Why do we need another dose of the Covid vaccine if the first one is effective

It would seem logical to take it further: if the first dose generates an immunity strong enough to safely wait for the second, and if the urgency of the health crisis justifies contradicting the manufacturer’s indications in terms of deadlines, does itwhy not skip the second dose directly and double the availability of vaccines? Even more when they arrive single-dose drugs, such as Janssen’s, with an apparently less effective but perfectly acceptable.

However, the possibility of canceling the second inoculation is not on the table; the CEO of Pfizer indicated that, in fact, we will surely need a third one in a year. “Why are the messenger RNA vaccines, Pfizer and Moderna, given two doses and not one? Well, because in phase I or phase II trials it was observed that after the first dose only half of the participants generated neutralizing antibodies or were produced at very low levels“, Explain Pepe Alcamí, from the AIDS Immunopathology Unit of the National Center for Microbiology.

Alcamí is one of the participants in the ‘CombiVacs’ study coordinated by the Carlos III Health Institute (ISCIII) and 600 patients from five large Spanish hospitals. The aim is to see if people who received a first dose of AstraZeneca before it was discouraged for their age group due to a rare risk of thrombi would benefit from a second dose with Pfizer compared to the results of a single ‘prick’. The threshold of success of this reinforcement or boost it would be the “intensification significant of the humoral immune response “; namely, a 33% increase in antibodies in the blood plasma.

“Clinical trials are research tools put at the service of public health but they are not exactly health care. They aim to solve questions, if possible clearly and clearly,” explained Jesús Antonio Frías Inhiesta, coordinator at the ‘CombiVacs’ presentation of the clinical research network of the Carlos III Health Institute and Head of the Clinical Pharmacology Service of the Hospital Universitario La Paz. “The questions, in this case, are: if a second dose of AstraZeneca is necessary; if so, when; and what magnitude of immunization is gained by this boost or souvenir dose“.

As Cristian Smerdou, director of the Gene Therapy and Regulation of Gene Expression Program at the University of Navarra, explained to EL ESPAÑOL, the booster dose or booster aims intensify the immune response and prolong it over time. However, the approaches to achieving this are not the same, and at such an early stage of vaccination globally, it is unknown which would be the most effective. Current drug application guidelines, as Alcamí recalled, are based on clinical trials. But the actual effectiveness in countries like Israel of the first dose has surprised even the most optimistic.

Thus, the ‘recall’ of AstraZeneca is performed with the same adenovirus from the first dose, but a study published in The Lancet validated the use of a different adenovirus to achieve broader immunity in the second, as the Russian Sputnik-V vaccine already does. Studies such as ‘CombiVacs’ will help determine the suitability of these boosts less orthodox. “Given the variants of the coronavirus, the efficacy of some vaccines is superior to that of others. Using heterologous guidelines, that is, mixed, can allow better coverage“, explains the Dr. Magda Campins, Head of Preventive Medicine and Epidemiology at the Vall d’Hebron University Hospital.

First row antibodies

While Alcamí’s team will assess the qualitative response of the first dose to the coronavirus, the quantitative work corresponds to Dr. Mayte Pérez Olmeda, rResponsible for the serology laboratory of the National Center for Microbiology. “Vmasters to measure participants’ antibody levels using a commercial chemiluminescence-based assay. The antibodies that may be present in the serum sample will recognize the antigen and, through an enzymatic reaction, will produce light. “

Is about the so-called anti-RBD antibodies, generated after the first administration of the vaccine and those with “the greatest neutralizing capacity,” according to Pérez Olmeda. “The test will allow us to determine the antibodies at 12 months, and that is very important. We will be able to know if this increase occurs, when it occurs, if it is going to be maintained constantly or if it is going to occur. sometime a decline. “

Although the first results of ‘CombiVacs’ will be available in 14 days and the decision on the boost heterologous will be taken within 28 days, follow-up for one year will allow assessing whether the reinforcements and the combination of doses are essential before the appearance of new variants, which will continue to appear as a reaction to increasing population immunization. The fight against the pandemic, in any case, is a long-distance race.


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