Covid vaccine, when should the second dose be taken?

Covid vaccine, when should the second dose be taken?

In the UK, it was preferred to give more people the first dose of vaccine while delaying the second: does that make sense?

(Image: Unsplash) With vaccination campaigns starting in many countries of the world on the one hand, and the growth of infections that for now shows no signs of slowing down on the other, healthcare institutions are now faced with a rather complex problem of optimizing resources. The issues at stake are many: the limited stocks of vaccines, the planning necessary for their distribution and administration, the circulation of new variants of the virus for which it is not yet clear how effective the approved vaccines are. But something must be decided, and quickly. The United Kingdom, for example, has decided to step on the accelerator, delaying the administration of the second dose of the vaccine to those who have already received the first to facilitate the distribution of the first to those who are still waiting. A strategy that has pros and cons, and on which - which is nothing new - we have not yet gathered enough evidence to establish whether it is right or wrong. The question can be reformulated as follows: given that the first dose of the vaccine, by itself, already confers partial immunity, priority must be given to the administration of vaccines in full (first and second dose) or it is better to administer to as many people as possible the first dose and then, further on, inoculate the second?

Let's start with certainties. The first, and the most solid, is that the body's immune response is not completely protective after a single dose of the vaccine. In fact, all clinical trials have shown that the total coverage given by the vaccine, the one we are referring to when we talk about efficacy, comes only between seven and ten days after receiving the second dose of the drug. Many other vaccines work like this: the one for hepatitis B, for example, includes three doses. The reason lies in the fact that the first administration initiates the immune system response, while the second (and any subsequent ones) serve to consolidate and enhance this response, structuring the body's immune memory to ensure that it lasts over time. >
Let the numbers do the talking. According to the report published in the New England Journal of Medicine by the researchers who made the Pfizer-BioNTech vaccine, for example, the average efficacy associated with a single dose reaches 90% immediately after administration (the short-term efficacy), but it then drops to 52.5% (with a very wide confidence interval, between 29.5% and 68.4%) before the second administration. A value far from 94.8% which is reached, however, seven days after the administration of the second dose.

It is for this reason that the European Medicines Agency (Ema) has recently reiterated, entering in the ongoing debate, that the maximum time frame of 42 days to elapse between the first and second dose of the Pfizer-BioNTech vaccine must be respected in order to obtain maximum protection: "Any modification to this provision", said the Agency, "would require a variation of the marketing authorization, as well as additional clinical data to support the modification, otherwise it would be considered an off-label use", which among other things entails fewer responsibilities for vaccine producers.

As for Moderna's vaccine (which, like Pfizer and BioNTech's, is an mRna vaccine), just approved by the EMA, at the moment, as explained by Antonio Cassone, member of the American Academy of Me crobiology, "the experimental design does not allow us to evaluate how effective the vaccine is after the first dose, given the low number of cases detected". Indeed, our Italian Medicines Agency (Aifa) also stresses that "immunity is considered fully acquired starting two weeks after the second administration, rather than one".

On the vaccine from AstraZeneca and Oxford situation is even more confusing. The decision to delay the second dose, made by the Medicines & Healthcare products Regolatory Agency (Mhra), the UK drug regulatory body, is based on an exploratory analysis of the vaccine results by AstraZeneca and Oxford. The British scientists, in particular, put together data from different efficacy trials of the preparation, comparing the vaccinated subjects and those who had received a placebo 22 days after the first administration and before receiving the second dose. In this way, as Science says, they found that the first dose of the vaccine was 73% effective in preventing the onset of Covid symptoms. Which seems strange enough, since AstraZeneca itself had declared a lower efficacy, equal to 62%, after the administration of both doses. In any case, this consideration led the British agency to endorse the delay in administering the second dose.

In addition to the EMA, however, the FDA is not of the same opinion. The US regulatory agency has recently made it known that delaying the administration of the second dose of the vaccine (referring specifically, however, to Pfizer's preparation) would be "premature and not solidly rooted in the evidence available to date". For their part, Pfizer and BioNTech confirmed that "the safety and efficacy of the vaccine was not evaluated over different administration times: most of the trial participants received the second dose within the window specified in the design of the study, and therefore there are no data to demonstrate that the protection obtained after the first administration is maintained even after 21 days ".

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