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Why do we need boosters for some vaccines and not others?

Question: Why do we need boosters for some vaccines and not others?

Answer: Our immune systems are complicated and there’s still so much about them that we don’t fully understand.  When it comes to booster shots, there are two main reasons that we need them for certain vaccines: 

  1. Immune memory needs an update because the virus/bacteria has mutated.  For example, the influenza virus mutates rapidly, causing “antigen drift.”  These changes accumulate to such a degree that the surface of the virus appears different than before and the antibodies your body has made in response to previous vaccine (or infection) no longer recognize the new appearance.  In this case, the booster provides an update (think: latest version of security software).
  2. Immune memory needs a reminder.  As time goes by, the immune system’s memory may need a boost.  This varies based on at least three factors.  
    • First, it depends on the type of vaccine given (for more on vaccine types, see Q&A of 1/21/21).  For example, protein subunit vaccines (like HepB) usually produce a weaker immune response, necessitating a booster dose.  
    • Second, it likely depends on how quickly disease develops after infection; that is, how quickly your body needs to mount an effective immune response.  For infections that replicate rapidly, quickly causing disease, your body needs a rapid immune response.  And to mount a rapid response, your body needs ample B-cell antibodies circulating.  Over time, however, B-cell antibodies become depleted.  For slow moving infections, this depletion matters less because your body has time for T-cells to kick in and prompt the production of more antibodies.  But to counter fast moving infections, your body needs the booster shot to ensure that B-cell antibodies remain robust.  
    • And finally, it depends on the age and strength of an individual’s immune system.  For individuals who are older and/or immunocompromised, a booster dose may be required to ensure adequate immune response.  This is why, for example, CDC recommends that adults ages 65+ are  vaccinated against pneumococcal disease.

If you’d like to read more, CDC offers a helpful overview, as does Cedars-Sinai.