An estimated 22,000 Montana residents with weakened immune systems are eligible to receive third doses of the Moderna or Pfizer-BioNTech Covid-19 vaccines, the state health department said Wednesday.

The announcement from the Montana Department of Public Health and Human Services followed new guidance from the federal Centers for Disease Control and Prevention, which recommends third doses for immunocompromised people — including organ transplant recipients, those with AIDS, those undergoing cancer treatment and others who take medications that suppress their bodies' immune response.

"This is great news because we know the impact Covid-19 has with high-risk groups," DPHHS Director Adam Meier said in a statement. "The updated recommendations will ensure those most vulnerable to complications from Covid-19 can get as much protection as possible."

While the vaccines have proved to be more than 90% effective in protecting against most variants of the coronavirus, people with moderately to severely compromised immune systems don't always develop the same level of immunity against Covid-19.

The CDC estimates that around 3% of people eligible for vaccination are in those groups. And nationwide, fully vaccinated immunocompromised people account for more than 40% of breakthrough infections that have required hospitalization. Immunocompromised people also are more likely to transmit the coronavirus to others in their households.

The CDC recommends third shots for:

• People receiving treatment for cancerous tumors or blood cancers.
• Organ transplant recipients who take medicine to suppress the immune system.
• Those who have received stem cell transplants within the past two years or are taking related medicine to suppress the immune system.
• People with moderate or severe primary immunodeficiency disorders, such as DiGeorge syndrome or Wiskott-Aldrich syndrome.
• Those with advanced or untreated HIV infection.
• Those receiving treatment with high-dose corticosteroids or other drugs that may suppress the immune response.
• Third doses are available through the same vaccine providers as before, including clinics, pharmacies and local health departments. DPHSS recently notified providers of the new guidance. Patients can self-attest and don't need to prove their diagnosis to be eligible for a third shot.

Health officials said additional doses should be administered at least four weeks after a person receives their second dose of either the Pfizer or Moderna vaccines. Kids 12 and older may receive the Pfizer vaccine; the Moderna vaccine is available to those 18 and older.

The updated guidance on third doses does not apply to the vaccine made by Johnson & Johnson.

Dr. Maggie Cook-Shimanek, Montana's acting state medical officer at DPHHS, said people should talk to their doctors about their medical conditions to find out whether a third vaccine dose is appropriate.

"People with weakened immune systems are especially vulnerable to Covid-19 because they are at higher risk of serious, prolonged illness," Cook-Shimanek said. "An additional dose could help prevent serious and possibly life-threatening Covid-19 infection in immunocompromised people. The vaccines continue to be very effective at preventing severe illness, hospitalization and death from Covid-19 in individuals who are not immunocompromised."

While the vaccines offer protection, DPHHS said immunocompromised people should still wear masks, maintain 6 feet of social distance, and avoid crowds and poorly ventilated indoor spaces.

The guidance for immunocompromised people is separate from the Biden administration's announcement on Wednesday that booster shots will be widely available starting in September for all Americans who have gone at least eight months since receiving their second dose.

DPHHS reported 203 people in Montana were hospitalized with Covid-19 on Wednesday. The disease has killed 1,749 people in the state.