Researchers hope that a mixed approach to COVID-19 vaccines will help alleviate these problems and create more flexibility in the vaccination schedules available to people.
All over the world, different pharmaceutical companies have taken different approaches to developing vaccines. Pfizer-BioNTech and Moderna have created mRNA vaccines. Oxford-AstraZeneca and Johnson & Johnson used what are called viral vectors. The Novavax COVID-19 vaccine is protein-based.
So mixing the vaccines could mean more than just switching manufacturers, like from Pfizer for dose one to Moderna for dose two. You may be using a different way of boosting your immune response if you take a first dose of AstraZeneca and a second dose of Moderna.
The most obvious benefits of treating different brands and types of COVID-19 vaccines as interchangeable are logistical – people can get any vaccine available without worry. By accelerating the global rollout of immunization, vaccine mixing and matching could help end this pandemic. The researchers also hope that combining different vaccines will trigger a more robust and long-lasting immune response compared to receiving two doses of a single vaccine. This approach can better protect people against emerging variants.
Biological effects of a mixed approach
Scientists suspect that there are several ways that receiving two different COVID-19 vaccines can result in a stronger immune response.
Each company used slightly different regions of the SARS-CoV-2 spike protein in their formulations. It is to the spike protein of the virus that your immune system responds, so exposure to different parts of the spike protein should mean that your body will produce a range of corresponding antibodies that can ward off future infection. The range of antibodies should then offer better protection and increase the likelihood that you will be protected against variants with changes in the spike protein.
And different vaccine technologies activate unique aspects of the immune system through the way they present their portion of the spike protein.
Pfizer and Moderna vaccines are made from a small extract of mRNA, genetic material that contains the recipe for making a region of the SARS-CoV-2 spike protein. Wrapped in a fatty coat, the mRNA sneaks into the cells of a vaccinated person where it directs the production of the viral protein. The person’s immune system then recognizes the foreign spike protein and makes antibodies against it.
Several other COVID-19 vaccines rely on a viral vector. In these cases, the researchers modified an adenovirus that typically causes the common cold to provide the DNA instructions to produce part of the SARS-CoV-2 spike protein. The modified virus is safe because it cannot replicate in humans. Along with J&J and AstraZeneca, examples of COVID-19 viral vector vaccines in use around the world include the Russian Sputnik Van vaccine and the CanSino Biologics vaccine.
Your immune system may develop an immune response to the viral vector vaccine itself, which could reduce the effectiveness of the coronavirus vaccine. Experts hope that combining vaccine platforms, for example using an mRNA-based vaccine or a vaccine including a different viral vector for the second dose, could reduce this risk.
Investigate the safety and effectiveness of combos
Around the world, studies are underway in animals and humans to investigate the safety, types of immune response generated, and duration of immunity when a person receives two different COVID-19 vaccines.
Results from a Spanish trial of more than 600 people indicated that vaccination with the AstraZeneca viral vector and Pfizer-BioNTech COVID-19 mRNA-based vaccines elicits a robust immune response against the SARS-CoV-2 virus.
Preliminary results from a German study that has not yet been peer reviewed found that getting the AstraZeneca vaccine first followed by the Pfizer vaccine resulted in the production of more protective antibodies and offered better protection against variants of concern compared to two doses of AstraZeneca.
The Com-COV study in the UK is also investigating the safety and effectiveness of administering to patients a combination of AstraZeneca and Pfizer-BioNTech injections. Preliminary results indicate that people who received one injection of each type were more likely to report mild to moderate side effects than those who received two doses of the same vaccine. The final results of this study, including the effectiveness of this approach, are expected in June 2021. The expanded study Com-CoV2 is testing other combinations of COVID-19 vaccines, notably from the platform of Moderna mRNA and the Novavax protein platform.
Combos could be a good anti-variant strategy
Emerging variants of the coronavirus are one of the most intriguing reasons to consider mixing vaccines. The administration of vaccines targeting different variants would provide broad herd immunity and limit the emergence of new, possibly more dangerous strains.
People who are currently fully vaccinated may need a third injection to address the genetic differences in the new variants. Switching platforms for this booster – for example, if your first round was based on a viral vector, switching to mRNA or a protein-based one – could help boost your immune response.
Influenza vaccines routinely protect against multiple strains of the influenza virus, but they are usually made by the same company. In the future, this approach could lead to vaccines containing multiple regions of SARS-CoV-2 to protect against multiple variants, or regions of both the coronavirus and influenza proteins, protecting against both viruses in one. single injection.
What is allowed so far
For now, however, the Centers for Disease Control and Prevention in the United States only allows mixing of Pfizer and Moderna mRNA-based injections in “exceptional situations”, such as a limited vaccine supply or if a patient does not know which vaccine they originally received.
Canada’s public health agency recently approved mixing different COVID-19 vaccines if a limited supply prevents a person from receiving their second dose of the same vaccine, or if someone is worried about a second dose of AstraZeneca due to ‘mediated side effects.
EU countries are so far awaiting the results of further studies before allowing mixing of vaccine doses.
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