Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. These viruses circulate globally each year, causing seasonal outbreaks. Vaccination plays a central role in public health efforts to reduce the impact of influenza, aiming to protect individuals and communities from severe illness.
How Flu Vaccines Work
Flu vaccines prepare the body’s immune system to recognize and fight off influenza viruses. They typically contain inactivated or weakened forms of the influenza virus, or specific proteins from the virus. Once administered, the vaccine introduces these components, prompting the immune system to produce antibodies.
These antibodies bind to the influenza virus, neutralizing it and preventing infection if the body is exposed to similar viral strains in the future. This trains the immune system without causing illness. After vaccination, it generally takes about two weeks for the body to develop protective immunity.
Why Vaccination Doesn’t Guarantee Immunity
Even with vaccination, it is possible to contract influenza due to several factors. Influenza viruses are constantly changing through antigenic drift, where small mutations accumulate in their surface proteins. This means the vaccine formulation, which is developed annually based on predictions of circulating strains, may not perfectly match the viruses that spread during a flu season. An antigenic mismatch can reduce the vaccine’s effectiveness.
An individual’s immune response to the vaccine can also vary. Factors such as age, pre-existing immunity from past infections or vaccinations, genetic background, and underlying health conditions can influence how strongly a person responds to the vaccine. Older adults or those with compromised immune systems may not mount as robust an antibody response. If exposure to the influenza virus occurs shortly before or within the two-week period it takes for immunity to develop after vaccination, a person might still become ill. While flu vaccines target specific strains, other influenza strains not included in the vaccine can also circulate and cause illness.
Benefits Even When Illness Occurs
If a vaccinated individual contracts influenza, the illness is typically milder, and symptoms are less severe. Vaccination still offers significant benefits. The immune system, primed by the vaccine, responds quicker and more effectively to the virus. Vaccination reduces the risk of serious complications like pneumonia, hospitalization, and death. Vaccinated adults hospitalized with flu had a lower risk of intensive care unit admission and death compared to unvaccinated individuals.
Beyond personal protection, widespread vaccination contributes to community immunity, sometimes referred to as “herd immunity.” When many people are vaccinated, the virus spreads less easily, protecting vulnerable individuals like infants or those with certain medical conditions who cannot be vaccinated.
Differentiating Flu from Other Viral Illnesses
Many other respiratory viruses cause symptoms similar to influenza, leading people to mistakenly believe they got the flu despite vaccination. Viruses such as rhinoviruses (which cause the common cold), respiratory syncytial virus (RSV), adenoviruses, and coronaviruses can all lead to fever, cough, and body aches. While symptoms can overlap, the flu typically presents with more sudden, intense symptoms, including high fever, chills, significant body aches, and fatigue, compared to a common cold.
It can be challenging to distinguish between these illnesses based on symptoms alone. A person might experience a different viral infection after their flu shot and mistakenly attribute it to vaccine failure. Laboratory testing is often necessary for a definitive diagnosis to identify the virus causing the illness.