In-Depth: Mixing vaccines could help boost immunity against COVID-19

Research across Europe shows promising results
Posted at 5:58 AM, Aug 05, 2021
and last updated 2021-08-05 13:46:25-04

SAN DIEGO (KGTV) - Research from across Europe shows that mixing COVID-19 vaccines may provide stronger immunity than getting a 2-dose regiment of a single vaccine.

"It is an interesting and promising idea," says Dr. Alessandro Sette from the La Jolla Institute for Immunology.

Dr. Sette says mixing shots could be the best way to help countries facing a vaccine shortage.

This week, the World Health Organization (WHO) placed a hold on booster shots. Officials say the world needs to make sure there are enough doses for everyone to get their first or second shot before considering a third. It underscores the idea that many poorer countries do not have enough vaccines.

Being able to mix different types of vaccines could be a way to help stretch the supply.

Now, studies also show it could also provide stronger immunity.

"The rationale is that the different vaccine platforms may stimulate the immune system in slightly different fashions," says Dr. Sette.

He calls it the "Prime Boost Strategy," and it's based on the fact that different vaccines work in different ways.

"Viral Vector" vaccines, like the ones produced by Johnson and Johnson or AstraZeneca, introduce a harmless portion of the virus into our bodies. The immune system fights it off and develops antibodies to protect against future infection.

Meanwhile, mRNA vaccines like Moderna and Pfizer teach the cells to make a spike protein similar to the coronavirus. They then get rid of that spike protein. The body remembers the instructions to recognize and fight the virus later.

"By mixing and matching two different vaccines, you get a more complete activation of the immune system," says Dr. Sette, noting that mixing the two types can give the immune system the best of both worlds.

New research from across Europe supports the "Prime Boost Strategy" idea.

In May, researchers in Madrid presented the results of their "Combivax" study. They found that patients who got one dose of AstraZeneca and then one dose of Pfizer had a 150-fold increase in antibody titers compared to those who just got one AstraZeneca shot.

The "Com-Cov" study at the University of Oxford compared four vaccine combinations: two AstraZeneca shots, two Pfizer shots, one of each shot with AstraZeneca first, and one of each shot with Pfizer first.

The study found two Pfizer doses gave the highest level of antibodies. But it also discovered that both mixed vaccine combinations gave a better immune response than AstraZeneca alone. And they found people who got the AstraZeneca shot first produced higher antibodies and T-Cells than people who got the Pfizer vaccine as their first shot.

A third study from Berlin followed 340 healthcare workers from December through May, giving them one dose of the AstraZeneca vaccine and one dose of Pfizer.

In that mix, when given 12 weeks apart, the mixed vaccines produced "higher immune responses" than traditional two-dose vaccines given three weeks apart.

In all three studies, side effects in people who got mixed vaccines were similar to those of people who got the same vaccine.

"The sum of the two is synergistic," says Dr. Sette. "You get more than each of the two separately when added together."

Dr. Sette told ABC 10News studies like this can be done relatively quickly because data from a few weeks or months can give hints about long-term results.

He also says that because other studies already show the vaccines are safe on their own, we can assume they'll be safe together.

Still, the CDC does not recommend mixing vaccines, as the US has enough supply to ensure everyone gets a total dose of the same vaccine.

No studies have been published about mixing the Johnson and Johnson vaccine, which is the only "viral vector" vaccine approved for use in the US.

But Dr. Sette says the European studies are essential for the safety of the world.

"Ultimately, nobody is safe until the whole world has gotten over the pandemic," he says.