Immunocompromised people should get a third dose of a COVID-19 vaccine, a Centers for Disease Control and Prevention committee recommended today. After taking only a standard, two-dose series of the Pfizer / BioNTech or Moderna vaccine, patients may not generate enough antibodies against the coronavirus.

“I have known of patients who are immunocompromised and in the hospital, they did all the right things. They’re just suffering from a lack of good vaccine protection,” says Camille Kotton, clinical director of transplant and immunocompromised host infectious diseases at the Massachusetts General Hospital, during the CDC’s Advisory Committee on Immunization Practices (ACIP) meeting today.

The decision follows the Food and Drug Administration’s announcement late Thursday night that it has amended the emergency use authorization for both vaccines to allow the additional third shot for people who have received solid organ transplants or who have conditions that impair the immune system to a similar degree. There are no changes for the Johnson & Johnson one-shot vaccine.

Around 7 million adults in the United States are immunocompromised. They’re more likely to get seriously ill from COVID-19, are more likely to have breakthrough infections even if vaccinated, and the vaccines aren’t as effective for them. People considered moderately or severely immunocompromised include people who have received organ transplants, in treatment for some cancers, and who take immunosuppressive medications like high-dose corticosteroids.

Immunocompromised people shouldn’t need a doctor’s note or prescription to get a third dose, says Kathleen Dooling, a medical officer at the CDC. “This is self-attesting,” she told attendees at the meeting.

Image: Centers for Disease Control and Prevention

A third shot increases the proportion of immunocompromised people who produce antibodies, research shows. However, it’s still not clear if or how much that antibody boost translates to prevention of COVID-19 for this group.

Even with a third dose, people who are immunocompromised should still take precautions against COVID-19. While the boost will likely give them some additional protection, they’d still be at high risk for a severe case of COVID-19 should they contract the disease. “Immunocompromised people should continue to wear a mask, stay six feet apart from others they don’t live with, and avoid crowds and poorly ventilated indoor spaces,” Dooling says.