Your questions, answered "I am a senior and I have received three doses of the Moderna covid vaccine because I am immunocompromised. I understand that my third dose was not considered to be a 'booster,' but instead was the final dose of an initial three dose series. The FDA has now authorized a Moderna booster for seniors and others at high risk from covid-19. Does that authorization only apply to people who received the typical two-dose series, or will people in my situation be eligible for a fourth dose as a booster?" — Kenneth in Texas You've explained a key distinction in your question. For most people, two doses of a Pfizer or Moderna mRNA vaccine are sufficient to be fully vaccinated against the coronavirus. Those Americans who are 65 and older, and others, are eligible for a booster shot given six months later. (You can check booster eligibility here.) People with moderate to severe immune deficiencies, meanwhile, should get multiple doses in sequence: two of the Pfizer or Moderna mRNA vaccine plus a third dose at least 28 days later, per the Centers for Disease Control and Prevention. This group includes organ transplant recipients and people with blood cancer or some chronic diseases. All told, the CDC estimates this population to be 2.7 percent of adults, or 7 million people. The CDC does not consider these third shots to be boosters, as you rightly note. That's because those third shots are part of the first series that jumpstarts the immune system, allowing it to churn out coronavirus-fighting cells and antibodies. If the immune system is an engine, imagine additional first shots as the extra yanks required to start a stubborn lawnmower. A booster, on the other hand, is maintenance for an already-running engine. Crucially, certain immunocompromised people may need even more shots as part of a first round, said Johns Hopkins University surgery and epidemiology professor Dorry Segev. In a small study, Segev and his colleagues found that some organ transplant recipients required four doses to generate protective antibodies. "Probably half of transplant patients don't have solid immunity even with three shots," he said. But now that boosters are becoming more widely available, what happens for people who already had at least three shots? It's a great question, and the answer may change with time. But, for now, the CDC does not recommend boosters for people who have had a three-dose series. Virologist Shane Crotty, who studies infectious disease at La Jolla Institute for Immunology in California, predicts that boosters will eventually be recommended for some groups of immunosuppressed and immunocompromised people. Because immune impairments range from mild to severe, those recommendations may be tailored to people with certain conditions, he said. (Here's a good time to remind readers: If you have specific questions about your health, please consult a trusted medical professional.) Segev agrees immunocompromised people will probably need boosters at some point, too. One reason there is not official guidance yet may be because, he said, from the regulators' point of view, agencies still have months to review data and make a decision. The FDA authorization and CDC recommendation for third shots happened in August. If people with weakened immune systems had the last in their series of first shots in August, a six-month booster shot would be given in February 2022. A booster at six months assumes antibody levels wane at the same rate in immunocompromised people as they do in everyone else. It is not clear this is the case, Segev said, but those rates might indeed be similar. Stay tuned for further research. Segev and other scientists recently embarked on a clinical trial, supported by $40 million in NIH funding, to see how additional coronavirus vaccines might help people who did not have a protective immune response after their first shots. | 
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