In July 2021, the chief scientist of the World Health Organization, Sumaya Swaminathan, warned in a press conference of the danger of leaving the decision to mix the vaccine schedule to individuals, and stressed the need to leave this decision to the competent authorities according to the information available at the present time, and later commented in a tweet via Twitter that the world is awaiting results Vaccine schedule blending studies to assess the efficacy and safety of this practice (2) Now that the results of some studies are beginning to emerge, we can be a little optimistic.
Somaya Swaminathan - Chief Scientist of the World Health Organization
The biggest motivation behind the experience of mixing vaccines was what happened several months ago with the “AstraZeneca” vaccine, when very rare cases of blood clotting problems appeared, after obtaining the first dose of the vaccine, which prompted several European countries such as Sweden, France and Spain to stop Its use in some age groups, which left a large group of citizens receiving only one dose of the vaccine, and their vaccination schedule had to be supplemented with a different type of vaccine instead of leaving them with half immunity (1).
One of the most important of these is a study known as "Com-Cov", which is an acronym for "Covid-19 Vaccine Schedule Group Comparison". The study was conducted at Oxford University and led by Matthew Snape, associate professor in the Oxford Vaccine Group, where the researchers used a different type of vaccine for the second dose than that used for the first dose, to study the effect of this combination on rates of immunity against the virus. The study also investigated potential side effects of vaccine combinations, with the aim of using the findings to determine new vaccine policies in the UK (3).
The study stipulated that the volunteers be 50 years old and over, and that the participant not know the type of vaccine he will receive in either of the two doses, whether the two doses are the same or different, provided that the period between the two doses varies from 4 weeks for some cases and 12 weeks in others. The study is currently testing the two vaccines, "AstraZeneca" and "Pfizer", as each of them works by a different mechanism from the other, which is likely that the combination of them will have a stronger and perhaps more sustainable immunological effect than only one type (4).
But first let's take a closer look at how these two vaccines work. Pfizer's vaccine is called a messenger RNA vaccine, which is simply a tiny message that instructs your body's cells to create a small, harmless portion of the SARS-CoV-2 virus protein that causes Covid-19 called the Al-Hasakah protein. Spike protein), and when your immune system detects this foreign protein, it creates antibodies to kill it. Thus, if you have already contracted the virus, your immune system will have previous experience with it and the ability to synthesize antibodies more quickly, which may prevent symptoms altogether or reduce their strength (5).
On the other hand, the AstraZeneca vaccine works in a different way, as it uses a harmless adenovirus vector, which is just a vessel to transmit a message made up of a small part of the DNA of the SARS-CoV-2 virus. This message is sent to the cells of your body to begin to produce the Al-Hasakah protein, and when your immune system recognizes it, it begins to produce immune "T-cells", and if you become infected, these immune cells kill the cells infected with the virus and prevent it from spreading (6).
In the preliminary results of the "COM-COV" study, the researchers found that the antibody response was highest in participants who received the identical schedule of "Pfizer" vaccine and those who received the varied vaccine schedule with a dose of "AstraZeneca" followed by a dose of "Pfizer" in almost equal proportion.
But that's not all, as the response of "T cells" resulting from this diverse schedule was twice that resulting from the schedule of the replica "Pfizer" vaccine, "T cells" are another representative of immunity besides antibodies, and this means that mixing the DNA vaccine mRNA (eg, virus) with an adenovirus-based vaccine as the vector (eg AstraZeneca) may provide a dual immunological gain (7).
On the other hand, a separate research paper published in "The Lancet", and based on the "COM-COV" study, indicated that the simple and moderate side effects of the vaccine were observed at greater rates in the research samples that received the varied vaccine schedule than those on the vaccine schedule. The vaccine schedule is identical, but these symptoms were short-lived with no long-term health implications. So despite the positive results of mixing the schedule of vaccines in terms of immune response, the study indicates an increase in absenteeism from work the day after receiving the mixed schedule vaccine (8).
The promising results of mixing vaccines are not limited to the Oxford study. In another large Spanish study called “Combivacs” also published in “The Lancet” last July, a sample of 676 volunteers aged 18-60 years who had already received the dose The first group received the “AstraZeneca” vaccine 8-12 weeks before the experiment, then the participants were divided in a ratio of 2:1, where two thirds of the sample received a dose of the “Pfizer” vaccine, while the last third did not receive any vaccine, and blood samples were examined after 14 days to measure the level of Antibodies in the body (9).
