Influenza, commonly known as the flu, is a respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. While often confused with a common cold, the flu can lead to more serious complications, including hospitalization and even death, particularly for vulnerable populations. Annual vaccination remains a primary public health measure to prevent influenza and its associated severe outcomes.
What Makes it Quadrivalent
A quadrivalent influenza vaccine is designed to protect against four different influenza virus strains. This differs from older trivalent vaccines, which target only three strains. Specifically, quadrivalent vaccines typically include components from two influenza A viruses—A(H1N1) and A(H3N2)—and two influenza B viruses, one from each of the two main B lineages (Victoria and Yamagata). The inclusion of a second B strain aims to provide broader protection against circulating flu viruses, as both B lineages have historically co-circulated worldwide.
Each year, health organizations like the World Health Organization (WHO) predict which strains are most likely to circulate, guiding vaccine strain selection. This annual adjustment is necessary because influenza viruses constantly evolve through a process called antigenic drift, which can alter their surface proteins, making previous immunity less effective. While quadrivalent vaccines have been the standard, for the 2024-2025 flu season, some regions are transitioning back to trivalent vaccines due to the sustained absence of the B/Yamagata lineage virus.
How the Vaccine Protects
The quadrivalent influenza vaccine introduces inactivated or weakened viral components to the immune system. These components cannot cause the flu illness itself. Upon vaccination, the immune system recognizes these viral components, known as antigens, as foreign. This recognition prompts the immune system to produce specific antibodies and immune cells.
These antibodies are proteins designed to target and neutralize the four specific influenza strains included in the vaccine. If a vaccinated person later encounters the actual influenza virus, these pre-existing antibodies are ready to defend the body, preventing infection or significantly reducing the severity of illness. The process of building this immune response takes about two weeks after vaccination.
Vaccination Recommendations and Timing
Annual influenza vaccination is recommended for nearly all individuals six months of age and older without specific contraindications. This helps prevent virus spread, especially to vulnerable populations. Certain groups are particularly encouraged to receive the vaccine due to a higher risk of developing serious complications from the flu. These include young children, older adults (especially those 65 years and above), pregnant individuals, and people with chronic health conditions.
For most people, the ideal time to get vaccinated is during September or October, before flu activity typically peaks in the winter months. However, vaccination can still be beneficial throughout the flu season as long as influenza viruses are circulating. Children aged six months through eight years may require two doses of the vaccine, administered at least four weeks apart, if they have not previously received at least two doses of flu vaccine.
Safety Profile and Common Misconceptions
The quadrivalent influenza vaccine has a well-established safety record, with most side effects mild and temporary. Common reactions include soreness, redness, or swelling at the injection site. Some individuals might experience low-grade fever, headache, or muscle aches, which typically resolve within one to two days. These reactions indicate that the immune system is actively building protection, rather than signifying an illness.
A common misconception is that the flu shot can cause the flu. This is incorrect because injectable vaccines contain inactivated viruses or specific viral components that cannot cause infection. Another common misunderstanding is that annual vaccination is unnecessary if one received a shot in the previous year.
Influenza viruses constantly change, and the protection from the vaccine can decline over time, necessitating yearly vaccination to ensure protection against the most current circulating strains. Furthermore, some people believe that healthy individuals do not need the flu vaccine, but anyone can contract and spread the flu, even without showing symptoms.