Many Vaccines Require More Than One Dose
A single dose of vaccine provides only partial protection. The number of doses needed to achieve immunity depends on whether the antigen in a vaccine is alive or not. Because they contain living bacteria or viruses, live-attenuated vaccines can provide enduring protection with only two doses. By contrast, non-live vaccines typically require at least three doses to achieve protection that fades over time and must be restored with booster doses.
Live-attenuated vaccines
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- Offer long-lasting, even lifetime protection.
- Could cause a life-threatening infection in someone with a weak or suppressed immune system.
- Require two doses to achieve maximum immunity.
- Examples include the chickenpox vaccine and the MMR (measles, mumps, and rubella) combined vaccine, which children should receive around their first and fifth birthdays.
Non-live vaccines
- Protection fades over time.
- Safer for people with weak immune systems.
- Require three or more doses to achieve maximum immunity.
- For example, the DTaP vaccine requires repeated doses to achieve and maintain protection from diphtheria, tetanus, and pertussis (whooping cough):
- Infants receive doses at 2 months, 4 months, 6 months, and 18 months of age.
- Children get one booster dose around the time they first enter school and another when they begin middle school.
- Adults should get a tetanus booster once every 10 years or during each pregnancy.
Certain vaccines must be updated periodically to protect against mutation-prone viruses that cause waves of infections months or years apart. To stay protected, people must get the updated vaccines even if they got an earlier version.
- The seasonal flu vaccine is reformulated each year to target the four strains expected to be most common and most dangerous.
- The updated COVID-19 vaccines were developed to deal both with fading immunity and a fast-evolving virus.
Everyone Should Get Recommended Vaccines on Schedule
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History shows that vaccines are the safest, most effective way to protect yourself and your family from many preventable diseases.
Everyone should get all recommended vaccines at the recommended times. It is especially important for children and adolescents to get catch-up doses of any missed vaccines or vaccine doses as soon as they can. Adults should get all recommended vaccines for their age or other risk factors such as health condition or occupation. All adults should get tetanus boosters, seasonal flu and COVID-19 vaccines, and any vaccines missed in childhood.
Vaccines Strengthen the Body’s Natural Defenses
To be immune is to be partially or fully resistant to a specific infectious disease or disease-causing organism. A person who is immune can resist the bacteria or viruses that cause a disease, but the protection is never perfect.
Immunization is the process of being made resistant to an infectious disease, usually by means of a vaccine.
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Immunity is protection against a disease, and it can be passive or active, natural or vaccine induced.
Active immunity comes from being exposed to a disease-causing organism.
- Natural immunity results from being infected by a disease-causing organism, whether the infection is symptomatic or not.
- Vaccine-induced immunity results from being exposed to killed or weakened bacteria or viruses—or even just important pieces of them—through vaccination.
Either way, active immunity takes longer to develop but lasts longer than passive immunity.
Passive immunity is provided by antibodies produced by another human being or animal.
- Full-term babies acquire passive immunity from their mother’s antibodies during the final months of pregnancy.
- Patients can acquire passive immunity through antibody-containing blood products derived from human or animal sources.
Passive immunity provides protection that is immediate but fades within weeks or months.
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