The CDC has cleared up one of the ambiguities in its recent COVID-19 booster shot recommendations: If you got a third shot of one of the mRNA vaccines because you have a weakened immune system, that third shot is not considered your booster. You can still get a booster, if you fall into one of the categories for whom boosters are recommended, and that will be your fourth shot. The clarification was published in an update to the clinical considerations for COVID-19 vaccines.
(There is still no specific recommendation for immunocompromised people who got the Johnson & Johnson vaccine, but everybody who got that vaccine is recommended to get a booster dose if it’s been at least two months since your initial dose.)
So why do immunocompromised people get as many as four shots? Well, there’s the initial two doses that everyone is supposed to get (three weeks apart for Pfizer, four weeks apart for Moderna), but it turns out that some people’s immune systems don’t respond well enough to those two doses, and a third is necessary to get the protection to fully take. That third dose should be given four weeks after the second.
People with cancer and people who have had organ transplants are most likely to need the third shot, but the option is open to anyone who is “moderately to severely” immunocompromised. This includes people who have primary immune deficiencies, or who are taking medications that suppress the immune system. The CDC has more information here on who needs a third dose and why.
Once you’ve had your three doses, your “primary series” is complete and any future shot would be considered a booster. We have an explainer here on what exactly defines a booster, although, to be honest, it probably hasn’t been easy for regulators to decide which is which — the third dose for immunocompromised people is being called an “additional” dose rather than a booster. That said, for the purpose of vaccination requirements from an employer or organisation, you are still considered “fully vaccinated” after your second dose, just like everyone else.