Everything To Know About the Influenza Vaccine & Its Importance 

Introduction

Seasonal influenza, commonly known as the flu, is a highly contagious respiratory illness that affects millions of people worldwide. According to the World Health Organization (WHO), it causes about one billion infections worldwide (including both symptomatic and asymptomatic), resulting in 3 to 5 million severe cases and nearly 290,000 to 650,000 deaths related to respiratory complications each year¹.  

While many individuals recover without complications, influenza can lead to serious health outcomes, especially among those with existing medical conditions or weakened immune systems. In severe cases, it may even result in pneumonia, sepsis, or a worsening of chronic diseases¹. 

Flu vaccinations that are both safe and effective have been used for more than 60 years. They continue to be the best defence against influenza and its consequences². This article explores everything you need to know about the influenza virus vaccine, from how it works to who should get it. 

What is the Influenza Vaccine?

Wondering what is influenza vaccine used for?  

The influenza vaccine, commonly known as the flu shot, is a preventive measure designed to protect against infection from influenza viruses. It works by stimulating the body’s immune system, particularly the antibody-mediated response, and in some cases, cell-mediated immunity to produce antibodies that target the hemagglutinin (HA) and neuraminidase (NA) proteins on the virus surface, without causing an illness. This helps the body recognise and fight off the virus if exposed later³. 

Influenza vaccines are available in various forms, including inactivated influenza vaccine, nasal spray, as well as recombinant options⁴. These types will be discussed in detail in the following section. 

Types of Influenza Vaccines

Influenza vaccines for adults and children are available in different forms. The main types include: 

1. Inactivated Influenza Vaccine (IIV)

This is the most common form of the flu vaccine, which is grown inside an embryonated egg (typically a chicken or hen’s egg)⁴. 

Inactivated influenza vaccine also includes other forms such as⁴:  

• High-dose vaccines (for those 65 years and older, containing 4 times more antigen to trigger a stronger immune response) 
• Adjuvanted vaccines (contain an ingredient to boost immune response; also, for older adults) 
• Cell-based vaccines (grown in mammalian cells instead of eggs, reducing egg exposure; however they may still use egg-derived reference strains, so may not be entirely egg-free in all cases) 

2. Recombinant Influenza Vaccine (RIV)

These vaccines are produced with recombinant DNA technology. While they do utilise parts of the influenza virus genome, they are not made using whole influenza viruses and do not involve egg-based production³. 

3. Live Attenuated Influenza Vaccine (LAIV) (Nasal Spray)

This type of vaccine contains weakened live viruses and is sprayed into the nose⁴. 

Note: Remember that accurate dosing depends on the influenza vaccine dosage chart guidelines. Always consult with a doctor to determine the best flu vaccine option for you. 

Why is the Influenza Vaccine Important?

The influenza virus vaccine plays a vital role in protecting individual and public health. Here’s why the flu vaccination is so important: 

• Reduces Hospitalisations and Healthcare Burden: Flu vaccination significantly lowers the risk of flu-related hospital admissions, easing the pressure on healthcare systems, especially during peak flu seasons⁵. 
• Prevents Severe Complications: Influenza can lead to serious complications, including pneumonia and the exacerbation of chronic diseases such as asthma or diabetes. The vaccine helps reduce the severity and duration of illness in those who do get infected⁵. 
• Protects Vulnerable Populations: Vaccination helps protect individuals who are at higher risk of complications⁶. Additionally, public health authorities worldwide advocate for the influenza vaccine in pregnancy to protect maternal and infant health, as they come under high-risk groups. 
• Minimises Community Spread: By reducing the number of infections, flu vaccination also decreases transmission in the community, helping to avoid outbreaks³. 

Who Should Get Vaccinated?

The Advisory Committee on Immunization Practices (ACIP) recommends that everyone aged 6 months and older receive a flu vaccine each year unless they have specific medical reasons that contradict it³.  

Vaccination is especially important for people at higher risk of serious complications from influenza and those who may spread the virus to vulnerable individuals. These include¹: 

• Older Adults (65 years and older) 
• Pregnant Women (can safely receive the IIV at any stage of pregnancy) 

Note: Vaccination during pregnancy also helps protect the newborn for the first few months after birth. 

• Children (Aged 6 months to 5 years) 

Note: Vaccination is recommended for all eligible children, not limited to age 5. 

• People with Chronic Health Conditions (such as diabetes) 
• People with Weakened Immune Systems (including those with cancer) 

Note: The LAIV is not recommended for immunocompromised individuals. 

• Healthcare Personnel 

When and How Often to Get the Flu (Influenza) Shot

For most people, the influenza vaccine schedule lies between September and October as they are generally considered the ideal months to get vaccinated. The main goal is for everyone to be immunised by the end of October before flu activity begins to rise significantly⁷.  

It takes about two weeks after vaccination for the body to develop protective antibodies, which is why getting vaccinated before the peak flu season is strongly recommended. 

Note: The influenza virus mutates frequently, resulting in changes to the circulating strains each year. Therefore, the influenza vaccine composition is updated annually to match the most common circulating flu strains^1,3. 

How the Vaccine is Given

The influenza vaccine is administered using two primary methods, depending on the type of vaccine and the recipient’s age and health status: 

• Injection: The most common way to receive the flu vaccine is through an intramuscular injection, typically into the upper arm for adults and older children. For infants and toddlers, the injection is usually given in the outer front part of the upper thigh^3,8. 

Note: Administering the influenza vaccine in pregnancy is considered safe and effective at any stage of gestation. Children may need a different influenza vaccine dose than adults. Therefore, always verify the correct influenza vaccine dose before vaccination. 

