Influenza (flu) – immunization

Influenza, often called “the flu”, is a highly contagious viral infection. It spreads through contact with fluids from coughs and sneezes, and each year it causes widespread illness in the community.

Getting vaccinated is the single most effective way to protect yourself, your family, and the people around you. The flu vaccine reduces the number of infections and the risk of severe illness, complications, and deaths.

How the flu spreads and how to prevent it#

The flu virus passes easily from person to person, especially when someone coughs or sneezes nearby. Alongside vaccination, you can help slow the spread by:

• washing your hands well and often
• covering coughs and sneezes
• staying home when you are unwell

Who should be vaccinated#

Influenza immunization is recommended for everyone aged 6 months and over.

Some people are at higher risk of complications from the flu and are often given priority for vaccination. These can include older adults, young children, people who are pregnant, and people with certain underlying medical conditions or reduced immunity. Conditions that can increase the risk of serious illness include:

• heart disease
• chronic respiratory conditions
• conditions that weaken the immune system
• blood (hematological) disorders
• chronic metabolic disorders
• chronic kidney disease
• chronic neurological conditions

Children aged 5 to 10 years who are on long-term aspirin therapy may also be advised to have the vaccine. Speak to your immunization provider to find out whether you or someone in your care is in a priority group.

In many countries, the flu vaccine is provided free of charge to priority groups through national immunization programs. People who are not eligible for a free vaccine can usually buy it from a doctor or a pharmacist immunizer, where a service fee may apply. Some community health services, hospitals, and maternity services also offer the vaccine, and some workplaces run flu vaccination programs for their staff.

When to get vaccinated#

Yearly vaccination before the start of the local flu season is recommended. Vaccines are usually made available shortly before each season begins.

Current evidence suggests the best protection occurs within the first few months after vaccination. However, while the flu virus is still circulating, it is never too late to be vaccinated.

The flu virus is able to change its surface structure from year to year. This means that even if you had the flu or a flu vaccine one year, your immune system may not recognize the changed version of the virus circulating the following year. For this reason, a new seasonal flu vaccine is produced each year.

How the vaccine works#

The seasonal flu vaccine includes protection against four strains of influenza virus.

The vaccine cannot give you the flu because it does not contain live virus. Some people may still catch the flu after vaccination, because the vaccine does not always protect against every strain circulating in the community.

Formulations for different ages#

There are different formulations for people under 65 years and for people aged 65 years and over. Check with your immunization provider that the formulation used is right for your age or for the age of the person in your care.

Children#

Young children, particularly those aged 6 months to under 5 years, are at higher risk of complications from the flu, and even healthy children can become seriously ill. The flu vaccine is recommended for children in this age group and can be given at the same time as routine childhood vaccines.

Children under 9 years who are being vaccinated against the flu for the first time usually need two doses, given about 4 weeks apart, for an adequate immune response.

Pregnancy and breastfeeding#

The flu vaccine is strongly recommended in every pregnancy and is safe to receive at any stage. It can also be given safely while breastfeeding.

It can be given at the same time as other vaccines used in pregnancy, such as the whooping cough (pertussis) vaccine and the COVID-19 vaccine. Vaccinating during pregnancy also protects the baby against the flu for the first 6 months of life, because antibodies pass from the mother to the baby before birth.

People who may need a second dose#

Some people are advised to have a second dose of flu vaccine within the same year. These can include:

• children under 9 years receiving the flu vaccine for the first time, who need two doses about 4 weeks apart
• people who have had a hematopoietic stem cell transplant or a solid organ transplant and are receiving the flu vaccine for the first time after the transplant
• people who were vaccinated before becoming pregnant, where vaccination during pregnancy helps maximize protection for the mother and the baby in the first months of life
• travelers who may benefit from an additional dose suited to the flu season at their destination

Check with your immunization provider to find out whether any of these apply to you.

Having the flu vaccine with other vaccines#

The flu vaccine can be co-administered, meaning it can be given on the same day as other vaccines, one after the other. This includes whooping cough, respiratory syncytial virus (RSV), pneumococcal, and COVID-19 vaccines.

Before vaccination, your immunization provider will go through a pre-immunization checklist with you. Tell them if you or your child:

• are unwell with a fever over 38.5°C (101.3°F)
• have had a serious reaction to any vaccine
• have had a severe allergy to anything
• are under 6 months of age
• have had Guillain-Barré syndrome

Side effects#

Most side effects of the flu vaccine are mild and temporary, usually appearing in the first 2 days after vaccination. In children under 5 years, reactions may be more noticeable. Common side effects include:

• drowsiness or tiredness
• muscle aches
• pain, redness, and swelling at the injection site
• occasionally a small lump (nodule) at the injection site that may last several weeks but needs no treatment
• a low-grade fever

Managing side effects#

Specific treatment is not usually needed. To ease mild side effects you can:

• drink extra fluids
• avoid overdressing if there is a fever

Routine use of paracetamol (acetaminophen) after vaccination is not recommended, but if pain or fever are present it can be given. Check the label for the correct dose, or ask your pharmacist, especially before giving paracetamol to children.

If side effects are unexpected, persistent, or severe, or if you are worried about your own or your child’s condition after vaccination, see your doctor or immunization nurse as soon as possible, or go directly to a hospital.

Rare side effects#

There is a very small risk of a serious allergic reaction (anaphylaxis) to any vaccine. This is why you are advised to stay at the clinic, medical practice, or pharmacy for at least 15 minutes after vaccination, in case further treatment is needed. Other extremely rare side effects include febrile convulsions.

A small increase in Guillain-Barré syndrome was seen following one flu vaccine campaign in 1976. Since then, surveillance has shown that any risk is limited to roughly one case for every million doses, if it occurs at all.

If any reaction is severe and persistent, or if you are worried, contact your doctor for further information. You can also ask your immunization provider how to report any problems.

Key points#

• Yearly flu vaccination is recommended for everyone aged 6 months and over.
• The vaccine reduces the risk of severe illness and flu-related complications.
• The flu spreads by contact with fluids from coughs and sneezes; good hand hygiene, cough etiquette, and staying home when unwell also help.
• The flu virus changes each year, so a new seasonal vaccine is produced and a yearly dose is needed.
• Vaccination is the best way to protect yourself and those around you.

Where to get help and trusted information#

For evidence-based global health guidance, see Source: World Health Organization (WHO).