Influenza, commonly referred to as the flu, is a contagious viral infection that can cause significant illness, especially in vulnerable populations such as the elderly, young children, and individuals with chronic diseases. In Singapore, annual flu vaccinations are widely encouraged as part of preventive healthcare. Yet, many people find themselves asking: “Why did I still get the flu even after taking the flu vaccine?”
This question is both valid and common. While the flu vaccine is a critical tool in reducing the risk and severity of infection, it does not offer absolute protection. Understanding how the flu vaccine works, and why it may not always prevent infection, can help manage expectations and reinforce the importance of yearly vaccination.
Let’s explore the reasons behind post-vaccination influenza cases and clarify how the flu vaccine still plays a crucial role in protecting public health in Singapore.
Understanding How the Flu Vaccine Works
The flu vaccine stimulates your immune system to produce antibodies against the most likely influenza strains for that year. Because influenza viruses constantly mutate, the vaccine is updated annually based on predictions by global health authorities, including the World Health Organization (WHO).
Most flu vaccines in Singapore protect against four strains of the influenza virus, two A strains and two B strains. These quadrivalent vaccines are developed several months in advance of flu season and are based on circulating strain data from the opposite hemisphere.
When administered, the flu vaccine takes about two weeks to become effective, giving your body time to build the necessary antibodies.
Why You Might Still Get the Flu After Vaccination
While the flu vaccine significantly reduces the risk of illness, it’s not foolproof. Here are several reasons why you might still contract influenza even after being vaccinated:
1. Weakened Immune Response
Certain groups, such as older adults, people with weakened immune systems, or those with chronic illnesses, may not develop as strong an immune response to the flu shot. While the vaccine still offers protection, it may not be enough to completely prevent infection. However, even partial immunity can make a significant difference in reducing the severity and complications of illness.
2. Infection by a Non-Influenza Virus
Flu-like symptoms can be caused by other respiratory viruses, such as rhinoviruses, adenoviruses, or respiratory syncytial virus (RSV). These viruses are not covered by the flu shot, so it’s possible to feel like you “have the flu” when in fact it’s a different illness altogether. A proper diagnosis, often through a swab test, can distinguish influenza from other viral infections.
3. Exposure Before Immunity Develops
If you’re exposed to the virus shortly before or within two weeks of receiving the influenza vaccine, your immune system may not have had enough time to produce protective antibodies, resulting in an infection. This is why timing matters. The flu vaccine is most effective when taken ahead of the flu season, typically between April and August in Singapore.
4. Vaccine-Strain Mismatch
Every year, scientists make educated predictions about which influenza strains will be most prevalent. In some years, the circulating strains differ slightly or significantly from the strains included in the vaccine.
This is called a “vaccine-strain mismatch.” In such cases, the vaccine may offer reduced protection or only partial immunity, which means you can still get infected, but your symptoms may be milder.
What the Flu Vaccine Still Does Right
Even if you get the flu after being vaccinated, the flu vaccine still offers several key benefits:
1. Reduces Severity of Illness
Vaccinated individuals who contract the flu tend to have milder symptoms, lower risk of complications like pneumonia, and reduced need for hospitalisation. This can be life-saving for high-risk individuals.
2. Lowers the Risk of Spreading the Virus
Vaccination contributes to community (herd) immunity, protecting those who cannot be vaccinated, such as infants or immunocompromised individuals, by reducing the overall spread of the virus.
3. Prevents Severe Complications
Studies have shown that flu vaccines reduce the risk of serious flu-related complications, including heart attacks, strokes, and worsening of chronic conditions such as asthma or diabetes.
At any reputable health screening clinic in Singapore, the flu vaccine is offered as part of comprehensive preventive care, especially for older adults and individuals with underlying health conditions.
Common Misconceptions About the Flu Vaccine
Let’s address a few common myths that may cause confusion:
“The flu vaccine gave me the flu.”
This is not possible. The flu vaccine does not contain live virus. Mild side effects such as low-grade fever or muscle aches are signs that your immune system is responding—not that you’re infected.
“If I still get sick, the vaccine doesn’t work.”
As explained earlier, no vaccine offers 100% protection. Getting sick after vaccination doesn’t mean the vaccine failed, it likely prevented a more severe case.
“Healthy people don’t need the flu vaccine.”
Even healthy individuals can catch the flu and transmit it to others. Flu-related complications can affect anyone, and vaccines help protect not just yourself, but your community.
Should You Still Get the Flu Vaccine?
Yes, absolutely. Despite the possibility of post-vaccination infection, the benefits of getting the flu vaccine far outweigh the risks. In Singapore, where population density and public transport usage increase the chances of virus transmission, vaccination plays a vital role in public health. MOH and healthcare providers recommend annual flu vaccination, particularly for:
- Adults over 65
- Children aged 6 months to 5 years
- Pregnant women
- Individuals with chronic illnesses (e.g., heart disease, asthma, diabetes)
- Healthcare workers
- Travellers to regions with high influenza activity
At a well-equipped health screening clinic, the flu vaccine is often bundled with other essential immunisations and health checks to support holistic care.
How to Maximise Your Protection
Even with the flu vaccine, you should continue practicing good hygiene to reduce your risk of infection:
- Wash your hands frequently with soap
- Avoid touching your face
- Cover your mouth when coughing or sneezing
- Stay home if you’re unwell
- Avoid close contact with sick individuals
If you suspect you’ve contracted the flu despite being vaccinated, visit a GP early. Antiviral medications can reduce the severity and duration of symptoms if taken within the first 48 hours.
Conclusion
It can be frustrating to fall ill after doing your part and getting vaccinated. However, it’s important to understand that the flu vaccine is not a guaranteed shield, it’s a scientific tool that significantly lowers your risk of infection, complications, and hospitalisation.
In Singapore’s humid and densely populated environment, the flu circulates year-round, with peaks during certain seasons. Staying vaccinated and vigilant is the best approach to keeping yourself and your loved ones safe.
Stay Protected Year-Round
At Keystone Clinic & Surgery, we provide professional, evidence-based preventive care tailored to your needs. Our team of experienced doctors offers annual flu vaccinations as part of our commitment to helping you stay ahead of illness, not just treat it. Whether you’re due for your shot or need advice after catching the flu post-vaccination, our doctors are here to support you with clear guidance and compassionate care.
References
Cdc.gov. Key Facts About Seasonal Flu Vaccine. https://www.cdc.gov/flu/vaccines/keyfacts.html
Medstarhealth.org. Why You Can Still Get the Flu–Even If You Got a Flu Shot. https://www.medstarhealth.org/blog/why-you-can-still-get-the-flu-even-if-you-got-a-flu-shot
Bangkokhospital.com. Why Do You Still Get the Flu After Getting Vaccinated?. https://www.bangkokhospital.com/en/bangkok/content/why-do-we-need-influenza-vaccine