Influenza is an infection caused by a virus called the influenza virus. It is usually a more severe illness than the common cold which is caused by other respiratory viruses. Influenza typically has a sudden onset with headache, chills and cough followed rapidly by a fever, appetite loss, muscle aches, and tiredness. On the other hand, cold symptoms are limited to the upper respiratory tract with runny nose, sneezing, watery eyes, and throat irritation. Gastrointestinal symptoms, such as nausea, vomiting or diarrhoea, can sometimes accompany influenza, especially in children, but these symptoms are rarely prominent. The term "stomach flu" is often incorrectly used to describe gastrointestinal illness that is due to other causes.

There are three main types of influenza:

a) influenza A,
b) influenza B, and
c) influenza C.

Influenza A and influenza B are associated with annual outbreaks and epidemics. Influenza A usually causes more severe illness than influenza B and can result in pneumonia, hospitalization or even death, especially in the elderly and those with chronic illnesses. Influenza A has also been responsible for all previous pandemics (worldwide epidemics of influenza).

Influenza C is very rarely seen and is associated with only mild sporadic illness.

The "flu" virus is spread from person to person very efficiently through droplets of saliva or phlegm (spit) that travel through the air, usually propelled along by the action of coughing or sneezing. This way of spreading the virus happens most easily indoors and in crowded places. Influenza can also be passed on from person to person by touching contaminated surfaces, for example touching an object like a doorknob or telephone receiver that has been recently handled by someone who is infected with influenza.

The period when an infected person is contagious depends on the age of the person. Adults may be contagious from one day prior to becoming sick and for three to seven days after they first develop symptoms. Some children may be contagious for longer than a week.

While the majority of people infected with influenza will recover completely within a week to 10 days after experiencing their first symptoms, some people are at greater risk of experiencing more severe and long lasting complications. In particular, very young children and those over 65 years of age as well as those already suffering from certain medical conditions including: chronic respiratory or cardiac disease, chronic kidney (renal) disease, diabetes or depressed immune systems (due to cancer, HIV infection, steroid treatment, etc.) are at increased risk from complications following influenza infection.

Unlike temperate countries, influenza infections happen throughout the year in Singapore. However, there are 2 seasonal peaks, one from May to July, the other from Nov to Jan, coinciding with the winter seasons in the Southern and Northern hemispheres respectively.

Hygienic practices such as hand washing may help to reduce the risk of being infected, especially after contact with persons who may have the flu and after handling soiled tissues. People can build up their body resistance to influenza by having a proper diet and adequate exercise and rest. They can also avoid close contact with persons who are ill with an influenza-like infection.

Children who exhibit symptoms of influenza such as fever, runny nose, cough, chills and muscle ache should be excused from school.

Proper hygiene practices include:

a)	Regular and proper handwashing. Proper handwashing requires the use of soap and water. Rinse hands under running water and dry them thoroughly.When handwashing facilities are not available, use a waterless antiseptic cleanser, following the manufacturer's directions for use.
b)	All body fluid spills must be cleaned thoroughly prior to disinfection. Mops and equipment used should be soaked in the disinfectant solution after use and rinsed thoroughly with water. The solution should then be promptly disposed off down a drain pipe. An appropriate disinfectant is the use of diluted household bleach.
c)	Toys should be kept clean. To prevent transmission of any pathogens, it has been recommended that toys that are mouthed routinely be cleaned and disinfected. Other toys should be cleaned and disinfected if they become contaminated by body fluids.
d)	Supplies of disposable gloves and towels and disinfectants should be readily available and accessible to all staff.

People who develop influenza-like symptoms are advised not to go to work or school and avoid crowded places to minimize the transmission of the infection to others. They should also cover their mouth and nose with a tissue when coughing or sneezing.

They should increase fluid intake (water, juice, soups etc.) and get plenty of rest for the 7 to 10 days that symptoms might continue. Strenuous physical activities like running and jogging, and smoking and alcohol should also be avoided during the illness until complete recovery.

Persons who are at risk of complications from influenza infection should see their family doctor for a medical evaluation when they have symptoms of influenza. See question below on "Who should get vaccinated" for a list of persons who are at increased risk of complications.

