CDC document offers more evidence that booster shots could help immunocompromised people in the face of Delta

• Delta will make breakthrough cases more common among immunocompromised people, a CDC document says.
• The document says mRNA vaccines are far less effective at preventing hospitalizations among people with weakened immune systems.
• US regulators may eventually recommend booster shots for immunocompromised people.

Disease experts have long anticipated that coronavirus vaccines may be less effective for immunocompromised people - but the Delta variant may hinder that vaccine protection even more.

A leaked internal document from the Centers for Disease Control and Prevention, first obtained by The Washington Post, suggests that Delta will make breakthrough cases (COVID-19 diagnosed after someone is fully vaccinated) more common among immunocompromised people.

Vaccines are already considerably less effective for transplant patients, people on immunosuppressive medications such as chemotherapy, and people with kidney or autoimmune diseases.

"Those are the kind of individuals that, if there's going to be a third boost, which might likely happen, would be [first among] the vulnerable," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN's State of the Union on Sunday.

The CDC document said that Pfizer's and Moderna's mRNA vaccines lower the risk of hospitalization due to COVID-19 by 59% for immunocompromised people, according to a preprint study that hasn't yet been peer reviewed. For people who aren't immunocompromised, vaccines lower that risk by 91%, the study found.

The document also cited another study that found Pfizer's vaccine lowered the risk of symptomatic COVID-19 by 75% among immunocompromised people - compared to 94% among all vaccine recipients. Pfizer's vaccine also lowered immunocompromised people's risk of getting the virus by 71%, compared with 90% in the overall population.

Emerging data like this might push US regulators to recommend boosters for immunocompromised people, Fauci said on Sunday.

Immunocompromised people are more likely to get sick and spread the coronavirus

A CDC advisory committee met earlier this month to discuss whether immunocompromised people might need booster shots. The group noted that relative to the average person, immunocompromised people are more likely to get severely ill from COVID-19, spread the virus to their household contacts, and develop a breakthrough case.

The committee also cited research indicating that immunocompromised people represent 44% of hospitalized breakthrough cases in the US and 40% of hospitalized breakthrough cases in Israel.

But boosters seemed to help: Among people who didn't develop antibodies after two doses of an mRNA vaccine, 33% to 50% developed an antibody response after a third dose, the CDC said. The side effects of that third shot generally weren't much different than the first two for immunocompromised people.

But the committee ultimately decided that more data was needed before boosters could be rolled out to people with weakened immune systems. The Food and Drug Administration would also have to authorize a third shot for this specific purpose.

However, Israel and France have both already authorized boosters for immunocompromised people. In April, France said people who are "severely immunocompromised" could receive a third dose four weeks after their second shot. And earlier this month, Israel said people with organ or stem-cell transplants, blood cancer, or autoimmune diseases, as well as people taking immunosuppressive medications, can get a booster, too.

The UK's National Health Service has also suggested that an additional dose for immunocompromised people over 16 years old be rolled out between September and December. But regulators haven't approved that plan yet.