In both cases, individuals may have benefited from a booster (third dose), restoring high anti-spike IgG levels 4. The rapid dissemination of the Omicron VoC has generated interest in the duration of immunity induced only by complete vaccination (two doses) or followed by a viral infection, referred to as hybrid immunity. There was widespread use of CoronaVac or ChAdOx1 for the first two doses followed by BNT162b2 or ChAdOx1 for boosting and very few people received BNT162b2 as the first dose. Furthermore, many Omicron lineages have been identified throughout 2022, including BA.1, BA.1.1, BA.2, BA.2.75, BA.3, BA.4, BA4.6, BA.5, and BQ.1, and BQ.1.1, which show common antigenic characteristics 3.Īs in many other countries, Brazil’s COVID-19 immunization program implemented heterologous vaccination.

From September 2020 to 2021, several variants of concern (VoCs) emerged, exhibiting mutations in the spike protein and displaying a high potential for transmission and immune evasion. Low adherence to social distancing measures, unequal global distribution of vaccines, and low vaccination rates have favored SARS-CoV-2 dissemination and the emergence of virus variants. In turn, adenovirus-based vaccines, such as ChAdOx1 (AstraZeneca) and the single-dose (Janssen) and inactivated virus vaccines, such as CoronaVac (Sinovac Biotech), showed intermediate protection rates of around 70% 2. Such vaccines have proven safe, but differences are observed in their efficacy in preventing symptomatic COVID-19, with mRNA vaccines such as BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) exhibiting the highest vaccine efficacy, usually over 90% 2. Most approved COVID-19 vaccines fall into three groups: inactivated virus, adenoviral vector, and messenger RNA (mRNA). Initially, immunization strategies relied mainly on the availability of vaccines approved by government agencies after controlled clinical trials.

The introduction of vaccination against Coronavirus disease 2019 (COVID-19) in December 2020 has dramatically reduced the pressure on health services worldwide by preventing morbidity, hospitalizations, and death 1.

Waves 13 complete