Second dose with a vaccine other than AstraZeneca, how it works –

In Italy who is under 60 years old and has already made it first dose with AstraZeneca receive another vaccine, in this case RNA (Pfizer or Moderna). After the change of course of the CTS, which evaluated the updated data on the side effects of the Anglo-Swedish vaccine on the range of young women, the so-called mix of vaccines was given authorization.

Relative studies

Practice not yet authorized officially by European regulatory bodies, but not new in medicine and, according to the first studies that are being launched all over the world, it would also involve advantages from the point of view of the immune response. There are at least three main studies that have published preliminary results on this option.
A recent Spanish survey monitored 670 volunteers aged 18 to 59 who had already received a first dose of AstraZeneca and who subsequently had Pfizer: side effects were no different, or more or less serious and frequent, on the other hand, it seems that the mix increased and enhanced the antibody response.
A German study of 300 people based on the same mix showed one good tolerability, with side effects comparable to homologous vaccination and a slight increase in the immune response.
In the UK, the Com-Cov study, launched in February, is looking into the possibility of using the AstraZeneca, Pfizer, Moderna and Novavax vaccines in various combinations. The results suggest that those who were given the mixed regimen had a median level of antibodies nearly four times higher neutralizing. There have also been higher rates of side effects vaccine-related commonalities, such as fever, compared to people who received two doses of the same vaccine (particular that did not emerge in Spain and Germany). However, all symptoms were short-lived.

Because it can be better

Combining various components of the vaccine is a common practice for enhancing antibody response, experts say. The different technologies involved in vaccines make it possible to present the same target to the immune system, but in a slightly different way, causing a more diverse immune response. a bit like giving the immune system two images of the virus, one from the front and one from the side. The two vaccines whose combination is currently being tested are in fact of different types: Pfizer a Messenger RNA and transfers within the body the part of the genetic material of SARS-CoV-2 that serves to build the Spike protein that activates our immune response, AstraZeneca is based on viral vector, which thanks to a harmless virus (a chimpanzee adenovirus) transfers the part of the genetic material (DNA) necessary to build the same protein into the individual. Researchers hope to trigger stronger and more robust immune responses than two doses of a single vaccine and this advantage could serve vaccination campaigns when there are uncertainties and doubts about any particular vaccine, but also when it is necessary to deal with floating supplies.

What we don’t know

The study on the vaccine mix is ​​also being undertaken in view of the third possible dose, but there are at least two aspects that we do not yet know: side effects and efficacy against coronavirus of the heterologous proposal. The adenovirus and mRNA vaccines are indeed new platforms (the former never used on a large scale and the latter completely new), but the first data are positive. It was expected that using different vaccines would work well, because all the vaccines we are using take anyway the same protein as a target (the so-called spike, ed). The technology that helps the vaccine enter cells may eventually change, as is the case between AstraZeneca and Pfizer, he explains. Antonella Viola, immunologist and professor of Pathology at the University of Padua.
The combinations
between preparations would therefore all possible and also theorder of administration between one and the other vaccine could be exchanged as needed. They are, however, deductions made on the basis of scientific assumptions that still need confirmation from specific studies. What we do not know for sure are the data on the protection offered by the vaccine mix, although we are confident – adds Viola -: mRNA vaccines activate the production of antibodies well, while those to adenovirus the response of the T cells of the immune system. so the combination could be strategic.

The precedents and the adenovirus node

The heterologous combination has also been implemented for vaccines against other diseases, such as Ebola: the first dose is a viral vector, the same one used by AstraZeneca, and the second dose a modified version of a poxvirus. In the case of vaccines (such as AstraZeneca) based on a viral vector (in this case an adenovirus), thinking about a second dose with vaccines designed with messenger RNA technology might be even better, dato that some people may develop an immune response against adenoviruses which could weaken the future efficacy of the drug – explains the expert -. AstraZeneca works by inducing an anti-adenovirus response, while the messenger RNA used by Pfizer only causes the specific anti-Covid response. the limit of adenovirus-based vaccines (they tend to be less and less effective, because the immune system mounts a response against adenovirus) that could therefore be overcome.
RNA vaccines, however, tend to trigger stronger side effects with additional doses and this has yet to be verified.
Normally the results of controlled clinical studies and the approval of regulatory agencies are expected, but, based on the knowledge of immunology, the choice can be anticipated, as Italy is doing and as some other countries have already decided, including Great Britain itself, where anyone under 40 can ask for a second dose (after AstraZeneca) different from the first.


June 12, 2021 (change June 12, 2021 | 15:51)



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