this period, representing slightly more than 1% of hospitalized cases and a case fatality rate of 6.7 per 100,000 cases. The WHO declared an end to the pandemic in August 2010.


  • Classic influenza includes symptoms of fever, chills, headache, malaise, myalgia, and anorexia. Respiratory symptoms include sore throat, dry cough  and nasal discharge. Elderly patients may present with confusion.
  • Well recognised pulmonary complications include primary viral pneumonia and secondary bacterial pneumonia (most often Staphylococcus aureus, Streptococcus pneumoniae and Haemophilus influenzae).
  • Other complications include croup, exacerbation of chronic pulmonary disease, myocarditis, Guillain-Barré syndrome and Reye’s syndrome.


  • Rapid diagnosis can be performed using antigen detection by immunofluorescence (IF) on nasopharyngeal aspirates/swabs, throat swabs, nasal or bronchial washings. Rapid detection kits using the membrane enzyme immunoassay (EIA) method and the immunochromatographic method are available for point-of-care testing.
  • Reverse-transcriptase PCR
  • For epidemiological purposes, serology by complement fixation test is carried out on paired sera taken 2 weeks apart.
  • Viral culture using the MDCK (Madin Darby canine kidney) cell line in shell vial cultures. Results are available after 5 days of incubation.

NB: Diagnosis can be made on epidemiological grounds in the setting of an influenza outbreak.



Seasonal influenza is not a notifiable disease in Singapore.



  • Symptomatic treatment with antipyretics and anti-histamines and adequate bed rest and fluids are usually sufficient in the management of acute symptoms of influenza. Avoid salicylates in children because of the risk of Reye’s syndrome.
  • Neuraminidase inhibitors (zanamivir and oseltamivir) if administered within 48 hours of illness onset, reduce duration of symptoms for both influenza A and B by one day. Recommended dose and duration:
    • Zanamivir 2 puffs (10 mg) bd x 5 days (age 5 years and above)
    • Oseltamivir 75 mg oral bd x 5 days (age one year and above). Weight- based dosing for those less than 40 kg.
  • Amantadine can shorten the duration and severity of influenza A illness if used within 48 hours of onset. It is approved for use in people age one year and above. The dose recommended for adults is 100 mg twice daily for 3 days. Weight-based dosing for those aged 1-8 years old.
  • Rimatadine is approved to treat influenza A infection in people age 13 years and above. The drug is not registered in Singapore.
  • Antibiotics are used for proven bacterial superinfections. Other complications are managed by supportive treatment.


  • Surveillance of influenza activity is routinely carried out in Singapore. This is based on weekly polyclinic attendances for influenza-like illnesses and clinical
  • specimens positive for influenza viruses reported to MOH by SGH and other selected laboratories.
  • Health education through the mass media to prevent rapid spread of infection by indiscriminate coughing and sneezing and instructions on proper hand-washing is carried out during an outbreak.
  • Immunoprophylaxis with inactivated vaccine or chemoprophylaxis with an influenza-specific anti-viral drug (oseltamivir or amantadine) can reduce the impact of influenza.



  • The trivalent inactivated influenza vaccine (TIV) is recommended annually. It is prepared from the prevailing strains of influenza A and B. The vaccine is updated every year based on the predicted predominant strain for the season.
  • One dose of inactivated vaccine is used if previously vaccinated or age ≥ 9 years. In children ≤ 9 years not previously vaccinated, 2 doses at least 4 weeks apart should be given.
  • In some countries, a live, intranasal influenza vaccine (LAIV) is available for healthy people 2-49 years of age. LAIV is not presently registered in Singapore.

Routine influenza vaccination is now recommended for all persons aged 6 months and older. It is especially important that certain people get vaccinated either because they are at high risk of having serious influenza-related complications or because they live with or care for people at high risk for developing influenza-related complications.

Target groups for vaccination

  • Persons > 65 years old.
  • Residents of nursing home or chronic care facilities.
  • Adults or children with chronic pulmonary or cardiovascular disorders including children with asthma.
  • Adults and children with chronic metabolic diseases.
  • Children and teenagers (6 months - 18 years of age) receiving long-term aspirin therapy and therefore at risk for developing Reye’s syndrome.
  • Healthcare workers and others providing care to persons at high risk.
  • Household members of persons in high-risk groups.
  • Women who will be in the second or third trimester of pregnancy during  the influenza season.


  • Oseltamivir 75 mg once daily for adults. Weight-based dosing for those less than 40kg.
  • Zanamivir 1 puff (5mg) bd for those 5 years and above
  • Amantadine 5mg/kg/day, up to 150mg /day in 2 divided doses (age 1-10 years); 100 mg bd (age 10 years and above)

Duration of chemoprophylaxis depends on the period of influenza activity in the community.



  • Encourage vaccination and chemoprophylaxis for the hospital staff.
  • Instruct staff members who develop illness to stay away from work.
  • Restrict visitors with any illness.
  • Isolate (in single rooms) or cohort patients with acute illness.
  • Observe standard and droplet precautions against cross-infection:  gowns,  masks and hand-washing.
  • Postpone all elective surgery because anaesthesia may add to the risk of pulmonary complications.





References

  1. Nicholson KG, Wood JM, Zambon M. Influenza. Lancet. 2003; 362:1733-45.
  2. US Centers for Disease Control and Prevention. Prevention and control of influenza with vaccines, Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep 2010; 59(RR-8): 1-62.
  3. World Health Organization. Statement by WHO Director-General Dr Margaret Chan 29 April 2009. Available at http://www.who.int/mediacentre/news/statements/2009/h1n1_20090429/en/index.html. Accessed October 2009.
  4. Cutter JL, Ang LW, Lai FYL et al. Outbreak of pandemic influenza A (H1N1-2009) in Singapore, May to September 2009. Ann Acad Med Singapore 2010;39:273-82.





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