Why mixing vaccines may hold promise. Scientists are studying mix -and- matching vaccines for Covid-19 and other diseases.

In the last two decades, as new vaccine technologies have emerged, the idea of using different kinds of vaccines against the same pathogen has gained momentum.

The approach - known among scientists  as ''heterologous prime- boost'' - has been explored as in rodent experiments to develop vaccines against Ebola [ now authorised for use by European regulators], tuberculosis, and even cancers associated with the EpsteinBarr virus.

Mouse experiments of his approach have even been tested for other coronaviruses in the past, such as the original SARS virus and the coronavirus that causes Middle East Respiratory Syndrome.

At the start of the Covid-19 pandemic, it was unclear whether researchers would be able to create a single working vaccine, which makes it all the more surprising that the latest immunization dilemma arises from having multiple vaccine options.

Because of unpredictable supply and some concerns about an exceedingly rare but serious clotting risk from the AstraZeneca vaccine, public officials in some parts of the world that have relied heavily on that shot have recently issued new guidance on mixing and matching different Covid-19 vaccines.

Recently, for example, Canada's National Advisory Committee on Immunization updated its guidance to say that people who received AstraZeneca vaccine as their first dose can receive that same vaccine as their second dose or get a follow-up shot of Pfizer-BioNTech or Moderna instead.

The committee also said that it was possible to receive the Pfizer-BioNTech and Moderna vaccines interchangeably as first and second doses.

Countries ranging from France, to Finland to China to Bahrain have also outlined possible scenarios for combining different vaccines. Even the Centers for Disease Control and Prevention has interim guidance saying this is acceptable in ''exceptional situations,'' such as if the same vaccine is not available.

While the guidance may seem confusing, especially when the initial vaccine guidance told people to get the same shot for both doses, it does provide an opportunity to understand the safety of using mismatched vaccines, and to measure whether mismatched vaccines offer any advantage.

One recent small and not yet  peer reviewed study of 26 people who received the AstraZeneca shot followed by one from Pfizer BioNTech suggested, based on blood tests, that those with mismatched vaccines had at least as strong an immune response as people who get both Pfizer shots.

The National Institutes of Health recently began a clinical trial that will examine the effects of different combinations of Covid-19 vaccines.

In Britain a trial of this kind is already underway for the AstraZeneca and Pfizer BioNTech vaccines, and the scientists behind it have released early data on side effects. They found more reports of feverishness, chills, fatigue and headache among people who received a dose that was different from their original shot compared with people who received identical shots.

Scientists want to know whether that indicates that the immune system was more stimulated by the different vaccine, and could develop added protection. It's still too soon to say, but more results from the trial are expected this month.

This is not the first time scientists have investigated what seems like an unconventional way of vaccine dosing, and it's not necessarily something to fear. It may be our best hope against pathogens.

The Publishing of this Opinion continues to Part 2, in the future. The World Students Society thanks author Roxanne Khamsi, a science journalist covering Covid-19.


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