The Centers for Disease Control and Prevention updated its guidance on Thursday on the administration of the two coronavirus vaccines currently approved for Emergency Use Authorization.

The guidance clarifies several questions public health practitioners have recently asked. Can the two doses of the Pfizer and Moderna vaccines be split, and how long do patients have to get the second dose of the vaccine?

Both doses of the vaccine require patients to return for a booster. For the Pfizer vaccine, the second dose is recommended three weeks after the first dose. The second dose of the Moderna vaccine is due four weeks later.

The CDC said that it continues to recommend that the same type of vaccine be administered to patients. But if only the opposite type of vaccine is available, the CDC said that the other vaccine can be administered and is considered effective.

The CDC also says that patients can wait up to 42 days to receive a second dose.

As of Friday, 19 million people have received at least one dose of the vaccine, according to the CDC.