Hong Kong is battling a particularly virulent flu season which has so far claimed 145 lives since it began on Jan 2.

Do you need to take a flu vaccination? How much does it cost and how effective is it? Here are six things to note about the flu vaccine.

1. Who should be vaccinated?

Singapore's Ministry of Health recommends getting a flu vaccination every year, especially for those who are at higher risk for flu-linked complications. These include people above 65 years old, children aged six months to less than five years, adults and children who have chronic disorders of the lung or heart systems, including asthma, and women at all stages of pregnancy.

The Health Promotion Board also suggests that those who live with people with low immunity, such as the elderly and the young, should take the vaccine.

Dr Yik Keng Yeong, a general practitioner who has been in practice for more than 30 years, says that frequent travellers should take an annual shot. This is especially so if you are travelling to an area which is having an outbreak, Dr Yik says.

2. How much does it cost?

The vaccination can cost between $30 and $40, according to the Health Promotion Board website. It also says that Medisave can be used for flu vaccinations for persons with higher risk of developing influenza-related complications.

3. How effective is the vaccine?

Dr Yik says that, in general, the vaccines are 70 to 80 per cent effective. It is fine for the vaccine not to be 100 per cent effective as the body will do the rest of the work, he adds. The effectiveness of flu vaccines may be delayed in individuals who are sleep deprived, according to the Singhealth website.

While the vaccination covers most flu viruses, the current flu A-type H3N2 virus spreading in Hong Kong is a recently-mutated form. The original H3N2, which also originated in the country, caused the 1968 flu pandemic which killed an estimated one million people worldwide.

The flu virus can be classified into three categories: A, B and C. A-type viruses are divided into subtypes and can originate from birds or pigs, which are then passed on to people.

B-types almost exclusively infect humans and are circulated seasonally, while C-types cause milder infections and are associated with minor outbreaks only.

4. When should the vaccination be taken?

Dr Yik recommends that those who are travelling should get vaccinated one month in advance, but says that two weeks is also acceptable.

5. What is the vaccine made of?

Dr Yik says that it is made up of dead flu virus or attenuated virus, which can be described as a tamed virus. Both of these help the body to develop resistance against the flu. The flu virus mutates frequently, he says, but the vaccine will be still partially effective as different strains of flu may share the same characteristics.

6. What happens if I get the flu?

In the first two days after getting the flu, an anti-viral medication called Tamiflu may be effective, but after that, doctors may prescribe antibiotics to fight any secondary bacterial infections in a patient. Such infections develop frequently because immunity is low due to the flu, Dr Yik says.

Sources: Health Promotion Board, Ministry of Health, Singhealth, Dr Yik Keng Yeong