The second dose with Pfizer’s Covid vaccine after Astrazeneca induces a “robust immune response“In the face of a reactogenicity profile (local or systemic reactions)”acceptable and manageable“. This is what emerges from the results of the clinical trial CombiVacs, one of the three studies on which the opinion of the Technical Scientific Commission (CTS) of the Italian Medicines Agency Aifa was based on heterologous vaccination for people under 60 years of age. The experiment, already available in preprint since last May 27, has passed the process of peer review – the peer review system adopted by the most authoritative scientific journals – and was published in The Lancet on 25 June 2021 in the article “Immunogenicity and reactogenicity of BNT162b2 booster in ChAdOx1-S-primed participants (CombiVacS): a multicentre, open-label, randomised, controlled, phase 2 trial”.

Pfizer after Astrazeneca increases the immune response

The study, conducted in five university hospitals in Spain (the Hospital Universitario de Cruces of Biscay; the Hospital Universitari Vall d’Hebron and the Hospital Clínic of Barcelona; the Hospital Clínico San Carlos and the Hospital Universitario La Paz of Madrid), evaluated the use of the Pfizer vaccine at 8-12 weeks from the first dose of Astrazeneca, involving a total of 676 adults aged between 18 and 60 years, of which 441 in the heterologous vaccination arm. Analysis, the first to evaluate the immune and cellular response of the vaccine mix against Sars-Cov-2, was supported by the Spanish research institute Carlos III, following the need for data in support of the heterologous strategy, the interest of which derives from the appearance of rare, but serious, associated thrombosis a platelet deficiency after vaccination with Astrazeneca, more frequently in young people.

As stated, the heterologous approach proved to be particularly advantageous. “Use of Pfizer’s vaccine as a second dose in subjects vaccinated with Astrazeneca induced a robust immune responsePointed out the researchers who observed a significant increase both the concentration of antibodies directed against the receptor binding domain (RBD) of Sars-Cov-2, and the immunoglobulin G specific for the Spike protein that the pathogen uses to infect cells.

To confirm the validity of the heterologous approach also the increase in cellular response, as well as the results relating to the safety profile, evaluated in terms of local and systemic side effects that occurred. Reactions were predominantly mild in most (68%) or moderate (30%) of people. “Most frequently they consisted of pain (88%) and induration (35%) at the injection site, headache (44%) and myalgia (43,) – reads the results of the study -. No serious adverse events were reported”.

“In summary – the researchers conclude – our study shows un robust humoral and cellular immune response 14 days after the second dose of Pfizer in individuals initially vaccinated with Astrazeneca 8-12 weeks earlier, with an acceptable and manageable reactogenicity profile. Further evaluations are ongoing, therefore the results of this and future studies comparing homologous and heterologous vaccination programs will allow for direct comparisons and substantiate decision making. on Covid-19 vaccination”.

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