People with moderately to severely compromised immune systems are especially vulnerable to COVID-19, and may not build the same level of immunity to 2-dose vaccine series compared to people who are not immunocompromised. The CDC recommends a third dose of COVID-19 vaccine for people who are immune compromised to improve immunocompromised people’s response to their initial vaccine.
For more information on who should receive a third dose of vaccine and why, details are available on this CDC page: COVID-19 Vaccines for Moderately to Severely Immunocompromised People