Can you mix COVID-19 vaccines?

Many vaccines for COVID-19 are currently being developed and several are currently being rolled out in countries around the world. With demand significantly exceeding supply, some wonder if mixing the first and second dose can become a reality. But can you mix COVID-19 vaccines? Is there any research that supports this?

mRNA vaccines

The Pfizer and Moderna vaccines are mRNA vaccines. mRNA stands for messenger ribonucleic acid. These vaccines teach the body to develop an immune response against the spike protein on the surface of the SARS-CoV-2 virus (3). When the body is exposed to COVID-19, it recognizes the virus and has the means to fight it off.

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

Viral vector vaccines

AstraZeneca's vaccine is the result of a partnership between AstraZeneca and Oxford University. Oxford University / AstraZeneca vaccine technology differs from that used in mRNA vaccines. It is a vector viral vaccine. The vector used in the vaccine is from a modified adenovirus that normally infects chimpanzees. Adenoviruses are common viruses that cause cold symptoms (5). Some of the COVID-19 spike protein is transmitted by the modified adenovirus, which triggers immune responses in humans (4).

The Johnson & Johnson / Janssen COVID-19 vaccine has the advantage of being a single vaccine that does not require a second dose. However, this vaccine is currently being postponed in the United States due to reports of severe blood clots in a small number of people. Because of this, the CDC has recommended that its use be suspended until scientists have been able to conduct a more detailed review of its safety and performance.

Vaccine distribution and scarcity

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

To address this supply problem, some countries have increased the time required between the two doses (8). The hope is that the starting dose of the vaccine will provide adequate protection while also giving more people the opportunity to get the first dose of a vaccine.

In addition to these struggles, some vaccine manufacturers warn that they may soon face a shortage of raw materials (9). Novavax has identified a shortage of plastic bags that are used to grow cells. Pfizer also lowered production targets due to raw material quality issues (10).

Due to possible vaccine shortages and the different vaccines available, the ability to mix COVID-19 vaccines could be another option.

Research on Mixing COVID-19 Vaccines

Researchers have already started studying whether or not COVID-19 vaccines can be mixed and matched. A mixed vaccine study is currently being carried out with the Russian COVID-19 vaccine Sputnik V and the AstraZeneca / Oxford vaccine (2). Study participants first received a dose of the Sputnik V vaccine, a vector vaccine. Three weeks later, participants receive a booster dose of the AstraZeneca / Oxford vaccine (2). The early efficacy results were encouraging.

Another clinical study examines the safety and effectiveness of mixing vaccines. The clinical study is a non-inferiority study designed to determine if mixing vaccines is worse than not mixing. The first part of the study, called the Com-Cov study, looked at the side effects of mixing and matching the AstraZeneca / Oxford and Pfizer vaccines. The study looked at different vaccine combinations and different lengths of time between doses.

Six new weapons have been planned since the start of the study. These arms will study the effects of mixing the Pfizer vaccine with the Moderna or Novavax vaccines and the AstraZeneca vaccine with the Moderna or Novavax vaccines. Two other arms compare these results to non-mixing normal doses from Pfizer / Pfizer and AstraZeneca / AstraZeneca (11).

Investigators hope the results will help more people get vaccinated, vaccination courses can be completed faster, and backup plans are made when a vaccine runs out of stock.

Currently, the CDC discourages people from the mix-and-match vaccine approach (12), except in extreme circumstances. A similar stance was taken in England in the Green Book Guidance by Public Health England (8). While vaccine mixing was common in routine vaccination in the past (13), COVID-19 vaccines did not yet have enough clinical trial data to support this practice.

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

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