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Influenza Vaccination

Why is seasonal influenza vaccination important?

Influenza occurs in Hong Kong throughout the year but is usually more common in periods from January to March/April and July to August. 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. 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.

When to get vaccinated?

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.

What are the possible side effects of the inactivated influenza vaccine administration?

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]

Who should not receive inactivated influenza vaccine?

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 seasonal influenza vaccination 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. 

Should I need 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 the next season. In addition, the vaccine compositions of the 2021-22 seasonal influenza vaccine are different from those in 2020-21 season.

Can pneumococcal vaccine be given 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.

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

There is currently limited information on the safety and effectiveness of using COVID-19 vaccine with influenza vaccine at the same time. It is suggested to have an interval of at least 14 days between administration of seasonal influenza vaccine and COVID-19 vaccines (BNT162b2 or Coronavac).