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FAQs by Doctors: Seasonal Influenza Vaccination for the 2021-22 Season in Hong Kong

FAQs by Doctors: Seasonal Influenza Vaccination for the 2021-22 Season in Hong Kong
2021-09-14
  1. What is the recommended composition of the 2021-22 seasonal influenza vaccine?
  2. How many shots of seasonal influenza vaccine will subsidised persons need?
  3. If a child under the age of 9 years is getting seasonal influenza vaccination for the first time and requires 2 doses, does the same type of vaccine have to be used for both doses?
  4. How to ensure accurate administration of correct dosage for children?
  5. When is the optimal time to receive influenza vaccination?
  6. What types of seasonal influenza vaccines are recommended for use in Hong Kong by Scientific Committee on Vaccine Preventable Diseases (SCVPD)?
  7. What kinds of seasonal influenza vaccines are covered in VSS?
  8. Who should not receive inactivated seasonal influenza vaccination?
  9. Is inactivated influenza vaccine safe? What are the possible side effects following the inactivated seasonal influenza vaccine administration?
  10. Who should not receive live attenuated influenza vaccine (LAIV)?
  11. What are the possible side effects following live attenuated influenza vaccine (LAIV) administration?
  12. Who should not receive recombinant influenza vaccine (RIV)?
  13. What are the possible side effects following recombinant influenza vaccine (RIV) administration?
  14. Does seasonal influenza vaccine cause influenza?
  15. Why do some people who have been vaccinated with seasonal influenza vaccine (SIV) still get influenza / influenza-like illness?
  16. Can COVID-19 vaccine be given together with seasonal influenza vaccine?

1. What is the recommended composition of the 2021-22 seasonal influenza vaccine?

The egg-based quadrivalent influenza vaccines recommended by the Scientific Committee on Vaccine Preventable Diseases (SCVPD) for the 2021-22 season contain the following:

  • an A/Victoria/2570/2019 (H1N1)pdm09-like virus
  • an A/Cambodia/e0826360/2020 (H3N2)-like virus
  • a B/ Washington/02/2019-like virus
  • a B/Phuket/3073/2013-like virus

The recombinant-based quadrivalent influenza vaccines recommended by the SCVPD for the 2021-22 season contain the following:

  • an A/Wisconsin/588/2019 (H1N1)pdm09-like virus
  • an A/Cambodia/e0826360/2020 (H3N2)-like virus
  • a B/ Washington/02/2019-like virus
  • a B/Phuket/3073/2013-like virus

If trivalent influenza vaccine is being used, the influenza B component shall contain a B/Washington/02/2019-like virus.

2. How many shots of seasonal influenza vaccine will subsidised persons need?

Persons aged 9 years or above are recommended to receive one dose of 2021-22 influenza vaccine. To ensure adequate immunity against seasonal influenza, children under 9 years old who have never received any seasonal influenza vaccine are recommended to receive 2 doses of seasonal influenza vaccine with a minimum interval of 4 weeks in the 2021-22 season. Children below 9 years of age who have received at least one dose of seasonal influenza vaccine in the 2020-21 season or before are recommended to receive one dose in the 2021-22 season.

3. If a child under the age of 9 years is getting seasonal influenza vaccination for the first time and requires 2 doses, does the same type of vaccine have to be used for both doses?

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.

4. How to ensure accurate administration of correct dosage for children?

The doctors should refer to the package inserts of individual products for the recommended age range of recipients and dosage.

5. When is the optimal time to receive influenza vaccination?

As it usually takes about two weeks after vaccination for antibodies to develop in the body and provide protection against seasonal influenza virus, eligible persons are advised to visit participating private medical doctors' clinics for vaccination as early as possible to protect from the possible winter influenza season.

6. What types of seasonal influenza vaccines are recommended for use in Hong Kong by Scientific Committee on Vaccine Preventable Diseases (SCVPD)?

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. The package inserts for individual products should always be referred to when deciding which vaccine to give.

7. What kinds of seasonal influenza vaccines are covered in VSS?

The VSS will cover the types of seasonal influenza vaccines recommended by the Scientific Committee on Vaccine Preventable Diseases (SCVPD) that are registered and supplied for use in Hong Kong. For the 2021-22 season, all available SIV in Hong Kong are quadrivalent SIV.

8. Who should not receive inactivated seasonal influenza vaccination?

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 inactivated influenza vaccines (IIVs) in primary care setting, such as health centres or clinics. 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, the vaccination should be deferred till recovery.

(Note: All doctors are advised to read carefully the product information of the vaccine they have procured, noting especially the contraindications, route of administration, suitable age group, dosage for children and expiry date.)

9. Is inactivated influenza vaccine safe? What are the possible side effects following inactivated seasonal influenza vaccine administration??

Inactivated influenza vaccine (IIVs) 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 (about 1 to 2 case 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 Guillain-Barré Syndrome 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)

10. Who should not receive live attenuated influenza vaccine (LAIV)?

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, such as health centres or clinics. 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.

Service providers should always refer to the package inserts of individual products for detailed information.

11. What are the possible side effects following live attenuated influenza vaccine (LAIV) administration?

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 increased risk of wheezing following administration.

12. Who should not receive recombinant influenza vaccine (RIV)?

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.

13. What are the possible side effects following recombinant influenza vaccine (RIV) administration?

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.

14. Does seasonal influenza vaccine cause influenza?

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.

15. Why do some people who have been vaccinated with seasonal influenza vaccine (SIV) still get influenza / influenza-like illness?

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.

16. Can COVID-19 vaccine be given together with seasonal influenza vaccine?

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.