Most vaccines require more than one dose over time to produce immunity and long-lasting protection. The number of doses needed depends on the type of vaccine.
There are two main types of vaccines: live attenuated vaccines and inactivated/subunit vaccines. Learn what these terms mean below, and how they affect the number of doses you need for full protection.
Live attenuated vaccines
Live vaccines are made from a weakened version of the germ (virus or bacteria) that causes the disease. When a person receives a live vaccine, the germ in the vaccine multiplies (grows) in the body and stimulates an immune response almost identical to that produced by natural infection. Because it is weakened, the vaccine germ cannot multiply enough to cause disease in people with healthy immune systems. However, live vaccines are not given to people with very weak immune systems as they may develop the disease the vaccine is meant to protect against.
One dose of a live vaccine usually produces immunity and long-lasting protection because the vaccine stimulates an immune response that is almost identical to that produced by natural infection. But because not all individuals respond to the first dose, a second dose is usually recommended. The measles, mumps, rubella (MMR) vaccine and chickenpox (varicella) vaccine are examples of live attenuated vaccines.
Inactivated vaccines are made from a killed germ (or parts of the germ for subunit vaccines) that causes the disease. Since the vaccine does not contain a live germ, there is no growth of the germ in the vaccinated person, and this leads to a weaker immune response compared to live vaccines. Due to the weaker immune response, inactivated/subunit vaccines almost always require more than one dose. In general, the first dose “primes” the immune system and the second and/or third dose produces a protective immune response. These vaccines are safe even for those with weakened immune systems, because the killed germ cannot cause disease.
Immunity from these vaccines can decline with time and booster doses are often needed to ensure continued protection. The tetanus, hepatitis B and pertussis vaccines are examples of inactivated/subunit vaccines.
Unlike other vaccines, a dose of the influenza (flu) vaccine is needed every year. That’s because every year, there are different influenza viruses that people can catch. As a result, a new version of the vaccine is created every year to provide best protection against the current influenza viruses. Most people need one yearly dose of influenza vaccine, but children under 9 years of age who have never had an influenza vaccine need two doses.
There are live and inactivated versions of the influenza vaccine. The inactivated version (given as an injection) is safe for people with weakened immune systems.