Influenza is an infectious viral disease. 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 and from July to August. The virus mainly spreads by respiratory droplets. The disease is characterised by fever, sore throat, cough, headache, muscle aches, runny nose 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 or even death in the most serious cases. Serious influenza infection can also occur in healthy individuals.
2. What is the difference between seasonal and pandemic influenza?
Seasonal influenza refers to the viruses that circulate in the human population and cause widespread illnesses during each influenza season. Pandemic influenza occurs infrequently, when a new influenza virus emerges which is markedly different from those already circulating in the human population. It causes disease in people and spreads easily between people because they have little or no immunity to it.
3. Why is seasonal influenza vaccination important?
Seasonal influenza vaccination is one of the effective means to prevent seasonal influenza and its complications together with reduction in influenza related hospitalisation and death.
4. Who should receive seasonal influenza vaccination?
Given 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 vaccine for personal protection.
5. Who should have higher priority for seasonal influenza vaccination?
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.
In 2016/17, the Scientific Committee on Vaccine Preventable Diseases recommends the following priority groups for seasonal influenza vaccination in Hong Kong:
*People with chronic medical problems mainly refer to those who have chronic cardiovascular (except hypertension without complication), lung, metabolic or kidney diseases, obesity# (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 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.
Given influenza vaccines offer approximately 70-90% 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 seasonal influenza vaccine for personal protection. Members of the public can consult their family doctors to receive seasonal influenza vaccination for personal protection.
7. Why should pregnant women receive seasonal influenza vaccination?
Influenza vaccination in pregnant women has shown benefits for both mother and child in terms of reduced acute respiratory infections. The World Health Organization considers inactivated seasonal influenza vaccine is safe in pregnancy and there is no evidence showing such vaccine 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 a live virus. Pregnant women should consult obstetrics and gynaecology doctors for any queries.
8. Why should obese persons (BMI = 30 or above) receive 2016/17 seasonal influenza vaccine?
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 expected to continue to circulate as seasonal influenza virus for some years, obese individual with BMI 30 or above is regarded as having a chronic medical problem and is therefore recommended for 2016/17 influenza vaccination.
9. Why should persons aged 50 or above receive 2016/17 seasonal influenza vaccine?
Persons aged 50-64 should receive seasonal influenza vaccination for the 2016/17 influenza season because (i) local influenza epidemiology showed that people aged 50–64 years, irrespective of chronic medical problems, were having a higher risk of intensive care unit admission and death associated with influenza A(H1N1)pdm09 infection, and (ii) the likelihood that influenza A(H1N1)pdm09 strain will continue to circulate in the 2016/17 season.
The Scientific Committee on Vaccine Preventable Diseases recommends seasonal influenza vaccination for elderly persons aged 65 years or above because of their high risk of complications, excess hospital admissions and death from influenza.
10. Why should health care workers receive seasonal influenza vaccination?
The Scientific Committee on Vaccine Preventable Diseases recommends health care workers to receive seasonal influenza vaccination to reduce morbidity and hence reduce absenteeism among health care workers related to respiratory infections. It is also recommended in order to reduce the risk of transmitting influenza to patients who are at high risk of complications and mortality from influenza.
Researchers previously detected influenza A(H1N1)pdm09 (formerly known as Human Swine Influenza) viruses in pigs sampled in Hong Kong. Local experts consider that the virus detected in pigs was transmitted from humans to pigs. Seasonal influenza vaccination of the pig-related industry workers would prevent emergence of new influenza A virus in either human or pig hosts.
The 2015/16 winter season was predominated by influenza A (H1N1)pdm09 and influenza B viruses. In general, the two viruses affect mainly the younger age group. The surveillance data showed that children were more affected in this season, including the facts that influenza-associated hospitalisation rates among children had been staying at a high level, and most of the notified institutional influenza-like illness outbreaks occurred in primary schools. Based on the latest local epidemiology of seasonal influenza, scientific evidence of effectiveness of seasonal influenza vaccine in children and overseas experience, the Scientific Committee on Vaccine Preventable Diseases has recommended expanding the priority group of children from “the age of six months to less than six years” to “the age of six months to 11 years (under 12 years)”.
13. What kinds of seasonal influenza vaccines are registered in Hong Kong?
Only inactivated seasonal influenza vaccine is currently registered in Hong Kong. Both trivalent and quadrivalent inactivated influenza vaccines are recommended for use in Hong Kong.
14. What is the recommended 2016/17 seasonal influenza vaccine composition?
The vaccine recommended by the Scientific Committee on Vaccine Preventable Diseases in 2016/17 contains the following:
If quadrivalent influenza vaccine is being used, it shall contain the above three viruses and a B/Phuket/3073/2013-like virus.
