COVID-19 Vaccines for People who are Moderately or Severely Immunocompromised – Centers for Disease Control and Prevention (.gov)

The safety, effectiveness, and benefit of the third primary dose in people who are moderately or severely immunocompromised continues to be evaluated. So far, reactions reported after the third primary dose of an mRNA COVID-19 vaccine are similar to those of the two-dose primary series. Fatigue and pain at the injection site were the most commonly reported side effects—and overall, most symptoms were mild and temporary.

As with the two-dose primary series, serious side effects are rare, but may occur.