(WETM) – Booster shots are on the way, but so far the FDA has only approved them for the Pfizer vaccine. And that approval is specifically for those 65 and older and high-risk people.

Johnson & Johnson reported that its booster shot prompts a strong immune response months after getting a first dose, but those results haven’t been peer-reviewed yet.

So if you’re looking for another shot, what happens if you mix and match the three vaccines?

First, mixing vaccines is nothing new. According to the European Commission, mix-and-matching vaccines started with HIV research in the 1990s.

In June 2021, the World Health Organization’s Vaccine Unit Director, Dr. Kate O’Brien said vaccine combos are probably going to work, but more research is needed.

“Based on the basic principles of how vaccines work, we do think that the mix-and-match regimens are going to work,” she said.

The CDC’s official statement warns against mixing vaccines.

For people who received either Pfizer-BioNTech or Moderna’s COVID-19 vaccine series, a third dose of the same mRNA vaccine should be used. A person should not receive more than three mRNA vaccine doses. If the mRNA vaccine product given for the first two doses is not available or is unknown, either mRNA COVID-19 vaccine product may be administered.

Center for Disease Control and Prevention

Scientists at Oxford University in the United Kingdom found, with limited data, that an AstraZeneca shot followed by a Pfizer shot seemed to be safe. They were testing combinations of the two-dose COVID-19 vaccines made by AstraZeneca, Moderna, Novavax and Pfizer-BioNTech.

The European Commission, the executive branch of the European Union, reported that a study of the same combination of vaccines showed “rather encouraging” results.

The study showed that the number of antibodies responsible for protecting your body from bacteria and viruses was much higher in those who received AstraZeneca and Pfizer than people who received two doses of just one vaccine.

However, like Dr. O’Brien, the authors of the study said “further studies need to address the safety and clinical efficacy of heterologous vaccination regimens.”

The European Commission also reported that mixing two different types of vaccines—particularly mRNA and adenovirus-based vaccines—has never been done before, but it could help fend off COVID-variants.

Dr. Ramil Sapinoro, associate professor of Pharmaceutical Sciences with St. John Fisher College explained the difference between these two types of shot.

Essentially, mRNA—something that our body produces naturally—gives our bodies the instructions on how to make a piece of the “spike” protein on the coronavirus.

Adenovirus-based vaccines use a “harmless cold virus” to deliver the instructions instead.

Lastly, the European Commission said that mixing vaccines might be able to simultaneously prompt a stronger immune response in fighting COVID variants and prevent your body from becoming immune to a specific vaccine.

What this means, in the case of adenovirus-based vaccines, is the immune system sometimes attacks that “cold virus”, which can weaken the effects of the shot. So having some variety might help improve that response.

The common thread coming from experts in the WHO, CDC and European Commission is that more data is needed. Even though it looks promising and doesn’t seem to be particularly dangerous, Dr. Pierre Meulien of the EC’s Innovative Medicines Initiative said “I think you really need to start from scratch. You have to do dosing regimens and all the usual things that you do in a precautionary way. You will have to do it with these new things because none of them have been tested together before.”