The influenza virus is constantly changing, so the flu vaccine changes every year to keep up with the most common strains.

The Therapeutic Goods Administration (TGA) has approved seven different vaccines for 2018: five quadrivalent vaccines (that is, they protect against four strains of flu) and two trivalent vaccines (which protect against three flu strains).

For babies over 6 months, children and adults up to 65, quadrivalent vaccines contain four influenza virus strains:

• A (H1N1): an A/Michigan/45/2015 (H1N1)pdm09-like virus
• A (H3N2): an A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus
• B: a B/Phuket/3073/2013-like virus
• B: a B/Brisbane/60/2008-like virus.

For adults 65 and older, there are two trivalent vaccines which have been boosted or augmented to provide enhanced protection. They contain:

• A (H1N1): an A/Michigan/45/2015 (H1N1)pdm09-like virus
• A (H3N2): an A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus
• B: a B/Phuket/3073/2013-like virus

These strains were determined last year, based on predictions for the coming season.

The World Health Organization (WHO) has 100 national influenza centres worldwide and four collaborating centres (including one in Melbourne) which analyse viruses from around the world. The WHO convenes two meetings each year, in February and September, to review the findings of the centres and recommend which vaccine formulations should be used in the northern and southern hemispheres respectively. It then takes about six months to manufacture the vaccines.

Last year's flu vaccine wasn't very effective, with around 30% overall effectiveness rating – worse for older adults but better for children and younger adults. Experts say that the H3N2 virus that caused many problems in 2017 in Australia and other countries is much better covered by the 2018 vaccine.

Many people mistake a bad cold for the flu, and may think they have the flu, "even though I got a flu shot".

Experts say the best time to get the flu vaccine is in late autumn, ideally late April to early May - the government's National Seasonal Influenza Vaccination Program, with free vaccination for eligible people, begins mid-April. This timing provides optimal protection for the peak flu season, which is June to September. That said, flu viruses are around at all times of the year, so even if you're a little late, it's still worthwhile.

The influenza virus components in the vaccine have been killed, so you won't get the flu as a result of having a flu shot.

Some people experience some mild flu-like symptoms for a day or so afterwards – it's not uncommon for the body's immune system to react to vaccination.

Possible symptoms include soreness, redness, discomfort and swelling at the injection site, tiredness, muscle aches and low fever.

Or perhaps, given that vaccination time is also the time of year when coughs and colds are starting to take hold, you could be suffering from another virus.

Health authorities are encouraging everyone who can have a flu shot to get one. High rates of immunity in the population – so-called herd immunity – help stop the spread of the disease, and protect those who can't be vaccinated.

Certain vulnerable groups are eligible for free vaccination as part of the National Immunisation Program:

• All adults aged 65 and over.
• Aboriginal and Torres Strait Islanders aged 6 months to 5 years, and 15 and over.
• People older than six months who have diabetes; cancer; asthma; low immunity; chronic respiratory, heart, kidney or metabolic disorders; and children on long-term aspirin therapy.
• Pregnant women.

But anyone who wants to avoid getting the flu should get a flu shot, and it's especially recommended for:

• public hospital outpatients and inpatients who are considered to be at high risk for complications of influenza
• residents of nursing homes, care hostels and other chronic-care facilities
• people such as doctors, nurses, daycare staff and nursing home staff who are in contact with at-risk groups
• family members caring for elderly people and/or people with chronic health conditions listed above.

If you've ever had a serious allergic reaction (anaphylaxis) to a flu vaccine or any of the ingredients in it, you should discuss this with your doctor.

The vaccine is considered safe for people with egg allergies, but if you're concerned, speak to your doctor and have the vaccine where there are staff who can recognise and treat anaphylaxis.

If you're already ill with a fever, wait until your symptoms subside before being vaccinated.

Babies under 6 months can't be given the flu vaccine. However, pregnant women can get the vaccine, and this will also protect the baby, including for the first few months after birth.

Children as young as six months can be vaccinated against the flu. There are different vaccines for different ages, so if you're booking an appointment, tell them the age/s of the child/ren to ensure the appropriate vaccine is at hand.