Covid-19: Why Mixing and Matching Vaccines Could Solve Many Problems

With the changes in the AstraZeneca vaccine, new coronavirus variants, and supply limitations, many people wonder if they can “mix and match” covid-19 vaccines.

This means, for example, receiving the AstraZeneca vaccine as the first dose, followed by a second dose of a different vaccine, such as the one from Pfizer, and boosters with other vaccines later.

Although there are many ongoing studies, data from mix-and-match trials in Spain and the UK have recently been published.

These data are very promising and suggest that mixed regimens can provide higher antibody levels than two doses of a single vaccine.

How does this formula work and why might it be a good idea?

What is the benefit of mixing and matching?

If the covid-19 vaccination program can mix and match vaccines, it greatly increases its flexibility.

Having a flexible immunization program allows us to be agile in the face of global supply limitations. If there is a shortage of a vaccine, instead of stopping the whole process to wait for the supply, you can continue with a different vaccine, regardless of the one that was given as the first dose.

If one vaccine is less effective than another against a certain variant, mixed schedules could ensure that people who have already received a dose of a less effective vaccine can receive a booster with a more effective vaccine against the variant.

Some countries are already using mixed vaccination schedules after changing the AstraZeneca vaccine recommendations due to the occurrence of thrombi as a very rare side effect.

Several countries in Europe are now advising younger people who were previously given this vaccine as the first dose to receive an alternative vaccine as the second dose, most commonly mRNA vaccines like Pfizer’s.

Spain, Germany, France, Sweden, Norway and Denmark are among the countries that advise mixed vaccination schedules for this reason.

It is safe?

In a UK study of vaccine combination published in the Lancet, 830 adults over the age of 50 were randomly assigned to receive either the Pfizer or AstraZeneca vaccine first, and then the other.

People who received mixed doses were found to be more likely to develop mild to moderate symptoms from the second dose of the vaccine, including chills, fatigue, fever, headache, joint pain, malaise, muscle pain and pain at the injection site, compared to those who received vaccines from the same company.

However, these reactions were short-lived and there were no other safety concerns . Researchers have now adapted this study to see if early and regular use of acetaminophen reduces the frequency of these reactions.

Another similar study (not peer-reviewed) carried out in Spain has revealed that most of the side effects were mild or moderate and of short duration (two to three days). They were similar to the side effects of receiving two doses of the same vaccine.

It is effective?

The Spanish work has shown that people had a much higher antibody response two weeks after receiving the Pfizer booster, following an initial dose of AstraZeneca.

These antibodies were able to recognize and inactivate the coronavirus in laboratory tests.

This response to the Pfizer booster appears to be stronger than the response after receiving two doses of the AstraZeneca vaccine, based on previous data from the trial. The immune response of receiving the Pfizer vaccine followed by the AstraZeneca vaccine is not yet known, but the UK will have the results soon.

There is no data yet on the effectiveness of mixed schemes to prevent COVID-19, but they are likely to work well. The immune response is similar, or even better, compared to studies using the same vaccine as the first and second doses. This indicates that they will be effective in preventing disease.

Could you speed up vaccination programs?

In Spain, those under 60 years of age who received the first dose of AstraZeneca can choose whether to continue with the same product or receive the Pfizer vaccine. Although the Ministry of Health recommends a second dose of Pfizer, many have chosen to stick with AstraZeneca.

Recent real-world findings from the UK suggest that after two doses, both vaccines are equally effective against the variants circulating in the British Isles.

The recommendations on changing the vaccine in different countries are due to concerns about the appearance of thrombi after the first dose of AstraZeneca, as well as the change in the age ranges of the citizens who receive it and supply problems.

This has led to widespread uncertainty and has meant that young people in some European countries who had already received a first dose were excluded from receiving a second dose.

The results of these mixed studies support the possibility of vaccinating those who have received the first dose of AstraZeneca with a different booster, if the need arises.

Further research is underway to evaluate mix and match schedules with Moderna and Novavax vaccines.

Don’t delay in getting vaccinated

While in some countries cases are decreasing, many others are experiencing an increase. Among them are Taiwan and Singapore , which were previously hailed as excellent examples of how to manage covid-19.

These examples highlight the difficulty of sustained virus suppression in the absence of high vaccination coverage. This will be exacerbated by the new, more transmissible variants.

Current cases in many regions of Europe, the United States, and Australia are caused by variant B.1.617.1 (“India”) . Both vaccines are effective against variant B.1.617.2, closely related to india (although slightly less than against B.1.1.7), and similar efficacy would be expected against B.1.617.1.

While we wait, it is essential that people do not delay their vaccination with the product that is offered to them. Vaccination is an essential part of the exit strategy from the pandemic.

The vaccination schedule is likely to change in the future, as boosters may be necessary. This is normal in immunization programs. We already do it every year with the flu vaccine. This should not be viewed as a policy failure, but rather as an evidence-based response to new information.

Fiona Russell is a researcher, pediatrician and epidemiologist at the University of Melbourne in Australia. J ohn Hart is clinical researcher of the Institute Murdoch Children’s Research.

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