Earlier in the pandemic, the concept of mixing COVID-19 shots from different companies was mostly discussed in regards to a national vaccine supply shortage.
Now, scientists across the globe are studying the method through a different lens. Can receiving a different COVID-19 shot for your second dose provide extra protection against more contagious coronavirus variants?
Although preliminary, data on mixing shots, scientifically known as “heterologous prime-boost vaccination,” offers some positive news. But experts say more research is needed to fully support this approach as the world grapples with COVID-19 case surges unseen since pre-vaccine peaks.
“Mixing and matching is a promising avenue to increase the efficacy of vaccines. It does appear that it may be effective and help improve vaccine efficacy and develop second-generation vaccines,” Dr. Amesh Adalja, senior scholar with the Johns Hopkins Center for Health Security, told McClatchy News in an email, speaking of the concept in relation to the delta variant. “More study is needed, however, to bring this to fruition.”
Dr. Dan Barouch, an immunologist at Harvard Medical School who helped develop the Johnson & Johnson COVID-19 vaccine, agrees.
“We don’t know what we don’t know,” Barouch told CNBC. “There’s limited data on the safety or efficacy of the approach, but theoretically, there is no reason that I can see that raises major safety concerns. But there are theoretical benefits in terms of the immune responses and potential improvement of efficacy.”
Both the Centers for Disease Control and Prevention and the World Health Organization advise against mixing COVID-19 vaccines due to a lack of data on its safety and efficacy, but some countries, including Canada, Spain, Italy and Sweden, have already given citizens the green light to do so.
“It’s a little bit of a dangerous trend here,” Dr. Soumya Swaminathan, the World Health Organization’s chief scientist, warned during a July 12 briefing. We’re “in a data-free, evidence-free zone here as far as mix-and-match. 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.”
What do studies show about mixing COVID-19 vaccines?
One study published in May found that people who received one dose of the two-shot Pfizer-BioNTech vaccine two weeks after getting one shot of the two-dose AstraZeneca COVID-19 vaccine — which is not authorized for use in the U.S. — produced a “robust” antibody response with mild side effects.
A separate study posted in June also found that mixing the Pfizer and AstraZeneca vaccines, in either order, conferred higher levels of antibodies against the coronavirus than two doses of the AstraZeneca shot.
Yet another paper published in July landed on similar results. Receiving the Pfizer or Moderna COVID-19 vaccines as a second dose after one dose of the AstraZeneca vaccine produced a better immune response than two doses of the AstraZeneca shot.
It’s important to note the studies to date were done in controlled laboratory settings, so more realistic data from the real world on mixing and matching is lacking. The studies also don’t test how effective this approach is when put up against the delta variant, though the variant may have been spreading when the studies were conducted.
The COVID-19 vaccines, so far, offer adequate protection against the delta variant. But emerging evidence from Israel on the Pfizer vaccine shows the shot’s effectiveness has dropped to 39% from an earlier estimate of 64% in just two weeks, thanks to delta’s rapid spread there. The vaccine still protects well against coronavirus hospitalization and death.
Why does mixing and matching COVID-19 vaccines appear to work?
Infectious disease experts say the method appears to work because the studied vaccines function differently, suggesting the mix-and-match method could teach the immune system to recognize the coronavirus in more ways than one.
The Pfizer and Moderna shots use a molecule called mRNA to trigger the production of antibodies, while the AstraZeneca and Johnson & Johnson vaccines are vector-based, meaning they insert a harmless modified virus into our bodies to complete the same job.
“The mRNA vaccines are really, really good at inducing antibody responses, and the vector-based vaccines are better at triggering T cell responses,” which are other components of the immune system that can help kill disease-causing germs, Leif Erik Sander, an infectious disease expert at the Charité University Hospital in Berlin, told Science.
This tag team approach to COVID-19 vaccination isn’t new.
The method actually got its debut in 1992 in a “landmark” experiment involving monkeys; it “became one of the most promising protection results” in scientists’ initial attempts at developing an HIV vaccine. The concept has also been studied as potential treatments for cancer.
Not to mention it’s well known — and accepted — that for some illnesses, multiple doses of the same vaccines, known as “homologous boosts,” are necessary for the shots’ success.
For example, up to five shots of the DTP (Diphtheria, Tetanus, Pertussis) vaccine may be needed in children, given three times in the first six months after birth, a fourth at age 2, and a last dose between ages 4 and 6. In adults, the Tetanus-diphtheria (Td) vaccine requires a booster shot every 10 years.
Despite the positive evidence collected so far, federal and international health officials say it’s too early to officially recommend mixing and matching COVID-19 vaccines to receive extra protection against variants.
The National Institutes of Health announced June 1 that it started a clinical trial to study the method.
“Although the vaccines currently authorized by the U.S. Food and Drug Administration offer strong protection against COVID-19, we need to prepare for the possibility of needing booster shots to counter waning immunity and to keep pace with an evolving virus,” National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci said in a statement. “The results of this trial are intended to inform public health policy decisions on the potential use of mixed vaccine schedules should booster doses be indicated.”