• Nasal Spray: An alternative to the injection is a nasal spray vaccine, available for healthy and nonpregnant individuals. It is administered using a single-use intranasal sprayer into each nostril while the recipient is in an upright position³. 

Note: Nasal spray should only be administered in healthy individuals from age 2 to 49 years. It should be avoided in pregnancy, people with weak immune systems, children with asthma (2 to 4 years), and in people with long term medical conditions³. 

What to Expect (in both cases)⁹: 

• A doctor will ask a few questions about your health history. 
• While there is no universal influenza vaccine dosage chart, doctors will usually follow established national immunisation guidelines to determine the appropriate dose based on factors like age, health status, vaccine type, and prior vaccination history. 
• You may be seated or asked to lie down during the injection or nasal spray administration. 
• You’ll be monitored briefly (usually for 15 minutes) for any immediate adverse reactions (which are rare). 

Safety and Possible Influenza Vaccine Side Effects

Influenza vaccine side effects are generally well-tolerated, and these may include³: 

• Soreness, redness, or swelling at the injection site 
• Low-grade fever 
• Fatigue or drowsiness 
• Irritability  
• Muscle aches (myalgia) 

Can You Catch the Flu Even After the Shot?

Yes, it is still possible to get the flu even after being vaccinated with the influenza virus vaccine. Here’s why: 

• Flu viruses change often (a process called antigenic drift), which means the vaccine may not always perfectly match the flu viruses spreading that year¹⁰. However, the vaccine can still offer some protection due to similarities between strains. 
• You might also get infected if you’re exposed to the virus before your body builds full immunity, which takes about 2 weeks after getting the shot³. 

Note: Even if the vaccine doesn’t prevent the flu entirely, it can still help your body fight it off faster and reduce the severity of the illness¹⁰. 

When to See a Doctor

Most influenza vaccine side effects (those mentioned above) are mild and short-lived. However, in rare cases, more serious reactions can occur.  

Seek medical attention immediately if you experience any of the following after getting the flu vaccine^3,10: 

• High fever  
• Hives or rash 
• Muscle weakness 
• Difficulty breathing or swelling of lips/face (anaphylaxis) 

These symptoms could indicate a serious allergic reaction or another medical concern that needs prompt evaluation. If you’re unsure whether a symptom is related to the vaccine, it’s always better to consult a doctor and get it checked. 

Also Read: Influenza A: What is It, Causes, Symptoms, Treatment & Prevention

Conclusion

Understanding what the influenza vaccine is helps to recognise why it’s important. It’s an annual shot designed to protect against the most common flu viruses circulating each year. Getting vaccinated every year helps lower your risk of infection and severe illness, and also contributes to reducing transmission, especially among vulnerable populations. While the influenza vaccine doesn’t completely stop the spread of the virus due to ongoing changes in flu strains (antigenic drift) and varying effectiveness, it significantly reduces the risk of serious complications and hospitalisations. Therefore, following the appropriate influenza vaccine schedule (getting vaccinated annually) ensures timely protection, especially for high-risk groups.

Frequently Asked Questions (FAQs)

References

1. World Health Organization. Influenza (seasonal) [Internet]. World Health Organization; [cited 2025 May 28]. Available from: https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal) 
2. World Health Organization. Influenza (seasonal) [Internet]. World Health Organization; [cited 2025 May 28]. Available from: https://www.emro.who.int/health-topics/influenza/influenza-vaccines.html 
3. Kalarikkal SM, Jaishankar GB. Influenza Vaccine [Internet]. StatPearls Publishing. 2025 Jan; [cited 2025 May 28]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537197/ 
4. Centers for Disease Control and Prevention. Different Types of Flu Vaccines [Internet]. CDC; [Cited Jun 03]. Available from: https://www.cdc.gov/flu/vaccine-types/index.html 
5. Trombetta CM, Kistner O, Montomoli E, Viviani S, Marchi S. Influenza Viruses and Vaccines: The Role of Vaccine Effectiveness Studies for Evaluation of the Benefits of Influenza Vaccines. Vaccines (Basel). 2022 May 1;10(5):714. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC9143275/ 
6. Rasmussen AL. Vaccination Is the Only Acceptable Path to Herd Immunity. Med. 2020 Dec 18;1(1):21-23. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC7831753/ 
7. Centers for Disease Control and Prevention. Who Needs a Flu Vaccine [Internet]. CDC; [Cited Jun 03]. Available from: https://www.cdc.gov/flu/vaccines/vaccinations.html 
8. Centers for Disease Control and Prevention. Seasonal Influenza Vaccine Dosage & Administration [Internet]. CDC; [Cited Jun 03]. Available from: https://www.cdc.gov/flu/hcp/vax-summary/vaccine-dosage-admin.html 
9. Centers for Disease Control and Prevention. Administer the Vaccine(s) [Internet]. CDC; [Cited Jun 03]. Available from: https://www.cdc.gov/vaccines/hcp/admin/administer-vaccines.html 
10. World Health Organization. 5 myths about the flu vaccine [Internet]. World Health Organization; [cited 2025 May 28]. Available from: https://www.who.int/news-room/spotlight/influenza-are-we-ready/5-myths-about-the-flu-vaccine 

Disclaimer: The information provided here is for educational/awareness purposes only and is not intended to be a substitute for medical treatment by a healthcare professional and should not be relied upon to diagnose or treat any medical condition. The reader should consult a registered medical practitioner to determine the appropriateness of the information and before consuming any medication. PharmEasy does not provide any guarantee or warranty (express or implied) regarding the accuracy, adequacy, completeness, legality, reliability or usefulness of the information; and disclaims any liability arising thereof.