In children, some emergency warning signs that need urgent medical attention include:

• High or prolonged fever
• Fast breathing or difficulty breathing
• Bluish skin color
• Refusing to drink water or other fluids
• Change in mental status, such as not waking up or not interacting; being so irritable that the child does not want to be held; or seizures
• Change in mental status, such as not waking up or not interacting; being so irritable that the child does not want to be held; or seizures
• Worsening of underlying chronic medical conditions (for example, heart or lung disease, diabetes)

In adults, some emergency warning signs that need urgent medical attention include:

• High or prolonged fever
• Difficulty breathing or shortness of breath
• Pain or pressure in the chest
• Near-fainting or fainting
• Confusion
• Severe or persistent vomiting

Seek medical care immediately, either by calling your doctor or going to an emergency room, if you or someone you know is experiencing any of the signs described above or other unusually severe symptoms. When you arrive, tell the receptionist or nurse about your symptoms. You may be asked to wear a mask and/or sit in a separate area to protect others from getting sick.

Yes. Respiratory infections due to viruses other than influenza viruses and the common cold viruses may occur. Viruses such as respiratory syncytial virus (RSV) and parainfluenza viruses can cause illnesses which may be mistaken for the "flu". These can also cause symptoms similar to those of the "flu" including fever, cough, and sore throat. While RSV and parainfluenza viruses are present every year and primarily affect young children, they can cause illness in adults as well.

Current influenza vaccines are made of dead fragments of influenza viruses. The process of making influenza vaccines involves initially growing the recommended virus strains in fertilized hens' eggs. The virus is then extracted from the egg and killed and further treated to break up the dead virus and purify it into small clean particles. These purified particles allow the body's immune system to recognise the genetic "blueprint" of the influenza virus without causing illness. Vaccines are updated annually to include purified vaccine particles which are similar to the most current influenza virus strains that have the greatest likelihood of causing severe and widespread illness.

After vaccination, the body's immune system produces antibodies against the inactivated virus in the vaccine. If you are exposed to the real virus, the antibodies prevent the infection or reduce the likelihood of severe illness should infection occur.

Current influenza vaccines are not recommended for children under 6 months of age due to the high incidence of febrile reactions in this age group. Influenza vaccine should not be given to people who had an allergic reaction to a previous dose or with known allergic reaction to eggs manifested as hives, swelling of the mouth and throat, difficulty in breathing, hypotension and shock.

Persons who are at higher risk of developing complications from influenza are advised to get themselves vaccinated against influenza every year. The Ministry of Health recommends that the following groups be vaccinated:

• Persons aged 65 years and older;
• Children aged 6 months to 5 years;
• Residents of nursing homes and other chronic care facilities;
• Adults and children who have chronic disorders of the lungs or heart, including asthma;
• Adults and children who have required regular medical follow-up or hospitalisation during the preceding year because of chronic metabolic diseases (including diabetes mellitus), kidney or blood disorders, or lowered immunity caused by medications or by the Human Immunodeficiency Virus (HIV);
• Children and teenagers aged 6 months to 18 years who are receiving long-term aspirin therapy;
• Women who are in the second or third trimester of pregnancy.

In general, adults and older children will require a single dose of the vaccine to protect them against influenza for that season. Previously unvaccinated children under the age of 9 years will require 2 doses given at least 1 month apart.

Unless there is a major change in the influenza virus strains by the Ministry, annual vaccinations would suffice.

Influenza vaccination is available at the travel clinics in Tan Tock Seng Hospital, National University Hospital, Kandang Kerbau Women's and Children's Hospital and Changi General Hospital, polyclinics and General Practitioners.

Travellers who are at higher risk of complications arising from influenza and will be travelling to temperate countries during the winter season are advised to go for influenza vaccination. The vaccination should be taken at least 1 to 2 weeks prior to departure as it takes time for the vaccination to take effect. (Children younger than 9 years old who have never been previously vaccinated will require 2 vaccinations at least 4 weeks apart; the second vaccination should be taken at least 1 week prior to departure.)

For persons living in Singapore, an annual vaccination can be taken a few weeks before either one of the 2 influenza infection peaks seen in May-July and Nov-Jan.

Soreness at the injection site lasting 2 days or less is the most common side effect of influenza vaccination, but this rarely interferes with one's normal activities. Fever, tiredness, and muscle aches may occur within 6 to 12 hours after vaccination and last 1 to 2 days.

Allergic responses are rare and are probably a consequence of hypersensitivity to some vaccine component, most likely residual egg protein, which is present in minute quantities.

