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, the two subtypes of influenza A virus, H1N1 and H3N2, 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 March/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 is one of the effective means to prevent seasonal influenza and its complications, as well as reduce influenza related hospitalisation and death. A person getting influenza and COVID-19 at the same time may be more seriously ill and has higher death rate. Influenza vaccination may reduce hospitalisation and length of stay. Therefore, it is important to receive both seasonal influenza vaccination and COVID-19 vaccination. For the arrangement of seasonal influenza vaccination and COVID-19 vaccination, please consult doctor.
B. People who should receive seasonal influenza vaccination
Given that influenza vaccines are safe and effective and severe cases can occur even in healthy persons, all members of the public aged 6 months or above, except those with known contraindications, should receive seasonal influenza vaccination for personal protection.
People who are in the priority groups are generally at increased risk of severe influenza or transmitting influenza to those at high risk. Therefore, they should have higher priority for seasonal influenza vaccination.
Influenza transmission may have been altered by the range of non-pharmaceutical interventions (NPIs) in place for COVID-19 or travel restrictions or border closures. Influenza transmission could increase when the NPIs and travel restrictions are lifted, leading to potential co-circulation of influenza and COVID-19. WHO has reconsidered the prioritization of risk groups of influenza vaccination during COVID-19 pandemic. They have prioritised health workers and older adults (above age of 65) as the highest priority risk group during the COVID-19 pandemic.
In the 2023/24 season, the Scientific Committee on Vaccine Preventable Diseases (SCVPD) recommends the following priority groups for seasonal influenza vaccination in Hong Kong:
- Healthcare workers (including care workers in Residential Care Homes for the Elderly [RCHE] and Residential Care Homes for Persons with Disabilities [RCHD])
- Persons aged 50 years or above
- Pregnant women
- Residents of Residential Care Homes (such as RCHE or RCHD)
- Persons with chronic medical problems*
- Children and adolescents aged six months to under 18 years (or aged 18 years or above studying in secondary schools in Hong Kong)
- Poultry workers
- Pig farmers and pig-slaughtering industry personnel
* 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 [BMI] 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 those who lack the ability to care for themselves.
# Obesity is considered as an independent risk factor for influenza complication and thus people with BMI 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).
Members of the public can consult their family doctors to receive seasonal influenza vaccination for personal protection.
According to the World Health Organization, when the vaccine strains closely match the circulating influenza viruses, efficacy of influenza vaccine in individuals younger than 65 years of age typically range from 70% to 90%. 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 for personal protection. Members of the public can consult their family doctors to receive seasonal influenza vaccination for personal protection.
Influenza vaccination in pregnant women has shown benefits for both pregnant woman and child in terms of reduced acute respiratory infections. The World Health Organization considers inactivated influenza vaccine (IIV) to be safe in pregnancy and there is no evidence showing that IIV can cause abnormality in foetus even if given during the first trimester. However, pregnant women should NOT receive live attenuated influenza vaccine because it contains live viruses. Recombinant influenza vaccine is not contraindicated in pregnancy. Pregnant women should consult a doctor for any queries.
Local and overseas evidence suggested that obesity is an independent risk factor for severe influenza A(H1N1)pdm09 infection including intensive care unit admission or death. As the influenza A(H1N1)pdm09 virus is a common type of seasonal influenza virus, obese individual with BMI 30 or above is regarded as having a high-risk condition and is therefore recommended for influenza vaccination.
Persons aged 50 to 64 should receive seasonal influenza vaccination because local influenza epidemiology showed that people aged 50 to 64 years, irrespective of high-risk conditions, were having a higher risk of intensive care unit admission and death associated with influenza A(H1N1)pdm09 infection. Influenza A(H1N1)pdm09 is a common type of seasonal influenza virus.
The SCVPD recommends seasonal influenza vaccination for elderly persons aged 65 years or above because of their high risk of complications, excess hospital admissions and deaths from influenza. WHO has recommended older adults as one of the highest priority group to receive influenza vaccine during the COVID-19 pandemic.
The SCVPD recommends HCWs to receive seasonal influenza vaccination to reduce morbidity and hence reduce absenteeism among healthcare workers related to respiratory infections. Seasonal influenza vaccination in HCWs can also reduce the risk of transmitting influenza to patients who are at high risk of complications and mortality from influenza. WHO has recommended healthcare workers as one of the highest priority group to receive influenza vaccine during the COVID-19 pandemic.
