Flu Vaccine FAQ

Influenza is an acute illness of the respiratory tract caused by influenza viruses. It can be caused by various types of influenza viruses. In Hong Kong, influenza A(H1), influenza A(H3) and influenza B virus are most commonly seen. Influenza occurs in Hong Kong throughout the year, but is usually more common in periods from January to April and from July to August. The virus mainly spreads by respiratory droplets. The disease is characterised by fever, sore throat, cough, runny nose, headache, muscle aches and general tiredness. It is usually self-limiting with recovery in two to seven days. However, if persons with weakened immunity and elderly persons get infected, it can be a serious illness and may be complicated by bronchitis, pneumonia, encephalopathy, or even death in the most serious cases. Serious infection or complications can also occur in healthy individuals.

Seasonal influenza refers to the viruses that circulate in the human population and cause widespread illnesses especially during each influenza season. Pandemic influenza occurs rarely, when a new influenza virus emerges which is markedly different from those already circulating in the human population. It spreads easily between people and causes disease because they have little or no immunity to it.

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

Given that SIV offers protection against clinical influenza and severe cases do occur in previously healthy persons, all members of the public aged 6 months or above, except those with known contraindications, should receive SIV annually for personal protection.

If the level of local seasonal influenza activity increases, members of the public, especially children, the elderly and persons with chronic medical problems, should receive seasonal influenza vaccination (SIV) as early as possible.

In addition, members of the public should maintain good personal and environmental hygiene against influenza and note the following:

• Surgical masks can prevent transmission of respiratory viruses from ill persons. It is essential for persons who are symptomatic (even if having mild symptoms) to wear a surgical mask;
• For high-risk persons (e.g. persons with underlying medical conditions or persons who are immunocompromised), wear a surgical mask when taking public transport or staying in crowded places. It is important to wear a mask properly, including performing hand hygiene before wearing and after removing a mask;
• Avoid touching one's eyes, mouth and nose;
• Wash hands with liquid soap and water properly whenever possibly contaminated;
• When hands are not visibly soiled, clean them with 70 to 80 per cent alcohol-based handrub;
• Cover the mouth and nose with tissue paper when sneezing or coughing. Dispose of soiled tissue paper properly into a lidded rubbish bin, and wash hands thoroughly afterwards;
• Maintain good indoor ventilation;
• When having respiratory symptoms, wear a surgical mask, refrain from work or attending classes at school, avoid going to crowded places and seek medical advice promptly; and
• Maintain a balanced diet, perform physical activity regularly, take adequate rest, do not smoke and avoid overstress.

Given that seasonal influenza vaccination (SIV) offers protection against clinical influenza and severe cases do occur in previously healthy persons, all members of the public aged 6 months or above, except those with known contraindications, should receive SIV annually for personal protection.

In the 2026-27 season, the Department of Health (DH) and the Scientific Committee on Vaccine Preventable Diseases (SCVPD) under the DH recommended the following priority groups for seasonal influenza vaccination (SIV) in Hong Kong:

• Health care workers [including care workers in Residential Care Homes for the Elderly (RCHEs), Residential Care Homes for Persons with Disabilities (RCHDs), and laboratory personnel handling influenza virus specimens]
• Persons aged 50 years or above
• Persons aged 18 to 49 years with chronic medical problems*
• Pregnant women
• Children and adolescents aged six months to under 18 years (or aged 18 years or above studying in secondary schools in Hong Kong)
• Residents of Residential Care Homes (e.g. RCHEs or RCHD)
• Poultry workers
• Pig farmers and pig-slaughtering industry personnel

Members of the public can consult their family doctors to receive SIV for personal protection.

According to estimates from the United States (US) Centers for Disease Control and Prevention, influenza vaccines reduced the risk of influenza illness by 40% to 60% among the overall population during seasons when most circulating viruses closely matched with the vaccine strains. Given that severe cases do occur in previously healthy persons, all members of the public aged 6 months or above, except those with known contraindications, should receive seasonal influenza vaccination (SIV) annually for personal protection. Members of the public can consult their family doctors to receive SIV for personal protection.

