COVID-19: Clarify your doubts about vaccines! (Part 2)

This article is the second part of a series of three articles designed to answer questions that may exist regarding COVID-19 vaccines. If you want, you can also read Part 1.

In this second part, I will answer very specific questions about the vaccines themselves and how they work.

In the case of vaccines against COVID-19, what is the current vaccination schedule?

Nowadays, the current vaccination schedules are as follows:

• Comirnaty (Pfizer) and Moderna vaccine: two doses, with an interval of 28 days between them;
• Vaxzevria® (AstraZeneca) vaccine: two doses, 8 weeks apart;
• Janssen vaccine: one dose.

What is the difference between mRNA and adenovirus vaccines?

First of all, it is important to mention that both mRNA vaccines and adenovirus vaccines have the same common goal: to give our bodies the necessary tools for our immune system to be able to respond adequately to a potential infection. . That is, putting in our body a genetic sequence that allows us to encode the spike protein of SARS-CoV-2, which is an essential protein for the virus to be able to infect our cells. The only difference is that mRNA vaccines have molecules in them that carry the information necessary for our body to synthesize the spike protein and, later, develop antibodies to defend against it. On the other hand, adenovirus vaccines consist of the spike protein itself, which will lead our immune system to recognize this viral component as a foreign substance, acting consequently in order to neutralize it. It is important, however, to emphasize that none of the vaccines that are authorized in Portugal introduce the virus into our body and, therefore, none of them can cause the infection.

Is heterologous vaccination safe?

The AstraZeneca vaccine is perhaps the one that has generated the most controversy in the general population and within the scientific community. In February, Portuguese health authorities recommended the suspension of this vaccine for those under 60 years of age. However, at the time of this decision, many individuals had already been inoculated with a first dose. According to the norm of the General Directorate of Health, the ideal would be for the vaccination schedule to be complete with a dose of an mRNA vaccine. That said, questions quickly arose such as: will immunity not be compromised? Will taking two vaccines from different brands be safe?

Well, it is known, in principle, that taking two different vaccines can cause greater reactogenicity. However, and despite the fact that studies are, in fact, scarce and quite recent, this recommendation is based on the "biological plausibility" of heterologous vaccination, since, despite the vaccines being different, they carry the same antigen - the spike protein of the virus. In addition, an increase in the immune response to SARS-CoV-2 has been demonstrated through heterologous vaccination, which, if confirmed, will speed up the vaccination process, reduce the impact of vaccine shortages, direct vaccination schedules. according to the specific needs of an individual and, perhaps, allow the better response of the immune system to the appearance of new variants of the virus.

Is the vaccine effective against the new variants?

Since the advent of COVID-19 and vaccines to fight it, we have seen the emergence of several variants of the SARS-CoV-2 virus. To date, little is known about the effectiveness of vaccines in responding to these changes. The truth is that current vaccines train the body to recognize the spike protein of the virus: the component of the virus on which fall, precisely, some mutations, associated with variants such as those in South Africa and the United Kingdom. Despite this, experts indicate that, even if there is a reduced response with regard to the antibody titer present in the vaccinated individuals, the response produced has been sufficient to neutralize the virus. Furthermore, neutralizing antibodies is not the only thing that protect us against the virus. In addition to humoral immunity, there is also cellular immunity that is conferred on us by specific T cells (cytotoxic and helper) against the virus.

If you would like to learn more about COVID-19 vaccines, you can also read Part 3 of this article.

Dr. Andreia Moreira