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COVID-19 vaccines are delivered in California. Al Seib/Los Angeles Times via Getty Images
  • The United Kingdom’s National Health Service (NHS) is conducting research to discover the effectiveness of combining two different vaccines.
  • The purpose of this 13-month trial is to see how well people’s immune systems respond once they’re “primed” with one type of vaccine and then boosted with another.
  • Experts say at this point it’s still just a trial, and it’s not clear it will be helpful.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

Even as COVID-19 vaccinations increase across the globe, experts have wondered whether it’s possible to get people vaccinated more quickly if vaccines can be mixed and matched.

Evidence of vaccine supply chain issues show that potentially allowing people to use different vaccines may help people get fully vaccinated more quickly.

In an attempt to create more flexible COVID-19 vaccination programs, the United Kingdom’s National Health Service (NHS) is conducting research to discover the effectiveness of combining two different vaccines.

It’s called the Com-COV study, and it’s raising important questions, like whether this can increase or decrease vaccine effectiveness.

“By using two different vaccines, there is not going to be a greater magnitude of protection against the disease,” Dr. Nikhil Bhayani, infectious disease specialist with Texas Health Resources, told Healthline.

However, Bhayani emphasized that at this time, “[the] use of two different vaccines is not recommended.”

Bhayani also cautioned that while the Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccines do offer a degree of protection against COVID-19, “as we see more variant strains the expectation of the vaccine protecting you from the disease could decrease. However, clinical trials will be warranted to prove this.”

According to the NHS, the purpose of this 13-month trial is to see how well people’s immune systems respond once they’re “primed” with one type of vaccine and then boosted with another.

Researchers want to see how strong the immune system response is when the second dose is separated from the first dose by different amounts of time.

Researchers will also use blood samples from trial volunteers to monitor the effect of different dosing regimens on participants’ immune system responses, and look for any additional adverse reactions to these combinations of vaccines.

The two vaccines used for this study are from AstraZeneca and Pfizer-BioNTech.

However, the NHS added that “if more vaccines are approved for use in the U.K. after this study starts, they may be added to the trial.”

It’s important to note, according to the most current guidelines from the Centers for Disease Control and Prevention (CDC), that a lack of available safety data means COVID-19 vaccines shouldn’t be mixed unless there’s an exceptional situation, like a shortage of the vaccine the person first received.

“We use this approach to protect against pneumococcal disease, which is serious illness caused by Streptococcus pneumoniae bacteria and can include pneumonia, bloodstream infection, meningitis, among other infections,” said Dr. Dana Mazo, hospital epidemiologist and assistant professor of medicine at Mount Sinai Hospital in New York.

She explained that in some instances, one type of vaccine can increase the effectiveness of another.

“There are two different types of pneumococcal vaccines that have different mechanisms of action, and in certain situations we recommend boosting one with the other,” she said.

According to Bhayani, scientific data has shown that getting several vaccines at the same time does not cause any chronic health problems.

When every new vaccine is licensed, “it has been tested along with the vaccines already recommended,” he added.

Mazo said the AstraZeneca and Pfizer-BioNTech vaccines use two different technologies to protect against COVID-19.

The Pfizer-BioNTech vaccine is an mRNA vaccine, but AstraZeneca’s vaccine is not.

“AstraZeneca uses adenovirus vector technology, similar to Johnson & Johnson,” Mazo explained. “[But] both vaccines contain instructions that tell our cells to make the spike protein of the SARS-CoV-2 virus that causes COVID-19.”

Mazo said this causes our bodies to then build an immune response against the coronavirus spike protein that helps protect us if we’re exposed to the virus.

She pointed out that the difference is how the two vaccines transport these instructions to our cells.

“Pfizer uses mRNA coated in lipid (fatty) material to get the information into the cells,” she said, “while AstraZeneca uses a weakened version of an adenovirus, a common cold virus, to do the same thing. Both vaccines currently require two doses.”

According to Bhayani, the altered virus in AstraZeneca’s vaccine can’t make you sick, but it carries a gene from the coronavirus’s spike protein, the portion of the virus that triggers an immune response.

“As a result, the immune system manufactures antibodies that work against COVID-19, teaching the body how to respond should one become infected,” he said.

“It is a new approach, which is why the U.K. is proposing to do a research study to evaluate it,” Mazo said.

She explained it’s possible that getting the information to the cells in two different ways (one via mRNA and one by adenovirus viral vector) could create a better response either in terms of stronger or longer protection.

But this approach could also compromise overall effectiveness.

“This question about whether it will be better or worse is why this approach is most appropriate in the context of a study,” Mazo said. “Since we do not know the answers, we want to learn from it.”

Without question, there are critical COVID-19 vaccine shortages.

How we resolve these issues to protect the majority of people from the pandemic and return to a more normal situation is a serious concern.

But is mixing different vaccines better than not receiving a needed second dose?

“Ideally you want to give the second dose to achieve the 90 percent efficacy,” Bhayani said.

“However, in dire circumstances, if there is a shortage of COVID vaccines, it would be better to get more people vaccinated with a single dose. At least some protection is better than none.”

Dr. Eric Cioe-Peña, director of global health at Northwell Health in New York, believes that at this time, there’s simply too little known about mixing vaccines.

“We don’t know if there’s an advantage to mixing vaccines or a disadvantage. Further study is needed of the effects of mixing. It’s not yet ready for prime time,” he said.

U.K. researchers are conducting a clinical trial to learn whether mixing the Pfizer-BioNTech and AstraZeneca vaccines can help increase the number of people who can be vaccinated with the needed two doses to protect against COVID-19.

Experts say there’s not enough data to know whether this approach is an effective solution, and it might even reduce the protection offered by these vaccines.

The CDC currently recommends against mixing vaccines except under exceptional circumstances.

According to experts, in situations when it’s either mix different COVID-19 vaccines or not receive the second dose, it’s better to use the same vaccine in one dose until we know more.