Seasonal influenza vaccination is recommended for all pregnant women for benefits in terms of reduced acute respiratory infection for both mothers and infants, and reduction of cardiopulmonary complications and the associated hospitalisations in pregnant women. The World Health Organization considers inactivated influenza vaccine (IIV) to be safe for use at any gestational age of pregnancy and there is no evidence indicating that IIV is teratogenic even when given during the first trimester. Recombinant influenza vaccine is not contraindicated in pregnancy. However, live attenuated influenza vaccine should not be used in pregnant women. Pregnant women should consult a doctor for any queries.

The Scientific Committee on Vaccine Preventable Diseases recommended seasonal influenza vaccination (SIV) for elderly aged 65 years or above as they pose a higher risk for complications, excess hospital admissions and deaths from influenza.

Persons aged 50 to 64 are also recommended to receive SIV because local influenza epidemiology showed that people aged 50 to 64 years, irrespective of chronic medical problems, were having a higher risk of influenza-related intensive care unit admission and death.

Pig farmers and pig-slaughtering industry personnel are recommended to receive seasonal influenza vaccination to prevent emergence of new influenza A virus in either human or pig hosts.

Seasonal influenza vaccination (SIV) is recommended for children aged 6 months to 11 years to reduce influenza-related complications such as excess hospitalisations or deaths. Local data have shown that SIV could effectively reduce the risk of severe influenza infection among children below 18 years of age. Overseas studies have shown that vaccinating young school children may potentially reduce school absenteeism and influenza transmission in the community.

Taking reference from overseas practice and local experience, SIV continues to be recommended to secondary school students or adolescents 12 to under 18 years of age for the 2026-27 season.

Seasonal influenza vaccination (SIV) is recommended for persons with chronic medical problems* in view of their increased risk of complications and death associated with influenza infection.

* People with chronic medical problems mainly refer to those who have chronic cardiovascular (except hypertension without complication), lung, metabolic or kidney diseases, obesity# (Body Mass Index 30 or above), who are immunocompromised^, children and adolescents (aged 6 months to 18 years) on long-term aspirin therapy, and those with chronic neurological condition that can compromise respiratory function or the handling of respiratory secretions or that can increase the risk for aspiration or impair their ability to take care of themselves. Of note, live-attenuated influenza vaccine should not be used in immunocompromised persons.

# Obesity is considered as an independent risk factor for influenza complication and thus people with body mass index of 30 or above are included in the priority groups for seasonal influenza vaccination. ^ People who are immunocompromised refer to those with a weakened immune system due to disease (such as HIV/AIDS) or treatment (such as cancer treatment).

The egg-based trivalent influenza vaccine recommended by the Scientific Committee on Vaccine Preventable Diseases (SCVPD) for the 2026-27 season contain the following:

• A/Missouri/11/2025 (H1N1)pdm09-like virus
• A/Darwin/1454/2025 (H3N2)-like virus
• B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus

The cell culture or recombinant based trivalent influenza vaccine recommended by the SCVPD for the 2026-27 season contain the following:

• A/Missouri/11/2025 (H1N1)pdm09-like virus
• A/Darwin/1415/2025 (H3N2)-like virus
• B/Pennsylvania/14/2025 (B/Victoria lineage)-like virus

The trivalent formulation includes antigens for two influenza A subtypes (H1N1 and H3N2) and one influenza B virus (Victoria lineage). Trivalent vaccine is effective in preventing influenza and its complications together with reduction in influenza-associated hospitalisation and death and has excellent safety profile. Based on the World Health Organisation surveillance data, B/Yamagata lineage viruses are no longer circulating in the population, only trivalent vaccines should be used in the 2026-2027 season.

Inactivated influenza vaccine (IIV), recombinant influenza vaccine (RIV) and live attenuated influenza vaccine (LAIV) are recommended for use in Hong Kong. Given B/Yamagata lineage viruses are no longer circulating in the population based on WHO surveillance data, only trivalent vaccines should be used in the 2026-27 season. Depending on individual brand, IIV is recommended for use among people aged six months of age or older, including healthy people and those with chronic medical problems. RIV can be used in adults and children from 9 years of age and older. The LAIV can be used for people aged 2 to 49 years, except those who are pregnant, immunocompromised or with other contraindications.

