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Updated on June 6, 2025
While different vaccines utilize different mechanisms and technology to protect against different diseases, the basic idea behind any vaccine is the same—a vaccine teaches the immune system to make antibodies that protect the body against an infection.
Antibodies are proteins that your body uses to fight off infectious agents and other foreign invaders (called antigens). Antibodies work by binding to the surface of an antigen, which allows them to be destroyed by immune cells called macrophages.
Antibodies are custom made for each infectious or harmful agent that they encounter. Antibodies that attach to a coronavirus will not attach to the bacteria that causes meningitis or to the toxin that causes poison ivy.
When you get a vaccine, you are essentially giving your immune system a head start. Instead of waiting for the body to encounter a specific antigen, a vaccine will prompt the immune system to make antibodies that can be ready and waiting in case that antigen arrives.
Boosters and updated vaccines
A booster is a dose of a vaccine that is given some time after the initial dose. Booster doses serve the same function as the initial doses of a vaccine, and they prompt the immune system to make antibodies. Boosters are given to strengthen immunity that has naturally weakened over time. For example, it’s recommended that adults get a tetanus and diphtheria (Td) booster every 10 years (and during every pregnancy) to maintain immunity against these severe bacterial infections.
Updated vaccines are a slightly different category. These are vaccines that have been developed to provide immunity against new strains of a virus that has emerged. For example, it’s recommended that a person get an annual flu shot (and updated vaccine) to protect against current strains of the influenza virus that are circulating. However, updated vaccines also boost existing immunity.
Updated COVID-19 vaccines provide better immunity against more current strains of the virus. The updated vaccines are given as an initial vaccination. They are also given to people who received an initial vaccination with a different COVID-19 vaccine.
Updated COVID-19 vaccines
In the United States, updated COVID-19 vaccines are recommended for people over the age of 65, and people 6 months and older with underlying medical conditions that increase the risk of severe illness from COVID-19. These underlying medical conditions include:
- Asthma
- Cancer
- Chronic kidney disease
- Chronic lung disease
- Chronic liver disease
- Cystic fibrosis
- Dementia, Parkinson’s disease, or other neurological conditions
- Diabetes (type 1, type 2, gestational diabetes)
- Disabilities
- Heart conditions
- Hemoglobin blood disorders (sickle cell disease, thalassemia)
- HIV infection
- Immunocompromised or weakened immune system
- Mental health conditions (depression, schizophrenia spectrum disorders)
- Overweight and obesity
- Physical inactivity
- Pregnancy
- Primary immunodeficiency disorders
- Smoking (former and current smokers)
- Stroke (cerebrovascular disease)
- Substance use disorders
- Transplant recipients (solid organ, stem cell)
- Tuberculosis
- Use of corticosteroid or other immunosuppressive medications
What vaccine doses should be used and when they should be administered will depend on a person’s age, if a person is immunocompromised, and what vaccines they received for their initial vaccination.
Recommendations for COVID-19 vaccines can vary depending on your source of information. As always, your healthcare provider or pharmacist will be your best source of information when making decisions about vaccines.
