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In Defense of Mixing Vaccines: Thai Health Officials Make Their Case

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By Silada Lydia Rojratanakiat

To maximize the usefulness of every tool available for the pandemic response, Thailand is exploring the advantages of combining doses of different Covid-19 vaccines. Thus, Thailand began to administer two doses of different Covid-19 vaccines as of July 19.

A week earlier though, Dr. Soumya Swaminathan, the WHO’s chief scientist, made a statement concerning risks involved in mixing and matching Covid-19 vaccines.

The remark was directed at individuals seeking adventure by mixing and combining Covid-19 vaccines without seeking appropriate medical advice. The chief scientist stressed that public health agencies should be the sole provider of guidance over mix and match regimens. Thailand has done it. Nevertheless, Thai people are still hesitant to embrace the new approach.

Typically, if a person requires two doses of a Covid-19 vaccine, he or she gets two of the same vaccine, for example, two AstraZeneca shots or two Sinovac shots. Not one and then the other.

Yet, the notion of combining two different vaccines is not new. This process is referred to as heterologous vaccination or “mix-and-match,” in which the first vaccine is delivered initially, followed by a second vaccine of a different kind several weeks later.

When individuals hear of changes, it is normal for them to feel concerned. Hence, vaccine skepticism is natural in the context of a new vaccine. However, it should be noted that, despite the novelty of the Covid-19 vaccine, heterologous vaccination is not unusual. Immunologists have previously mixed vaccines against a variety of diseases, including HIV, Ebola, Malaria, and Influenza.

In the case of HIV, researchers have long explored this approach because protecting against the virus requires a complex immune response, which is nearly impossible to achieve with a single type of vaccine.

One type of vaccine typically elicits only one type of immune response or stimulates only one set of immune cells. Until now, the most effective HIV vaccine has been created using a mix of various vaccine types.

In the instance of Covid-19, the mix-and-match strategy most likely started after the AstraZeneca vaccine was connected to extremely rare blood clots. As a consequence, many European nations, including Germany and France, advised that patients who had it as a first dose, should get a second dose of Pfizer or Moderna.

All Covid-19 vaccines have received emergency authorization. This implies that there is no data on the efficacy of mixed Covid-19 vaccines. On that account, Thailand’s public health authorities have paid close attention to their ability to enhance immunity and any possible adverse side effects. The combination must be safe, and the advantages must significantly exceed any possible safety issues.

Although combining Covid-19 vaccines is an idea that has not been officially recognized globally, some evidence indicates that having two distinct vaccines is better than having only one type of vaccine, since two distinct vaccines may widen or enhance the immune response.

In a nutshell, different vaccine platforms activate the immune system in different ways. Apart from the potential benefit to the immune system, mixing vaccines may also help prevent vaccines from becoming less effective in the face of new variants. As the virus evolves, the portion of it that a vaccine targets may change, reducing the vaccine’s effectiveness. If, on the other hand, two vaccines target distinct components of the virus, the immune system will have several weapons in its arsenal.

Thailand’s decision to mix two distinct kinds of vaccine is not only for scientific reasons but also for practical reasons. Mixing doses will provide Thailand with even greater flexibility.

As the country currently experiences a supply shortage as a result of a global supply bottleneck, the ability to mix vaccines from several producers may alleviate some of the strain on vaccine supplies. If there is a worldwide scarcity of a specific vaccine, the immunization campaign in Thailand may continue rather than be suspended.

The authorized Covid-19 vaccines around the world are all designed to stimulate the immune system, the body’s defense against infection. Hence, the mix-and-match regiment should work according to fundamental vaccine principles.

Although there is currently little evidence on whether the degree of protection remains consistent when doses are mixed, data on vaccines has been rapidly accumulating during the pandemic. Therefore, the administration of Covid-19 vaccines may change in the future.

About the Author

Silada Lydia Rojratanakiat (ศิลดา โรจน์รัตนเกียรติ) – a former CPD research assistant and a graduate of USC Annenberg School of Communication with focuses on crisis communication and public diplomacy. She obtained her bachelor’s degree in politics and international relations at Thammasat University in Thailand. As a Royal Thai scholarship’s recipient, she works at the Department of Public Relations in a position of communication officer. While her professional focuses are on “fake news and misinformation” and political communication, her interests expand to narratives, risk messages, and communication in tourism and hospitality settings.