Inactivated influenza vaccine (IIV) contains inactivated (killed) viruses. Recombinant influenza vaccine (RIV) contains haemagglutinin (a surface protein of influenza virus) that is produced by recombinant technology, and does not require egg or influenza virus in the production process. Both IIV and RIV are given by injection. Live attenuated influenza vaccine (LAIV) contains weakened viruses and is given by intranasal spray.

All these seasonal influenza vaccines (i.e. IIV, RIV and LAIV) are effective, but there are some differences in their indications.

People who have a history of severe allergic reaction to any vaccine component or a previous dose of any influenza vaccine are not suitable to have inactivated influenza vaccination (IIV). Individuals with mild egg allergy who are considering a seasonal influenza vaccination (SIV) with an egg-based IIV can be given in primary care settings, provided they undergo a 30-minute observation period following vaccination. Individuals with a history of anaphylaxis to egg should have SIV administered by healthcare professionals in appropriate medical facilities with capacity to recognise and manage severe allergic reactions. Influenza vaccine contains ovalbumin (an egg protein), but the vaccine manufacturing process involves repeated purification and the ovalbumin content is very low. Even people who are allergic to eggs are generally safe to receive vaccination.

Those with bleeding disorders or on anticoagulants should consult their doctors for advice. If an individual suffers from fever on the day of vaccination, vaccination should be deferred till recovery. Please consult your doctor for details.

People who have a history of hypersensitivity to any vaccine component of the RIV are not suitable to receive the vaccine. RIV contains no egg protein.

Those with bleeding disorders or on anticoagulants should consult their doctors for advice. If an individual suffers from fever on the day of vaccination, vaccination should be deferred until after recovery. Please consult your doctor for details.

Live attenuated influenza vaccine (LAIV) is a live vaccine and is generally contraindicated in the following conditions, taking reference from recommendations of the United States, United Kingdom and Canada:

• History of severe allergic reaction to any vaccine component or after previous dose of any influenza vaccine;
• Concomitant aspirin or salicylate-containing therapy in children and adolescents;
• Children 2 years through 4 years who have asthma or who have had a history of wheezing in the past 12 months**;
• Children and adults who are immunocompromised due to any cause;
• Close contacts and caregivers of severely immunosuppressed persons who require a protected environment;
• Pregnancy; and
• Receipt of influenza antiviral medication within previous 48 hours.

** The UK recommended the use of IIV instead of LAIV for children with increased wheezing and/or needed additional bronchodilator treatment in previous 72 hours. Also, specialist advice should be sought on giving LAIV for children who require regular oral steroid for maintenance of asthma control or who have previously required intensive care for asthmatic attack. Canada recommended that individuals with severe asthma (currently on oral or high-dose inhaled glucocorticosteroids or active wheezing) or those with medically attended wheezing in the 7 days prior to vaccination should not use LAIV.

Individuals with mild egg allergy who are considering an influenza vaccination can be given an egg-based LAIV in primary care settings, provided they undergo a 30-minute observation period following vaccination. Individuals with a history of anaphylaxis to egg should have seasonal influenza vaccine administered by healthcare professionals in appropriate medical facilities with capacity to recognise and manage severe allergic reactions. Influenza vaccine contains ovalbumin (an egg protein), but the vaccine manufacturing process involves repeated purification and the ovalbumin content is very low. Even people who are allergic to eggs are generally safe to receive vaccination.

Please consult your doctor for details.

Children suffering from lung diseases such as asthma are recommended to receive inactivated influenza vaccine (IIV) because of an increased risk of complications associated with influenza. However, children aged 2 to 4 years who have asthma or who have had a history of wheezing in the past 12 months should NOT receive live attenuated influenza vaccine.

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.

Guillain-Barré syndrome (GBS) is an acute paralysing illness, usually provoked by a preceding infection, surgery or rarely after immunisation. It is characterised by progressive weakness of all limbs and areflexia. Overseas studies have estimated that the risk of GBS following influenza vaccination is extremely rare (about one to two GBS cases per million vaccine recipients). Locally, one case of GBS was recorded among persons who had received SIV in the 2024-25 season (within the period of five days and six weeks after seasonal influenza vaccination). No case was reported in the 2025-26 season so far (as of 23 March 2026). In Hong Kong, the baseline number of GBS (all causes) admitted to public hospitals ranged from 33 to 88 cases per year between 2020 and 2025.

