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(NEXSTAR) – As the Delta variant spreads rapidly across the United States and other countries, some people may wonder if they can boost their immunity with a shot from a different manufacturer.

On Monday, chief scientist for the World Health Organization Soumya Swaminathan warned against mixing and matching different COVID-19 vaccines.

“It’s a little bit of a dangerous trend here,” Swaminathan said during an online WHO briefing Monday. “It will be a chaotic situation in countries if citizens start deciding when and who will be taking a second, a third and a fourth dose.”

The WHO’s statement caused a stir in some parts of the world, such as Canada, where health care providers have embraced a mix-and-match approach to vaccinating residents against COVID-19.

A spokesperson for Ontario Health Minister Christine Elliott said in a statement that “Ontario continues to follow the advice of the National Advisory Committee on Immunization (NACI) which recommends that it is safe to mix vaccines based on studies from the UK, Spain and Germany that have found that mixing vaccines is safe and produces a strong immune response.”

Swaminathan later clarified on Twitter that “individuals should not decide for themselves, public health agencies can, based on available data,” adding that health officials are still awaiting the outcome of mix and match studies using different vaccines.

Dr. Peter Chin-Hong, an infectious disease expert at the University of California at San Francisco School of Medicine, said he thought her words were not meant as personal advice, but came from a public policy standpoint and were aimed at avoiding “unregulated chaos around the world,” especially in countries where vaccines are not easily accessible.

“I think where she probably made a misstep originally in her comments, was saying that there was no data, but there is data,” Chin-Hong said. Researchers in the UK and other European countries have found that mixing doses from different manufacturers worked just as well when it came to protecting against COVID-19.

“The party line is if you don’t have to mix and match don’t do it, but if you did it for whatever reason, pragmatic or operational, then it’s going to be fine,” Dr. Chin-Hong said.

Enabling health care providers to mix and match doses could streamline the vaccination process in countries suffering vaccine shortages.

In the U.S., the CDC hasn’t given full approval pending further study. In January, however, the CDC did update its guidance to say that in rare circumstances someone could get a Pfizer injection and then a Moderna shot at least 28 days apart.

What if you got the Johnson & Johnson vaccine?

There are reports of people who received the single-dose Johnson & Johnson vaccine seeking out an mRNA “booster” shot of Moderna or Pfizer vaccine to bolster their immunity, including a Saskatchewan virologist who tweeted about the experience.

“We don’t know if a mRNA booster (or just 2 doses of J&J) will significantly boost protection, though it’s likely it would given boosters do for nearly every other vaccine,” Dr. Angela Rasmussen wrote. “But we shouldn’t wait to make recommendations about this. In the US and increasingly in Canada, there are ample supplies of mRNA vaccines that will expire before they can be shipped to other countries.”

The Johnson & Johnson vaccine was found to be 66.3% effective at preventing COVID-19 infection in a clinical trial, according to the CDC, which was lower than the 94-95 percent reported for mRNA vaccines. The vaccine was, however 100 percent effective at preventing hospitalization and death from the virus.

Johnson & Johnson has issued a statement July 1 saying that the vaccine produced antibodies that protected against COVID-19 and were effective at neutralizing the Delta variant, eight months after the shots were administered.

But is it safe to mix the J&J vaccine, a viral vector, with an mRNA vaccine like Moderna or Pfizer?

“In terms of getting Pfizer or Moderna after J&J I’m not worried about it,” said Chin-Hong. “It’s very different, it’s not like saying ‘I’ll get an AstraZenca after a J&J because that’s similar. But the Pfizer and Moderna haven’t really been linked with any of these autoimmune conditions like the neurologic syndrome Guillain-Barré or the rare blood clotting events. It’s a very specific side effect – very, very rare – but mainly associated with the vector vaccines, but not with the mRNA vaccines.”

He added that it is “very unlikely” anyone who received the J&J vaccine would experience either of the two side effects beyond three weeks after injection.

When it comes to seeking out multiple shots of COVID-19 vaccine from different manufacturers, Chin-Hong says there is a theoretical risk to overdoing it.

“In terms of a limit, I probably wouldn’t get too many vaccines,” he said. “Every time you get a vaccine you probably increase the chances of immune activation. So right now two or three seems to be kind of OK, but beyond a certain threshold you might get so many immune cells in your body they might start attacking your own body, so you definitely want to worry about safety beyond a certain point.”

Chin-Hong added that not many people have received more than three doses and there’s no documented case of such an immune response.