Cross-Reactivity between Half Doses of Pfizer and AstraZeneca Vaccines-A Preliminary Study.

Media reports have caused a significant drop in confidence in the AstraZeneca ChAdOx1 nCoV-19 COVID-19 vector vaccine (Vaxzevria, AstraZeneca Södertälje, Sweden). This has caused many people, already vaccinated with the first dose of AstraZeneca, to refuse vaccination with this product. On the other hand, the increased demand for mRNA vaccines has resulted in a greater shortage of mRNA vaccines and cases of people being vaccinated with the AstraZeneca vaccine after the first dose of the Pfizer/BioNTech BNT162b2 COVID-19 vaccine (Comirnaty, Pfizer/BioNTech, Mainz, Germany). Moreover, currently, 60.9% of the global population have received at least one dose of a COVID-19 vaccine, while only 10% of people in low-income countries have received at least one dose. Even less people are fully vaccinated. The present pilot study evaluated the administration of half doses of AstraZeneca and Pfizer vaccines and included the enrollment of 26 subjects who were vaccinated with a different vaccine the first and second time. The reference group included individuals undergoing vaccination with two full doses of the Pfizer vaccine (21-day interval) monitored for their antibody levels as part of a parallel study. The distribution of antibody levels was not significantly different between those who received the Pfizer vaccine alone and those receiving the AstraZeneca vaccine plus Pfizer or Pfizer and AstraZeneca. To prepare for the next pandemic waves, solving the problem of the matching of booster vaccine to the previously received doses would be advisable. The topic is important and emerging as most of the population in low-income countries is still not vaccinated. We strongly believe that vaccine equity is the most important aspect of vaccination strategies.