As it is unknown whether seasonal influenza vaccination is causally associated with increased risk of recurrent Guillain-Barré syndrome (GBS), precaution should be taken to ascertain the temporal relationship if there is a history of GBS. Please consult your doctor for advice before receiving vaccination.

The viruses in inactivated influenza vaccine are killed viruses and the haemagglutinin contained in recombinant influenza vaccine is a surface protein of the influenza virus, therefore both vaccines cannot cause influenza. The viruses in live attenuated influenza vaccine (LAIV) are weakened and cannot cause influenza in immunocompetent individuals. However, LAIV is not recommended for close contacts and caregivers of severely immunosuppressed persons who require a protective environment because of the theoretical risk of transmission of the live attenuated vaccine viruses to the severely immunosuppressed persons and cause disease.

Yes. Seasonal influenza vaccine is safe for breastfeeding mothers and their infants. Women who receive the influenza vaccination while pregnant or breastfeeding can develop antibodies against influenza that can be passed to their infants through their breast milk and provide some protection against influenza for infants.

he seasonal influenza vaccines currently supplied for use in Hong Kong do not contain mercuric compound (e.g. thiomersal) or aluminum.

No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. For prevention against influenza, vaccinated individuals should maintain good personal and environmental hygiene practices, balanced diet, regular exercise, adequate rest and no smoking.

The Scientific Committee on Vaccine Prevention Diseases (SCVPD) recommended that October-November is the most optimal time for vaccination. 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 onset of the main influenza season. However, even if seasonal influenza vaccination is not received before the recommended period, it can still be received as soon as possible.

According to systematic reviews, influenza vaccines reduced the risk of influenza illness by 40% to 60% among the overall population during seasons when most circulating viruses closely matched with the vaccine strains. Among individuals aged less than 65 years, the estimates ranged from 70 % to 90%.

In older adults, protection that inactivated vaccines offer in preventing influenza illness was modest. A review of existing studies suggested that, in this age group, recombinant influenza vaccines may be potentially more effective than standard-dose inactivated vaccines. For live attenuated influenza vaccine (LAIV), overseas studies and clinical experience had generally shown that LAIV is a safe vaccine, providing comparable protection against influenza to that offered by inactivated influenza vaccine.

All members of the public except those with known contraindications should receive seasonal influenza vaccination (SIV) annually before the arrival of the main influenza season, which is expected to offer protection for upcoming influenza seasons. For children aged 6 months to under 9 years who have never received any SIV before, two doses of SIV with a minimum interval of 28 days are recommended.

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.

To ensure adequate immunity against seasonal influenza, children aged 6 months to under 9 years who have never received any seasonal influenza vaccination (SIV) before are recommended to receive 2 doses of SIV with a minimum interval of 28 days in the current season. Children aged below 9 years who have received at least one dose of SIV before are recommended to receive one dose of SIV in the current season. For persons aged 9 years or above, only one dose of SIV is required in each influenza season.

No, the first and second doses do not have to be the same type. However, the two doses should be separated by at least 28 days.

Seasonal influenza vaccine (SIV) contains ovalbumin (an egg protein), but the vaccine manufacturing process involves repeated purification. The final ovalbumin content is very low. People who are allergic to eggs are generally safe to receive vaccination.

Individuals with mild egg allergy can be vaccinated in a primary clinic setting, provided they undergo a 30-minute observation period following vaccination.

Individuals with a history of anaphylaxis to egg should have SIV administered by healthcare professionals in appropriate medical facilities with capacity to recognise and manage severe allergic reactions.

Yes. Pneumococcal vaccine can be given with seasonal influenza vaccine at the same clinic visit, but should be administered with a different syringe and at a different injection site if inactivated influenza vaccine or recombinant influenza vaccine is used.

Yes. Seasonal influenza vaccine can be given with MMRV or MMR vaccine at the same clinic visit. For individuals receiving inactivated influenza vaccine (IIV) or recombinant influenza vaccine (RIV), they may be administered on the same day or at any interval with MMRV or MMR vaccine, and those vaccines should be administered with a different syringe and at a different injection site than that of MMRV or MMR vaccine. For individuals receiving live attenuated influenza vaccine (LAIV), MMRV or MMR vaccine should be administered at least 28 days apart.