Despite the absence of a sample for comparison between those who received a similar vaccine schedule and a different vaccine schedule, the results of the "Combivacs" study announced by researchers at the Carlos III Health Institute in Madrid were promising, as it confirmed that the proportion of antibodies in the bodies of participants who received a dose of the "AstraZeneca" vaccine. followed by a dose of "Pfizer" vaccine was 37 times higher than that of those who received only one dose of "AstraZeneca" vaccine, and the proportion of immune "T cells" was 4 times higher (10).
Similarly, the German University of Saarland has also worked on a study of the immune response to a diverse vaccine schedule using AstraZeneca and Pfizer (11). Some participants were given two identical doses of the "AstraZeneca" or "Pfizer" vaccine, while others received an initial dose of the "AstraZeneca" vaccine, followed by a booster dose of "Pfizer" 9-12 weeks later. The results indicated that the diversified vaccine schedule produced a greater amount of specialized antibodies against the Hasaka protein of SARS-CoV-2 and a greater number of "T cells", compared to the identical "AstraZeneca" vaccine schedule, while the results were close or slightly higher compared to the schedule of "AstraZeneca" vaccine. The replica "Pfizer" vaccine, which confirms the results of the "Com-Cov" study.
The only reason behind the plan to mix vaccines was not what happened with the “AstraZeneca” vaccine, but rather this plan may provide a suitable alternative for less fortunate countries or regions that struggle to obtain doses of a particular type of vaccine regularly, as these countries can now with some confidence It allows its citizens to obtain the vaccine that is currently available even if it does not match the type of vaccine they received in the first dose, instead of depriving them of the second dose and the protection it provides against the symptoms and complications of the disease.
The other reason is that mixing the schedule of vaccines may help speed up access to a larger number of citizens in a shorter time, as there is no need to wait for 12 weeks if citizens receive the first dose of a vaccine such as “AstraZeneca” and then get a second dose of Pfizer vaccine. In other cases, some countries, such as the UAE, decided to give a booster dose of one of the messenger RNA vaccines to those who received the Chinese “Sinopharma” vaccine, after the spread of the virus worsened despite the ongoing vaccination campaigns, in the hope that this combination would give additional immunity (12).
And according to what was published by the China Global TV Network, Cambodian Prime Minister “Hun Sen” decided that his citizens would receive a third booster dose of the Corona vaccine, so that citizens who received a similar vaccine schedule from the Chinese “Sinopharm” would receive a third dose of the vaccine. AstraZeneca, while citizens on an identical schedule of vaccines from AstraZeneca will receive a third booster dose of Sinopharm. While there are no adequate studies on the immune effect or side effects of this particular combination, the World Health Organization has not raised any objection to its adoption in Cambodia (13).
A vaccine blending strategy could provide the world with a window of hope in the face of pandemic mutants and the decline in vaccine-induced immunity over time. Recently, the National Institutes of Health in the United States of America began conducting a new study that includes 150 participants who received an identical vaccine schedule from "Johnson and Johnson", "Moderna" or "Pfizer", at least 20 weeks ago, provided that they receive a third booster dose exclusively of Moderna vaccine. The study aims to determine the safety and efficacy of booster doses for those who have already received a full vaccine schedule, in order to confront the mutations that occur in the virus, in addition to the waning of the body’s immunity after a period of time after receiving the vaccine of all kinds, and its results are expected to be published at the end of this summer (14).
In the absence of sufficient information on the effect of mixing vaccines against the delta mutant most prevalent in the world at the present time, a recent study published in The New England Journal of Medicine comes with optimistic results while we wait. Results of studies directed at the efficacy of combination vaccines against the delta mutant. The study indicates that receiving two doses of the "Pfizer" vaccine is 88% effective against the delta mutant, while getting two doses of the "AstraZeneca" vaccine is 67% effective against the same mutant (15).
These results are, of course, insufficient to predict the efficiency of mixing the schedule of vaccines in the face of different virus mutations, but the studies that have been conducted so far agree on one thing: mixing vaccines increases the individual's immunity against the virus, with a noticeable increase in test samples.
While we wait for the results of these studies to be translated on the ground among a larger number of individuals, it is inevitable to continue to follow the precautionary measures and receive the available “Covid-19” vaccine, of any kind, at the earliest opportunity, perhaps humanity is one step ahead of the virus this time.
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Sources
Mix-and-match COVID vaccines: the case is growing, but questions remain
WHO warns individuals against mixing and matching COVID vaccines
Com-CoV: Home
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