Three vaccines, three diseases, and the capital letters tell you the dose

The CDC recommends diphtheria vaccination for everyone. The vaccines are
very effective, and they are the best protection there is against diphtheria.

Diphtheria never comes on its own. There are three combination vaccines
that include protection against it:

Reading the name

The letters are not arbitrary, and the case of each one carries information:

• T is tetanus; D or d is diphtheria; P or p is pertussis
• **A capital letter is a full-strength component; a lower-case letter is a
  reduced dose.**

So DTaP carries a full diphtheria dose and is the childhood vaccine, while
Tdap carries a reduced diphtheria and pertussis dose and is the adolescent
and adult booster. Td is the tetanus-diphtheria booster without pertussis.

Once you can read the name, "which one am I due?" stops being a question only
the clinic can answer — and it explains why a person can be given Td one
decade and Tdap another without either being a mistake.

Talk to a vaccine provider if you have questions about which applies to you.

Source: Centers for Disease Control and Prevention.