Can you mix COVID-19 vaccines?

Many vaccines are now being developed to counteract COVID-19, and many are now being launched in countries around the world. Given the demand being more than the supply, some are beginning to wonder if mixing the first and second doses would be a reality. But can you mix COVID-19 vaccines? Is there any research supported to do this?

mRNA vaccine

Pfizer and Moderna vaccines are mRNA vaccine. The mRNA stands for messenger ribonucleic acid. These vaccines instruct the body to develop a resistance response against the spike protein found on the surface of the SARS-CoV-2 virus (3). When exposed to COVID-19, the body recognizes the virus and has tools to prevent it.

Both the Pfizer and Moderna vaccines require a second dose of mRNA vaccine given weeks after the first for the prevention and control of COVID-19 disease. The second dose of the vaccine increases the effectiveness of the vaccine to prevent COVID-19 infection.

Viral vector vaccines

AstraZeneca vaccine a result of a partnership between AstraZeneca and the University of Oxford. The vaccine technology at the University of Oxford / AstraZeneca is different from that found in mRNA vaccines. This is a viral vector vaccine. The vector used in the vaccine was obtained from a modified adenovirus that commonly infects chimpanzees. Adenoviruses are a common virus that causes cold -like symptoms (5). Part of the COVID-19 spike protein is carried by the modified adenovirus, which activates immune responses in humans (4).

The Johnson & Johnson / Janssen COVID-19 vaccine has the advantage of being a one-shot vaccine that does not require a second dose. However, this vaccine is currently withheld in the U.S. due to reports of severe bleeding in some people. Consequently, the CDC recommends discontinuing its use until scientists have been able to conduct a more detailed assessment of its safety and efficacy.

Vaccine distribution and scarcity

Admittedly, vaccines are not equally available around the world. Most of the approved vaccines are distributed in high-and-middle-income countries in the United States, the United Kingdom, the European Union, China, and India. More than half of all purchased doses of COVID-19 vaccine go to countries (7). This leaves other middle- and low-income countries with little access to protection against COVID-19. The current rate of vaccine distribution in low-income countries could only reach 60% vaccinated by 2023 or later (7).

To address this issue of provision, some countries have increased the length of time required between two doses (8). The hope is that the first dose of vaccine will provide adequate protection while allowing more people to receive the first dose of a vaccine.

Beyond these struggles, some vaccine makers warn that they may soon face a shortage of raw materials (9). Novavax has experienced a shortage of plastic bags used to grow cells. Pfizer also cut production targets due to raw material quality issues (10).

Given the potential shortages of the vaccine and the variety of vaccines that may be available, the possibility of mixing COVID-19 vaccines may be another option.

Research for mixing COVID-19 vaccines

Researchers have already begun to study whether COVID-19 vaccines can be mixed and paired. A joint vaccine study was conducted today using the Russian COVID-19 Sputnik V vaccine and the AstraZeneca / Oxford vaccine (2). Study participants were initially given a dose of Sputnik V vaccine, a vector vaccine. Three weeks later, participants received a booster shot of the AstraZeneca / Oxford vaccine (2). The consequences of early effectiveness are promising.

Another clinical trial is to study the safety and efficacy of mixing vaccines. The clinical trial is a non-invasive study, aimed at determining whether mixing vaccines is worse than not mixing. Called the Com-Cov study, the initial arm of the study investigated the effects of mixed and matched vaccines of AstraZeneca / Oxford and Pfizer. The study examined different vaccine combinations and different lengths of time between doses.

Since the study began, six new weapons have been planned. These arms investigated the effects of mixing the Pfizer vaccine with the Moderna or Novavax vaccine and the AstraZeneca vaccine with the Moderna or Novavax vaccines. Two other arms will compare these results with non -mixing, usual doses of Pfizer / Pfizer and AstraZeneca / AstraZeneca (11).

Investigators hope the results will help more people get vaccinated, allow vaccination courses to be completed more quickly, and create backup plans if a vaccine runs out of stock.

Today, the CDC bans people from mixing the vaccine (12), except in severe conditions. A similar stance is taken in England in the Public Health England Green Book instruction (8). While mixing vaccines has been a common practice over conventional vaccination in the past (13), COVID-19 vaccines do not have sufficient clinical study data to support this practice.

If the results from the current mixing and pairing of COVID-19 vaccines are positive, mixing with COVID-19 vaccines will become a common practice in the future. Until sufficient clinical data is obtained, people should continue with current advice and public health measures to help protect themselves from infection.


1. Centers for Disease Control and Prevention (CDC). Different Vaccines. [Online] April 13, 2021. [Cited: April 20, 2021.]

2. Cohen, Jon. Do you need to mix and match COVID-19 vaccines? Scientists are demanding answers. [Online] Science, February 12, 2021. [Cited: April 20, 2021.] Scientists-are-seeking-answers.

3. Centers for Disease Control and Prevention (CDC). MRNA vaccine. CDC. [Online] March 4, 2021. [Cited: April 20, 2021.]

4. Knapp, Alex and Rosenbaum, Leah. Here’s What You Need to Know About the Astra-Zeneca Covid-19 Vaccine. Forbes. [Online] November 23, 2020. [Cited: April 20, 2021.] know-about-astrazenecas-covid-19-vaccine/? sh = 1e00dbe27b3e.

5. Centers for Disease Prevention and Control. Adenoviruses. Centers for Disease Prevention and Prevention. [Online] [Cited: April 20, 2021.]

6. Mayo Clinic staff. Different types of COVID-19 vaccine: How it works. Mayo Clinic. [Online] April 16, 2021. [Cited: April 20, 2021.]

7. Mark McClellan, MD, PhD, et al. Reducing the Global COVID Vaccine Shortage: New Research and Recommendations for U.S. Leadership. Durham: Duke Margolis Center for Health Policy, 2021.

8. COVID-19 vaccines: what happens to evidence-based medicine? Quek, Eleanor and Tahir, Hasan. 2, sl: British Journal of Hospital Medicine, 2021, Vol. 82.

9. Tapper, James. The launch of the Global Covid vaccine threatened by a lack of essential ingredients. The Keeper. [Online] April 10, 2021. [Cited: April 20, 2021.]

10. Staines, Richard. Pfizer is targeting vaccine production due to lack of raw material – reports. pharmaphorum. [Online] December 4, 2020. [Cited: April 20, 2021.]

11. Covid-19: Moderna and Novavax vaccines to be tested in a combination test of vaccines. Mahase, Elisabeth. n971, sl: BMJ, 2021, Vol. 373.

12. COVID-19 in 2021-Ongoing Uncertainty. Carlos Del Rio, MD and Malani, MD, MSJ, Preeti. 13, sl: JAMA, 2021, Tomo. 325.

13. COVID-19 vaccines for low- and middle-income countries. Choi, Edward M. sl: Transactions of the Royal Society of Tropical Medicine & Hygiene, 2021, Vol. trab045.

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