15. Does the 2016/17 seasonal influenza vaccine contain human swine influenza virus?
As recommended by the World Health Organization, the 2016/17 seasonal influenza vaccine contains A/California/7/2009 (H1N1)pdm09-like virus (human swine influenza virus), A/Hong Kong/4801/2014 (H3N2)-like virus and B/Brisbane/60/2008-like virus. If quadrivalent influenza vaccine is being used, it shall contain the above three viruses and a B/Phuket/3073/2013-like virus.
16. Who should not receive inactivated seasonal influenza vaccination?
People who are allergic to a previous dose of inactivated influenza vaccine or other vaccine components 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 vaccine in primary care. Individuals with diagnosed or suspected severe egg allergy should be seen by an allergist/immunologist for evaluation of egg allergy and for administration of inactivated influenza vaccine if clinically indicated. 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.
17. What is Guillain-Barré syndrome? Can influenza vaccination cause Guillain-Barré syndrome?
Guillain-Barré Syndrome (GBS) is a rare neurological disorder causing paralysis and even respiratory difficulties. Most people recover completely but some have chronic weakness. GBS can also develop following a variety of infections, including influenza. So far, no clear association has been found between GBS and seasonal influenza vaccine.
18. Can I receive seasonal influenza vaccine if I had history of Guillain-Barré Syndrome?
As it is unknown whether influenza vaccination is causally associated with increased risk of recurrent Guillain-Barré Syndrome (GBS), precaution should be made to ascertain the temporal relationship if there is a history of GBS.
19. Does seasonal influenza vaccine cause influenza?
The viruses in the inactivated influenza vaccine are dead viruses. Therefore, it cannot cause influenza. However, some people develop side effects after vaccination.
Inactivated seasonal 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 and joint pains, 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 case per million vaccinees), meningitis or encephalopathy (1 in 3 million doses distributed) and severe allergic reaction (anaphylaxis) (9 in 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) is much higher than after influenza vaccination (1.03 per million).
(The Lancet Infectious Diseases. 2013 Sep; 13(9): 769-76)
21. How do seasonal influenza vaccines work?
The seasonal influenza vaccine induces development of antibodies against influenza virus infection in the body.
22. Does the seasonal influenza vaccine work right away?
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.
23. How much protection does the seasonal influenza vaccine provide?
When the vaccine strains closely match the circulating influenza viruses, efficacy of inactivated influenza vaccine in individuals younger than 65 years of age typically range from 70% to 90%. For prevention against influenza, vaccinated individuals should maintain good personal and environmental hygiene practices, balanced diet, regular exercise, adequate rest, and no smoking.
24. Is it necessary to get vaccinated against seasonal influenza every year?
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 next season. In addition, the vaccine compositions of 2016/17 seasonal influenza vaccine are different from those in 2015/16.
25. How many doses of seasonal influenza vaccination will my child need?
To ensure adequate immunity against seasonal influenza, children under 9 years old who have never received any seasonal influenza vaccine are recommended to be given 2 doses of seasonal influenza vaccine with a minimum interval of 4 weeks. Children below 9 years, who have received seasonal influenza vaccine in the 2015/16 season or before are recommended to receive one dose in the 2016/17 season.
No, the first and second doses do not have to match; trivalent or quadrivalent inactivated influenza vaccine can be used for either dose. The doses should be separated by at least 4 weeks.
27. Can pneumococcal vaccines be received together with seasonal influenza vaccine?
Yes. Pneumococcal vaccines 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.
A study has shown that there may be a small increased risk of febrile convulsions following concomitant administration of inactivated influenza vaccine and pneumococcal vaccine in children, but the overall risk remains acceptable. Given the obvious benefit of on-time vaccination with the two vaccines, it is recommended that the current immunisation schedule remains unchanged.
Injectable inactivated influenza vaccine does not interfere with the effectiveness of other vaccines, it can be given at the same time or at different time with either inactivated vaccine (e.g. Hepatitis B vaccine) or live vaccine (e.g. Measles, mumps and rubella vaccine). Different vaccines should be given at different injection sites if inactivated influenza vaccine and other vaccines are given at the same time.
29. My child has asthma. Should he avoid the influenza vaccination in case it gives him an attack?
Children having asthma are not contraindicated to receive inactivated influenza vaccine. People suffering from lung diseases such as asthma are recommended to receive inactivated seasonal influenza vaccine because of an increased risk of complications associated with influenza. However, children having asthma and people suffering lung diseases should not receive live attenuated influenza vaccine.
Both trivalent (IIV3) and quadrivalent (IIV4) inactivated influenza vaccines are recommended for use in Hong Kong. Trivalent influenza vaccine may potentially prevent majority of influenza burden in Hong Kong, while quadrivalent influenza vaccine may potentially offer additional protection against influenza B. Please consult your family doctor.
Both trivalent (IIV3) and quadrivalent (IIV4) inactivated influenza vaccines are recommended for use in Hong Kong. Trivalent influenza vaccine may potentially prevent majority of influenza burden in Hong Kong, while quadrivalent influenza vaccine may potentially offer additional protection against influenza B. Please consult your family doctor.