Dieu Thuan

Many individuals question why they experience flu-like symptoms or even contract the flu after receiving a flu vaccine. This common misconception stems from several factors, including the body's natural immune response to the vaccine and the complexities of flu viruses themselves.

It is important to understand that flu vaccines do not cause the flu. The vaccines typically contain inactivated (dead) virus particles or components, or a weakened live virus (in nasal spray forms), which are insufficient to cause illness. Instead, they stimulate the immune system to produce antibodies, preparing the body to fight off actual flu exposure. This immune response can sometimes lead to mild, temporary side effects, such as a low-grade fever, muscle aches, or fatigue, which are often mistaken for the flu itself. These symptoms are a sign that the body is building protection and usually resolve within one or two days.

Another reason why someone might feel unwell after vaccination is exposure to the flu virus before the vaccine has had time to become fully effective. It takes approximately two weeks for the body to develop a sufficient immune response after vaccination. If an individual is exposed to the flu shortly before or after receiving the shot, they could still contract the illness before their immunity is established.

Furthermore, the flu vaccine is designed to protect against the specific strains of influenza virus that scientists predict will be most prevalent in a given flu season. However, flu viruses are constantly evolving, and new strains can emerge that are not covered by the vaccine. If a vaccinated individual is exposed to a strain not included in the vaccine, they could still contract the flu. Additionally, the vaccine's effectiveness can vary from year to year and among different individuals, typically ranging from 40% to 60% when vaccine viruses are well-matched to circulating viruses. This means that while the vaccine significantly reduces the risk of infection, it does not offer 100% protection.

Finally, many other respiratory viruses circulate alongside the flu, causing symptoms that are very similar to influenza. Conditions like the common cold, respiratory syncytial virus (RSV), or other viral infections can cause fever, cough, and body aches. A vaccinated person experiencing these symptoms might mistakenly attribute them to the flu, even if they have been protected against influenza by the vaccine.

Despite these possibilities, flu vaccination remains the most effective way to prevent influenza and its severe complications. Even if a vaccinated person does contract the flu, the illness is typically milder, and the risk of hospitalization or death is significantly reduced. Public health experts consistently recommend annual flu vaccination for nearly everyone six months of age and older to protect both individuals and the community.