A: Most likely. In fact, this “mix and match” strategy can lead to a more robust immune response overall, because different vaccines often cause the body to generate an immune response targeted against SARS-CoV-2 in slightly different ways!
TL; DR Vaccines work by showing our immune system a W.A.N.T.E.D. poster of the SARS-CoV-2 virus so that if we encounter the virus again, our body knows what it looks like and can quickly arrest it. Since different vaccines target slightly different parts of the spike protein and/or stimulate different parts of the immune system (antibodies or T cells), getting vaccinated with doses of two different vaccines is like showing the body *two* W.A.N.T.E.D. posters-one with a forward facing picture and one with a profile shot. In other words, the body has multiple ways to recognize and react to the SARS-CoV-2 virus helping it to be arrested faster and more effectively. This phenomenon is known as a *heterologous prime boost* and is a strategy that has already been studied for other vaccines.
So what do we know about this “mix and match” strategy for different COVID-19 vaccines so far?
The original vaccine trials were focused on establishing the safety and efficacy for single vaccine types. Several studies are, however, now assessing the “mix and match” strategy. A few are detailed below.
✳️In a trial of over 600 people < 60 years old in Spain, who received the AstraZeneca vaccine for their first dose, 442 people received the Pfizer vaccine for their second dose between 8-12 weeks later and 221 received no second dose. The immune response generated by the Pfizer dose was even stronger than what was observed for two doses of the AstraZeneca vaccine in the original trial. In addition, the side effects were acceptable and no adverse events were reported. Efficacy data has not been reported yet.
✳️Another trial is also ongoing in the U.K., in which 830 volunteers aged 50+ years were randomly assigned to get two doses of AstraZeneca, AstraZeneca as first dose and Pfizer as second, Pfizer for both doses or Pfizer for first dose and AstraZeneca for second-with doses given 28 days apart. Researchers have reported that the proportion reporting side effects was highest in those who received mixed doses compared to two doses of the same vaccine. Data on immune response and efficacy for these various combinations has yet to be released. This trial has also been expanded to test combinations of these vaccines as well as Moderna and Novavax.
✳️The U.S. National Institutes of Health also recently launched a phase 1/2 trial in which 150 people aged 18+ years who have been fully vaccinated with the Pfizer, Moderna or J&J vaccine will receive a booster dose of the Moderna vaccine 12 to 20 weeks later. A separate group who have not yet been vaccinated will receive two doses of the original Moderna vaccine as well as a Moderna booster vaccine 12 to 20 weeks later. Initial trial results are expected in late summer 2021. All people will be followed for a year to monitor for safety, side effects and immune response against current and new variants in circulation. Anyone who develops COVID-19 during this time will also be tested to determine which variant caused the infection.
Point being, the “mix and match” strategy is being tested for multiple combinations of vaccines and we should have more information soon! Keep checking back for Nerdy Girl updates.
Are any countries already using a “mix and match” approach?
✳️In the U.S., The Centers for Disease Control and Prevention (CDC) has only said that mixing and matching vaccine doses is okay in “exceptional situations” such as if the vaccine administered for the first dose cannot be determined or is no longer available. In addition, if someone has an allergic reaction to the first dose of an mRNA vaccine, the Johnson & Johnson vaccine could be administered with people considered fully vaccinated 2 weeks after receiving that vaccine.
✳️The Canadian National Advisory Committee on Immunization recently stated that people who receive the AstraZeneca vaccine for their first dose can get either AstraZeneca OR the Pfizer or Moderna shot for their second dose and that any combination of two doses with Pfizer or Moderna is also okay.
✳️Many other countries across Europe and Asia, particularly where the AstraZeneca vaccine was initially offered, have also now said it is fine for people to get a second dose with a different vaccine.
As recommendations related to the “mix and match” strategy as well as the need for booster vaccines evolve, we will push out information as it becomes available. Stay tuned!
To hear Dr. Malia Jones and Dr. Chana Davis discuss the “mix and match strategy”, tune into our past FB live Q&A around minute 36.
For our past post on boosters and “mix and match” strategy, see here.
For more information on “mix and match” trials, see links below:
For more information “mix and match” strategies being employed across different countries, see below:
