The Centers for Disease Control and Prevention on Friday urged people with weak immune systems to take extra precautions to avoid Covid after the dominant omicron subvariants knocked out a key antibody treatment.

These precautions include wearing a high quality mask and social distancing when it’s not possible to avoid crowded indoor spaces, according to the CDC.

The guidance comes after the Food and Drug Administration on Thursday pulled its authorization of Evusheld, a combination antibody injection that people with weak immune systems took as an additional layer of protection to prevent Covid infection.

The FDA pulled Evusheld because it is not effective against 95% of the omicron subvariants circulating in the U.S. This includes the XBB subvariants which are now causing 64% of new cases, as well as the BQ family that is responsible for 31% of reported infections.

Although most Americans have largely returned to normal life as the Covid pandemic has ebbed, people with weak immune systems remain at higher risk of severe disease because they do not mount as strong of an immune response to the vaccines.

Still, it is important for people with weak immune systems to stay up to date on their Covid vaccines by receiving the omicron booster because the shots can slash the risk of severe disease, according to the CDC.

If you have a weak immune system and develop Covid symptoms, you should get tested as soon as possible and receive treatment with an antiviral within five to seven days, according to CDC.

Available antivirals include Paxlovid, remdesivir or molnupiravir, but patients should talk to their doctor to find out which treatment is best. Some people cannot take Paxlovid due to how it interacts with other drugs they are taking.

People with weak immune systems include cancer patients who are on chemotherapy, organ transplant patients who are taking medication for their transplant, people with advanced HIV infection, and those born with immune deficiencies.

Some 7 million adults in the U.S. have a condition, like cancer, that compromises their immune system, according to the CDC.