Poultry industry workers have higher chance of contacting avian influenza viruses. Seasonal Influenza vaccination can prevent seasonal influenza in humans, and hence minimise the chance of concomitant infections of human seasonal influenza and avian influenza viruses in humans. This in turn minimises the risk of genetic re-assortment and eventual emergence of a novel influenza A virus.
Researchers previously detected influenza A(H1N1)pdm09 (formerly known as Human Swine Influenza) viruses in pigs sampled in Hong Kong. Local experts considered that the influenza A(H1N1)pdm09 virus detected in pigs was transmitted from humans to pigs. Swine influenza viruses circulating in pigs might infect humans in rare occasions. Seasonal Influenza vaccination of the pig-related industry workers can prevent seasonal influenza among them, and hence minimise the chance of concomitant infections of human seasonal influenza and swine influenza viruses in either humans or pigs. This in turn minimises the risk of genetic re-assortment and eventual emergence of a novel influenza A virus.
Seasonal influenza vaccination is recommended for children 6 months to 11 years to reduce influenza-related complications such as excess hospitalisations or deaths. Overseas studies have shown that vaccinating young school-aged children may potentially reduce school absenteeism and influenza transmission in the community.
Taking reference from the latest World Health Organisation recommendation, overseas practice and local experience, seasonal influenza vaccination has also been expanded to secondary school students of 2023/24 school year or adolescents 12 to under 18 years of age for the 2023/24 season.
C. Seasonal influenza vaccine
The egg-based quadrivalent influenza vaccines recommended by the Scientific Committee on Vaccine Preventable Diseases (SCVPD) for the 2023/24 season contain the following:
- an A/Victoria/4897/2022 (H1N1)pdm09-like virus
- an A/Darwin/9/2021(H3N2)-like virus
- a B/Austria/1359417/2021(B/Victoria lineage)-like virus
- a B/Phuket/3073/2013(B/Yamagata lineage)-like virus
The recombinant-based quadrivalent influenza vaccines recommended by the SCVPD for the 2023/24 season contain the following:
- an A/Wisconsin/67/2022 (H1N1)pdm09-like virus
- an A/Darwin/6/2021 (H3N2)-like virus
- a B/Austria/1359417/2021(B/Victoria lineage)-like virus
- a B/Phuket/3073/2013(B/Yamagata lineage)-like virus
If trivalent influenza vaccine is being used, the influenza B component shall contain a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
Inactivated influenza vaccine (IIV), recombinant influenza vaccine (RIV) and live attenuated influenza vaccine (LAIV) are recommended for use in Hong Kong by SCVPD. For IIVs, quadrivalent IIV is preferred to trivalent IIV due to the additional protection against one more lineage of influenza B offered by quadrivalent IIV. Depending on individual brand, most IIVs are recommended for use among people six months of age or older, including those who are healthy, pregnant women and those with chronic medical problems. RIV is recommended for use in individuals of 18 years of age or above. For LAIV, which is a quadrivalent vaccine, it can be used for people 2 to 49 years of age except those who are pregnant, immunocompromised or with other contraindications (please refer to Question 19 for details). The package inserts for individual products should always be referred to when deciding which vaccine to give.
The IIV contains inactivated (killed) viruses. 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. LAIV contains weakened viruses and is given by intranasal spray. IIV, RIV and LAIV are effective, but there are some differences in their indications. Depending on individual product, most IIVs are recommended for use among persons aged 6 months or above, including those who are healthy, pregnant women and those with chronic medical problems. RIV is recommended for use in individuals of 18 years of age or above. LAIV can be used for people 2 to 49 years of age except those who are pregnant, immunocompromised or with other contraindications (please refer to Question 19 for details). Please consult your doctor for details.
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 seasonal influenza vaccination. Individuals with mild egg allergy who are considering an influenza vaccination can be given IIV in primary care setting. 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. 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.
RIV is recommended for use in individuals of 18 years of age or above. People who have a history of hypersensitivity to any vaccine component of the RIV are not suitable to receive RIV. 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 till recover. Please consult your doctor for details. RIV does not require egg in the production process.
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.
# According to Canadian guidelines, intramuscular influenza vaccination is the standard for children living with HIV. LAIV may be considered as an option for children 2 to 17 years of age with stable HIV infection on highly active antiretroviral therapy (HAART) and adequate immune function, and the decision should be made on a case-by-case basis.
Individuals with mild egg allergy who are considering an influenza vaccination can be given LAIV in primary care setting. 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 IIV because of an increased risk of complications associated with influenza. However, children 2 years through 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 (Please refer to Question 19 for details).
