Can you mix covid-19 vaccines from different technologies?

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(CNN Spanish) – Scientists have developed vaccines against covid-19 with different technologies or platforms. Those of messenger RNA, such as that of Pfizer / BioNTech, and that of viral vector, such as that of AstraZeneca.

Many wonder if these vaccines can be combined, especially those from two different technologies. In this episode, Dr. Elmer Huerta analyzes a study in Spain in which researchers answer precisely this question.

You can listen to this episode on Spotify or your favorite podcast platform or read the transcript below.

Hello, I am Dr. Elmer Huerta and this is your daily dose of information about the new coronavirus. Information that we hope will be useful to take care of your health and that of your family.

Today we will see a preliminary Spanish study – announced in an online press release from the Carlos III Health Institute – in which it is shown that a first dose of the AstraZeneca vaccine can be followed by a second dose of the Pfizer / BioNTech, with an excellent immune response and few side effects.

The combination of the covid-19 vaccine

As we heard in the March 16 episode, Europe was shocked when some people who received the first dose of the Oxford / AstraZeneca vaccine developed clots as a side effect.

As a result, several European countries, including Spain, suspended vaccination after millions of people had already received the first dose of the Oxford / AstraZeneca vaccine. In Spain they were just over 2 million.

One of the solutions proposed at the time was that the second dose of the vaccination be completed with a different brand than the first, and that of Pfizer / BioNTech was proposed.

The problem was that there was no scientific basis for making that important public policy decision.

The Spanish study

With this idea in mind, the Carlos III Health Institute decided to start a phase 2 study in five institutions in Spain on April 17, 2021, in people under 60 years of age previously vaccinated with the first dose of the Oxford / AstraZeneca vaccine to determine what the volunteers’ response would be to a second dose of the Pfizer / BioNTech vaccine.

The study, called Combivacs, randomly drew 673 people into two groups. The first, of 441 participants, received the Pfizer / BioNTech vaccine as a second dose. The second, out of 232 people, was the control group since they did not receive any vaccine.

The mean age in both groups was 44 years and 56% were women, and the researchers were careful to avoid selection biases in gender, age and participation center that could confuse the study.

The results

The results indicated that -after receiving the Pfizer / BioNTech vaccine, and compared to baseline, immunoglobulin G production was 150 times higher.

Similarly, antibodies directed specifically against the spike of the virus increased almost 42-fold, relative to the baseline level.

Finally, the researchers measured the effectiveness of the antibodies produced by vaccinated people, confronting them in neutralization tests against pseudoviruses that carry the spike protein of the new coronavirus.

This technique showed an increase in neutralizing antibody titers of more than 7 times after the administration of the Pfizer / BioNTech vaccine dose, an increase – say the researchers – greater than that described when the second dose is of the same type of vaccine. the first.

Were there any side effects from the COVID-19 vaccine combination?

As for the side effects, documented through an electronic diary, these were mild and similar to those reported in studies of the Pfizer vaccine, not causing extra medical attention or hospitalization in any case.

The most frequent side effects were:

Pain in the injection area, Headache (44% of all cases), General malaise (41%), Chills (25%), Mild nausea (11%), Mild cough (7%) And fever (2 ,5%).

All adverse events, which were perceived as serious by the participants, were evaluated in real time by physicians from the participating center, and in no case was this perceived severity confirmed.

A similar study, published by British researchers in The Lancet on May 12, compared groups of volunteers over the age of 50 who received various combinations in the order of administration of the AstraZeneca and Pfizer vaccines, some starting with one and ending with the other, he reports that, although the side effects were slightly greater when the second dose was of a different vaccine than the first, neither was serious enough to hospitalize the volunteers.

The results of the study will be available in June 2021.

Finally, and giving strength to the weight of scientific evidence focused on studies, the National Public Health Council of Spain, in a decision taken on May 18 and based on the study that we have just described, decided that the more than two million Spaniards under 60 years of age who have already received the first dose of the Oxford / AstraZeneca vaccine, complete their second dose with the Pfizer / BioNTech vaccine.

Do you have questions about the coronavirus?

Send me your questions on Twitter, we will try to answer them in our next episodes. You can find me at @DrHuerta.

If you think this podcast is useful, help others find it by rating and reviewing it on your favorite podcast app. We’ll be back tomorrow so be sure to subscribe to get the latest episode on your account.

And for the most up-to-date information you can always head to Thanks for your attention.

If you have any questions you can send them to Dr. Elmer Huerta through Twitter. You can also head over to for all episodes of our “Coronavirus: Reality vs. Reality” podcast. fiction”.

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