Nevertheless, in almost every scenario there remains the need of repeated vaccination, once again, driving the advantages of reducing and mixing the doses. As this was a pilot study, the main limitation is the sample size. 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. = 45) and 1230 BAU/mL (= 13) ITE in the reference and study group, respectively. Open in a separate window Figure 1 Boxplot (median, hinges: first and third quartiles, whiskers: the largest value no further than 1.5 * IQR from the hinge) showing humoral response to vaccination as measured with antibody levels in the full-dose and mixed-dose groups on the day of the second dose (A) and 8C10 days after the second dose for participants whose results did not exceed the test detection limit (B). 4. Discussion Rapid population vaccination appears ITE to be the only available solution leading to a return to normalcy. Currently, 63.8% of the global population have received at least one dose of a COVID-19 vaccine, compared with only 14.1% of people in low-income countries. Even less are fully vaccinated . This drives a conclusion that the main problem of vaccine strategy is the vaccine inequity [21,22]. Currently, the International Rescue Committee (IRC) notes that countries such as Burkina Faso CLTB and Niger, already requiring significant humanitarian assistance, saw a 31% and 22% increase in COVID deaths, respectively, whereas only 13% of doses contracted by COVID-19 Vaccines Global Access (COVAX, the program supporting global vaccination) have been delivered . Due to vaccine inequity, the Omicron variant spreads the world and wealthy nations consistently prioritize boosters for themselves. Meanwhile, people of low-income countries are still being ignored . For instance, it was calculated that high-income countries have already administered 69 times more doses than Bangladesh . Moreover, it was estimated that approximately 50 million doses in the rich-country stockpile are on the point of expiring before they can be donated and, due to over-buying, the unused stockpile will keep growing well into next year . With the support of the Strategic Advisory Group of Experts (SAGE) in Immunization and its COVID-19 Working Group, WHO published the statement encouraging research in vaccine reduction . We have already demonstrated no differences in humoral response between the groups vaccinated with half and the recommended dose of the BNT162b2 vaccine in subjects under 55 years of age . For now, the main goal is to test the possibility of using vaccines interchangeably. This could be useful in vaccination strategies especially in low-income countries. Currently, it is advised to preferentially use subsequent doses ITE in a vaccination series from the same manufacturer where possible . ITE On the other hand, for people aged 18 years and older, mRNA vaccines are recommended as a booster dose, regardless of the type of vaccine received for the first two doses. COV-BOOST, a multicenter, randomized, controlled, phase 2 trial of third-dose booster vaccination against COVID-19, was conducted in the United Kingdom. Participants were initially vaccinated with two doses of Pfizer/BioNTech BNT162b2 COVID-19 vaccine (Comirnaty, Pfizer/BioNTech, Mainz, Germany) or ChAdOx1 nCoV-19 COVID-19 vector vaccine (Vaxzevria, AstraZeneca, S?dert?lje, Sweden). All tested vaccines showed acceptable reactogenicity and side-effect profiles. However, this study did not evaluate the safety and efficacy of mixed vaccines for the first two doses . Nevertheless, conducted studies demonstrated that immunogenicity can be sustained with vaccines made by different manufacturers used in the same individual to complete a vaccination program for certain diseases . The present study compares the humoral response to the half dose of mixed vaccinesPfizer/BioNTech BNT162b2 COVID-19 vaccine (Comirnaty, Pfizer/BioNTech, Mainz, Germany) and ChAdOx1 nCoV-19 COVID-19 vector vaccine (Vaxzevria, AstraZeneca, S?dert?lje, Sweden)and the recommended dose of the BNT162b2 vaccine administered to adults up to 55 years old. The obtained results demonstrated that the distribution of antibody levels was not significantly different between those who received the Pfizer vaccine.