Inactivated influenza vaccine is very safe and usually well tolerated apart from occasional soreness, redness or swelling at the injection site. Some recipients may experience fever, muscle pain, and tiredness beginning 6 to 12 hours after vaccination and lasting up to two days. If fever or discomforts persist, please consult a doctor. Immediate severe allergic reactions like hives, swelling of the lips or tongue, and difficulties in breathing are rare and require emergency consultation.
Influenza vaccination may be rarely followed by serious adverse events such as Guillain-Barré Syndrome (GBS) (1 to 2 cases per million vaccinees) and severe allergic reaction (anaphylaxis) (9 per 10 million doses distributed). However, influenza vaccination may not necessarily have causal relations with these adverse events. Studies have shown that the risk of GBS after influenza infection (17.20 per million infected persons) is much higher than after influenza vaccination (1.03 per million vaccine recipients).
(The Lancet Infectious Diseases. 2013 Sep; 13(9): 769-76)
The most common side effects following RIV administration include tenderness and pain over the injection site. Recipients may also experience headache, fatigue, muscle pain and joint pain.
The most common adverse reactions following LAIV administration are nasal congestion or runny nose (in all ages), fever (in children) and sore throat (in adults). The safety in pregnant women has not been established. Children aged below five years with recurrent wheezing / persons of any age with asthma may be at an increased risk of wheezing following administration.
GBS is a rare neurological disorder causing paralysis and even respiratory difficulties. Most people recover completely but some have chronic weakness. GBS can develop following a variety of infections, including influenza. So far, there is no conclusive evidence supporting a causal relationship between seasonal influenza vaccine and GBS.
As it is unknown whether seasonal influenza vaccination is causally associated with increased risk of recurrent GBS, precaution should be made to ascertain the temporal relationship if there is a history of GBS. Please consult your doctor for advice.
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.
No. The influenza vaccines currently supplied for use in Hong Kong do not contain a mercuric compound (e.g. thiomersal) or aluminum.
D. Vaccine administration
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.
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, which usually begin in January in Hong Kong. However, even if seasonal influenza vaccination is not received before the winter influenza season, it can still be received as soon as possible.
According to the World Health Organization, when the vaccine strains closely match the circulating influenza viruses, efficacy of inactivated influenza vaccine (IIV) in individuals younger than 65 years of age typically range from 70% to 90%. For live attenuated influenza vaccine (LAIV), overseas studies and clinical experience had generally indicated LAIV provides comparable protection against influenza to that afforded by IIV.
Yes. The circulating seasonal influenza strains may change from time to time. In accordance with the circulating strains, the seasonal influenza vaccine composition is updated every year to enhance protection. The immunity built up in a vaccinated person in the prior season will decrease over time and may become too low to provide protection in the next season. In addition, the vaccine compositions of the 2023/24 seasonal influenza vaccine are different from those in 2020-21 season.
To ensure adequate immunity against seasonal influenza, children under 9 years of age who have never received any seasonal influenza vaccination before are recommended to receive 2 doses of SIV with a minimum interval of 4 weeks in the 2023/24 season. Children below 9 years of age who have received at least one dose of SIV before are recommended to receive one dose of SIV in the 2023/24 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 4 weeks.
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 is used.
Yes. Seasonal influenza vaccine can be given with MMR vaccine at the same clinic visit. For individuals receiving inactivated influenza vaccine (IIV), or recombinant influenza vaccine (RIV), they may be administered simultaneously or at any interval with MMR vaccine, and those vaccines should be administered with a different syringe and at a different injection site than that of MMR vaccine. For individuals receiving live attenuated influenza vaccine, MMR vaccine not administered on the same day should be administered at least four weeks apart.
Inactivated influenza vaccine (IIV) can be given at the same time or at any time before or after other inactivated vaccine (e.g. Hepatitis B vaccine, but do not include COVID-19 vaccines) or live vaccine (e.g. Measles, mumps and rubella vaccine). Different vaccines should be given at different injection sites if IIV and other vaccines are given at the same time. For individuals receiving live attenuated influenza vaccine, other live vaccines not administered on the same day should be administered at least four weeks 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, SIVs is 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 except those with known contraindications should receive SIV annually because influenza vaccination is one of the effective means in reducing the risk of influenza and its complications together with reduction in influenza-associated hospitalisation and death.
There are currently limited data on the simultaneous administration of COVID-19 vaccines with other vaccines. It is recommended to have a minimal interval of at least 14 days between administration of COVID-19 vaccine (Comirnaty or CoronaVac) and any other vaccine.