The inactivated influenza vaccine, widely recognized as the “flu shot,” is a preventative measure designed to protect individuals from influenza viruses. This vaccine contains virus particles that have been inactivated, meaning they are no longer alive and cannot cause influenza illness. It serves as a tool to prepare the body’s defenses against potential future encounters with the actual virus.
How an Inactivated Vaccine Works
Once injected, the vaccine introduces specific viral components, such as hemagglutinin (HA) antigens, into the body. The immune system recognizes these antigens as foreign, prompting various immune cells, including macrophages, T-lymphocytes, and B-lymphocytes, to initiate a response.
B cells are able to process these HA antigens and present them on their surface, while activated helper T cells interact with activated B cells to stimulate antibody production. This process leads to the creation of antibodies specifically designed to target the influenza virus. The body also develops memory B cells, which are prepared to respond quickly and effectively if exposed to the live influenza virus later. Inactivated influenza vaccines are produced using several methods, including traditional egg-based approaches where viruses are grown in fertilized chicken eggs, or through cell-based and recombinant technologies that do not rely on eggs.
Safety and Side Effects
The inactivated influenza vaccine is safe for most individuals. Following vaccination, the body exhibits common, mild reactions that indicate the immune system is building protection. These can include soreness, redness, or swelling at the injection site, typically the muscle in the arm. Some people may also experience low-grade fever, headache, muscle aches, or fatigue for one to two days.
While rare, more significant reactions can occur. A severe allergic reaction, known as anaphylaxis, is estimated to occur in about 1.3 to 1.35 cases per million vaccine doses. This type of reaction manifests quickly and requires immediate medical attention. Another rare condition, Guillain-Barré Syndrome (GBS), has been observed after influenza vaccination, with an estimated risk of about one to two cases per million doses. GBS is more frequently associated with influenza illness itself than with the vaccine.
Who is Eligible for the Inactivated Vaccine
Health authorities, such as the Centers for Disease Control and Prevention (CDC), recommend annual influenza vaccination for nearly everyone aged 6 months and older. Vaccination is particularly important for groups at increased risk of serious complications from influenza. These groups include pregnant people, adults aged 65 years and older, young children, and individuals with chronic health conditions like heart or lung disorders, diabetes, or weakened immune systems.
There are specific situations where individuals should consult a healthcare provider before receiving the inactivated vaccine. Those with a history of a severe allergic reaction to a previous influenza vaccine dose or any component of the vaccine should consult a healthcare provider. Similarly, individuals with a history of Guillain-Barré Syndrome within six weeks of a prior influenza vaccination should consult a medical professional to weigh the benefits against potential risks. Most people with an egg allergy can safely receive inactivated influenza vaccines, as severe allergic reactions are very rare and monitoring for symptoms is recommended.
Comparison with Other Flu Vaccines
The inactivated influenza vaccine differs from the Live Attenuated Influenza Vaccine (LAIV), commonly known as the nasal spray flu vaccine. The inactivated vaccine is administered as an injection into a muscle, and contains influenza viruses that have been killed and are therefore non-infectious.
In contrast, the LAIV is given as a nasal spray and contains live, but weakened, influenza viruses. While these weakened viruses do not cause the full-blown illness in healthy individuals, they can lead to mild, cold-like symptoms such as a runny nose, wheezing, or a sore throat. Due to its live virus composition, LAIV is not recommended for certain populations, including pregnant individuals, children younger than two years old, adults over 50, or those with weakened immune systems. The inactivated vaccine is a suitable option for these groups, providing protection without the use of live virus components.