The Centers for Disease Control and Prevention have tacitly changed their guidelines for Covid-19 vaccine shots, stating that it is now okay to mix up Pfizer and Moderna shots in “exceptional situations” and that it is okay too to wait up to six weeks to receive the second shot two-dose immunization from both companies.

While Pfizer and Moderna’s vaccines, both of which use messenger RNA technology, were approved 21 and 28 days apart, the agency now says you can get both vaccines as long as they are spaced apart, according to new information at least 28 days will be administered instructions published Thursday on its website.

Although “every effort” should be made to ensure that a patient receives the same vaccine, in rare situations “any available mRNA COVID-19 vaccine can be administered with a minimum of 28 days between doses” – if supply Is limited or the patient does not know what vaccine they originally received, the CDC says in new guidelines.

The agency says the two products are not interchangeable, admitting that it hadn’t yet investigated whether its new recommendations would alter the safety or effectiveness of either vaccine. But vaccine specialists speaking to CNBC said the two vaccines are set up so similarly that people shouldn’t worry about the rare occasions when the doses are mixed.

“The intent is not to suggest people do something else, but rather to give clinicians flexibility in exceptional circumstances,” said Jason McDonald, a spokesman for CDC, in an email to CNBC on Friday.

The CDC said health care providers should give patients a vaccination card detailing when they received their first shot and what type of shot it was, to ensure patients know which shot to receive the second time. The agency also recommends providers to record the patient’s vaccination information on their medical records and on the government vaccination information system.

Ramon Lorenzo Redondo, a molecular virologist at Northwestern University’s Feinberg School of Medicine who reviewed the CDC’s updated guidelines, said health care providers should try to stick to the recommended schedule. But, he added, there will be some “emergency situations”, like those described by the CDC, that may require switching shots.

While there’s no data on it and it’s certainly not the optimal route, he said, it’s unlikely to be harmful, so it’s a reasonable protocol.

The CDC’s new guidelines come after UK health authorities also updated their guidelines on Covid-19 vaccines earlier this month. If the maker of the first shot is unknown, or if a second dose of the shot that a patient first received is not available, it is “reasonable” to replace another shot, say UK health officials in their updated playbook. It is not the preferred practice, but it is allowed.

John Moore, a vaccine researcher at Cornell University who has reviewed the new CDC guidelines, said in a telephone interview Friday with CNBC that the change “makes sense”. He said the Pfizer and Moderna vaccines are very similar, and while they have not been studied, there is no immediately identifiable reason to believe that mixing doses would decrease effectiveness or decrease safety.

“The FDA and CDC allow mix and match. They don’t recommend it. I don’t think it will happen on a regular basis, but I can’t imagine anyone would harm anyone if this and that of course is first priority” , he said. “These are unusual times. At unusual times, you must accommodate unusual changes to the protocol.”

Moore, who specializes in HIV vaccine research, said it was common to use experimental vaccines one at a time to study the effect on HIV. He said it was unusual to do this outside of a research setting, but the country is currently in exceptional circumstances.

“There is no good reason to believe that there are safety concerns in this situation, and you would strongly expect the effectiveness to remain essentially unchanged,” he said.

Both companies need two doses to achieve maximum protection against the coronavirus. While both shots should be administered according to the guidelines originally recommended, the CDC said the second dose of both companies’ vaccine could be delayed for up to six weeks if necessary.

The updated guidelines come as some cities and counties across the country cancel vaccination appointments because they don’t have as many doses as they originally expected.

Wayne County, Michigan, for example, said last week it would be a priority to make sure people who got their first shot get their second shot on time. But the county said it had to cancel nearly 1,400 appointments for people to get their first shot.

The Pfizer and Moderna vaccines are “extremely similar,” said Dr. Bill Schaffner, an epidemiologist at Vanderbilt University, in a telephone interview. So there is no reason to believe that mixing the vaccines would react adversely. It is important for the CDC to issue these guidelines, Schaffner said, as vaccine registration systems operated by states vary in quality across the country. This can cause some information to be lost so that people do not know which vaccine they received. He added that some people may travel across state lines and have their second dose in another state, further complicating matters.

“While this mixing and matching, as we call it, has not been specifically studied, there is no biological reason that it does not work and it is warranted,” he said.

Dr. Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, was asked on Friday about the interval at which the two shots should be administered.

“The data we have is of a two-dose vaccine on the recommended schedule of 21 or 28 days,” she said at a virtual event hosted by Harvard TH Chan School of Public Health and National Public Radio . “At this point in time, we at CDC agree with what the FDA says, and the FDA has made it very clear that we should be using the approved regimen.”

“It’s deeply rooted in science and the available evidence, and doing anything else would not follow science and possibly not allow us to really get the full potential of these vaccines,” she said. “For now, from the CDC’s point of view, we think it has to be two doses on the recommended schedule.”