Guillain-Barre (Ghee-yan Bah-ray) syndrome (GBS) associated with influenza vaccination is a rare occurrence and has been observed in only a few influenza seasons over the last two decades. GBS is an inflammatory disorder of the peripheral nerves - those outside the brain and spinal cord. It is characterized by the rapid onset of weakness and, often, paralysis of the legs, arms, breathing muscles and face. The occurrence of GBS after influenza vaccination is uncommon. Recent U.S. studies of vaccine associated adverse events, estimated that the risk of vaccine-associated GBS is approximately one case per million persons vaccinated. In comparison, the risk of complications and deaths associated with influenza is much greater. Hence, the benefits of influenza vaccination clearly outweigh the risk of vaccine-associated GBS.

No. Influenza vaccination cannot cause influenza because the vaccine does not contain living virus. All current influenza vaccines available in Singapore are composed of killed, split and purified virus particles, which are incapable of causing influenza.

In general, protection acquired from influenza vaccination lasts 6-12 months. Furthermore, influenza viruses continually undergo genetic changes; thus one's immunity, acquired as a result of previous influenza infections or previous vaccination, would not necessarily protect against subsequent infections to new strains of the virus. As the influenza viruses change, the vaccine has to be updated on a yearly basis to include the most current strains.

A global network of laboratories identifies specific influenza viruses year-round as they appear. In Singapore, the National Influenza Laboratory sited at the Virology Department of Singapore General Hospital participates in this global network by determining the prominent influenza strains circulating in the Singaporean population. Singaporean information is used, together with similar information gathered from around the world, to predict which strains will most likely dominate the next influenza season. All viruses identified are reported to four World Health Organization (WHO) Collaborating Centres for Influenza. Twice a year (in Feb for the Northern Hemisphere winter season starting in November of that year, and in October for the Southern Hemisphere winter season starting in May of the following year), the WHO makes a recommendation regarding which strains should be included in the next season's influenza vaccine. Based on that recommendation, the global network of laboratories provides vaccine manufacturers with the "seed strains" (strains from which the virus is grown) needed to produce the vaccine.

All influenza vaccines sold in Singapore contain the strains recommended by WHO for that season.

All influenza vaccines that are sold in Singapore have been licensed by the Health Sciences Authority, and have been shown to be effective. You should discuss with your doctor on the vaccine brands available and on the most suitable one for you.

Yes. It is possible to become infected after vaccination; the following factors determine the success of influenza vaccination:

1. The health of the person's immune system.
2. The similarity between the virus they are exposed to and the vaccine they received.

It is possible that one dose of vaccine in a child younger than 9 years of age could provide protection against influenza. Studies in the 1970s suggested that between 10% and 40% of children 6 months through 12 years of age produced substantial antibody levels with one dose of influenza type A/H1N1 vaccine, and as many as 67% to 85% of children 6 months through 12 years of age produced substantial antibody levels to influenza type B and influenza type A/H3N2 viruses with one dose of vaccine. However, the degree of protection remains unknown because there are no vaccine efficacy studies showing how much protection is provided by one dose versus two doses of influenza vaccine in young, previously unvaccinated children. Since one dose of vaccine may not lead to optimal levels of antibodies, a second vaccination is recommended for children younger than 9 years old who have never been previously vaccinated.

There are antiviral medications available in Singapore which can be used to treat influenza. If taken early in the course of influenza illness (within 48 hours), these medications can also reduce the severity and duration (by about 1 to 1.5 days) of infection. The drugs which are used in treating influenza include amantadine, oseltamivir (Tamiflu) and zanamivir. Rimantadine is currently not registered for use in Singapore.

If you have any questions about the use of antiviral medications, and whether these medications are suitable for you, you should consult your doctor.

Yes, it is common for influenza strains to change with each season.  The degree of change and its impact on each season is unpredictable.

There is a naming convention which goes according to the city of the laboratory from which the strain was obtained.  However, other countries may have similar strains too.  New strains can be very similar to old strains, or may be different.

There are two  subtypes of influenza A and one type of influenza B in seasonal circulation.  Although there are many strains, these usually fall into groups which may be closely related to type strains used in vaccine production.  Singapore has a mixture of these subtypes and strains.  This includes those which are related to the vaccine strains, as well as those which are less related.

Emergence of new strains with different properties are expected, which is why influenza vaccines are updated every year.  Due to the mixture of strain types, immunization is never 100% effective, but does confer protection which is important in those people at risk of complications.   Influenza illness caused by any strain is a cause for concern as the infection can be complicated by lung or brain infection.