Inactivated influenza vaccine (IIV) and recombinant influenza vaccine (RIV) can be given at the same time or at any time before or after other non-live (e.g. Hepatitis B vaccine, COVID-19 vaccine and Human Papillomavirus Vaccine ) or live vaccine (e.g. Measles, Mumps, Rubella & Varicella (MMRV) Vaccine / Measles, Mumps & Rubella (MMR) Vaccine). Different vaccines should be given at different injection sites if IIV or RIV are given with other vaccines at the same time. For individuals receiving live attenuated influenza vaccine, any other live vaccines should be administered at least 28 days apart.

There are several reasons why someone may get influenza / influenza-like illness even after they have been vaccinated. Firstly, because the body takes about 2 weeks to develop antibodies after vaccination. If a person is infected with influenza virus shortly after vaccination, he or she may develop influenza before becoming immune. Secondly, infection of viruses other than influenza virus can also result in illness similar to influenza, so people may falsely think that they had influenza despite being vaccinated. Thirdly, small number of people experience side effects after receiving inactivated influenza vaccine, for example fever and myalgia, which may be confused with influenza virus infection. Lastly, seasonal influenza vaccines (SIV) are unable to provide complete protection against influenza, because vaccine effectiveness depends on factors including the similarity between the virus strains present in the vaccine and those circulating in the community, as well as the characteristics of recipients, for example, age. Even so, all members of the public aged 6 months or above except those with known contraindications should receive SIV annually because it is one of the effective means in reducing the risk of influenza and its complications together with reduction in influenza-associated hospitalisation and death.

COVID-19 vaccines can be co-administered with seasonal influenza vaccine on the same visit under informed consent for administrative convenience and achieving better coverage.

In general, common side effects of both vaccines are usually mild and temporary which include soreness, redness and swelling at the injection site. Some people may experience fever, muscle pain, and fatigue a few hours after vaccination. In most cases, these symptoms would subside within a few days. If symptoms persist, or if allergic reactions (such as hives or facial swelling) or serious side effects occur, you should seek medical advice promptly. For more information on co-administration of COVID-19 vaccines and seasonal influenza vaccination, please click here.

The general public is encouraged to register for eHealth prior to vaccination. Persons aged 18 or above must register for eHealth to be eligible for subsidised or free seasonal influenza vaccination. You may use "express registration for eHealth" at the vaccination sites. For e-health registration details, please refer to the eHealth website.

Giving eConsent on the eHealth App refers to the function of electronically submitting the consent form for the SIV under VSS. Eligible persons and their parents/ guardians can submit the consent form in advance using the eHealth App.

Under the Vaccination Subsidy Scheme, consent for seasonal influenza vaccination can be given in three ways. Which ones are available depends on the recipient’s age, and on whether the recipient is a mentally incapacitated person.

• Electronic consent by inserting a Hong Kong Identity Card into a card reader — available to every recipient. A person aged 18 or above consents for themselves. For a child under 18, and for a mentally incapacitated person, both the recipient and the parent or guardian insert their Hong Kong Identity Cards.
• Electronic consent through the eHealth App — a person aged 18 or above consents for themselves, and for a child under 18 the parent or guardian gives consent in the app. This method is not available for mentally incapacitated persons.
• Paper consent form — available to every recipient. A person aged 18 or above signs for themselves; for a child under 18, and for a mentally incapacitated person, the parent or guardian signs.

The vaccine recipients and the parents/ guardians will receive a pop-up message from the eHealth App, which displays “Consent form for this year has been submitted”. After logging into the app, the vaccine recipients and or their parents or guardians can find the message by tapping the bell icon in the top-right corner to enter the “Information Center”, and then selecting “eHealth Notification”.

Submitting eConsent using eHealth App for the Vaccination Subsidy Scheme is not applicable to MIPs aged 18 or older. Instead, consent shall be given either by inserting both the MIP's and guardian's Hong Kong Identity Cards into a card reader or by submitting a paper consent form.