Seasonal Influenza Vaccine

Details

Why should I get the flu shot?

Seasonal influenza vaccination is one of the effective means in preventing influenza and its complications together with reduction in influenza-associated hospitalisation and death.

What are trivalent and quadrivalent vaccines? Are they safe and effective?

The trivalent formulation includes antigens for two influenza A subtypes (H1N1 and H3N2) and one influenza B virus (Victoria lineage) while the quadrivalent formulation has antigens for two influenza A and two influenza B viruses (Victoria and Yamagata lineages). Based on the World Health Organisation surveillance data, B/Yamagata lineage viruses are no longer circulating in the population.

Both vaccines are effective in preventing influenza and its complications together with reduction in influenza-associated hospitalisation and death and have excellent safety profiles.

When should I get the flu shot?

Since it takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection, it is advised to get vaccinated at least two weeks before the arrival of winter influenza season. However, even if seasonal influenza vaccination is not received before the winter influenza season, it can still be received as soon as possible.

What is the recommended dosage for the flu shot?

Persons aged 9 years or above: Single dose per year.

For children aged 6 months to under 9 years who have never received any seasonal influenza vaccine before, two doses of seasonal influenza vaccine with a minimum interval of 28 days are recommended.

What are the common side effects of the flu shot?

The most common side effects following inactivated influenza vaccine administration are local reactions, which include pain, redness or swelling at the injection site. Some recipients may experience fever, chills, muscle pain and tiredness.

Side effects are generally mild and temporary. If you experience persistent fever, severe allergic reactions (e.g. difficulty in breathing, swelling of the lip or tongue, hives, etc.) or other adverse events after receiving vaccination, please consult a doctor immediately.

The most common side effects following recombinant influenza vaccine administration are local reactions, which include pain, redness and swelling at the injection site. Some recipients may also experience fever, chills, muscle pain and tiredness.

Side effects are generally mild and temporary. If you experience persistent fever, severe allergic reactions (e.g. difficulty in breathing, swelling of the lip or tongue, hives, etc.) or other adverse events after receiving vaccination, please consult a doctor immediately.

The most common side effects following live attenuated influenza vaccine administration are mild nasal congestion or runny nose, low-grade fever and sore throat. The safety in pregnant women has not been established. Children aged below five years with recurrent wheezing or persons of any age with asthma may be at an increased risk of wheezing following administration. As this is a live vaccine, there is potential for transmission of the vaccine viruses to immunocompromised individuals.

Side effects are generally mild and temporary. If you experience persistent fever, severe allergic reactions (e.g. difficulty in breathing, swelling of the lip or tongue, hives, etc.) or other adverse events after receiving vaccination, please consult a doctor immediately.

Do I need to get vaccinated for influenza every year?

Seasonal influenza vaccination requires annual administration. Evidence on repeated influenza vaccination shows that vaccination in the current and prior season provides better protection than no vaccination or being vaccinated in the prior season only. Furthermore, the circulating seasonal influenza strains may change from time to time and the vaccine composition is updated every year in accordance with the circulating strains to enhance protection.

Reference:
1. Centre of Health Proctection Frequently Asked Questions on Seasonal Influenza Vaccination for the 2025-26 Season in Hong Kong
https://www.chp.gov.hk/en/features/100764.html