In a recent study published in The Lancet Global Health journal, the researchers assessed the efficacy of homologous and heterologous booster doses of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines.
Coronavirus disease 2019 (COVID-19) vaccines have proved instrumental in curbing the mortality and morbidity caused globally by the pandemic. However, various studies have reported waning vaccine effectiveness against SARS-CoV-2 infections.
About the study
In the present study, the researchers investigated the effectiveness of CoronaVac, AZD1222, and BNT162b2 vaccine booster doses against COVID-19 infections, related hospitalizations, intensive care unit (ICU) admissions, and deaths in persons vaccinated with the primary series of CoronaVac.
The team used individual-level information obtained from an observational, prospective, national-level population from the Fondo Nacional de Salud (FONASA). Participants eligible for the study were individuals aged 16 years and above, who had an affiliation with FONASA, and were vaccinated with either the CoronaVac, AZD1222, or the BNT162b2 SARS-CoV-2 vaccines between 2 February 2021 and the end date of study of 10 November 2021, or had not been vaccinated during the specified period.
In Chile, individuals above 55 years could receive a booster dose of AZD1222, and those under 55 years could receive one BNT162b2 booster dose. The researchers analyzed the efficacy of homologous as well as heterologous vaccine booster doses by classifying the study participants into the following cohorts: (1) unvaccinated individuals; (2) individuals vaccinated with two doses of CoronaVac vaccine 14 days or more after the receipt of the second dose and before the third dose was administered, and (3) individuals vaccinated with three vaccine doses 14 days or more after receiving the third vaccine dose of either the homologous series of CoronaVac or a heterologous dose with either the BNT162b2 or the AZD1222.
The team mainly analyzed the effectiveness of booster vaccine doses in CoronaVac-vaccinated persons as opposed to the unvaccinated persons. They also compared the effectiveness of the homologous and the heterologous booster vaccines in comparison to the CoronaVac primary series of two vaccine doses.
The primary outcomes of the study included the number of confirmed COVID-19 cases, related hospitalizations, ICU admissions, and deaths. The team also considered the time between the beginning of the study to the date of COVID-19 symptom onset as the endpoint for the four primary outcomes. COVID-19 cases were defined as laboratory-confirmed SARS-CoV-2 infections and related deaths accompanied by code U07.1 in the International Classification of Diseases (ICD)-10-CM.
The study results showed that the analysis cohort comprised 11,806,589 individuals aged 16 years and above, among which a total of 11,174,257 individuals were eligible. As of 10 November 2021, almost 10,71,998 individuals were unvaccinated, and 678,331 were vaccinated with one dose of either the BNT162b2, CoronaVac, or the AZD1222 vaccine, while 94,23,928 had received two doses of either of the three COVID-19 vaccines. Among vaccinated persons with the primary series of COVID-19 vaccines, 74.5% received two doses of CoronaVac. Among these, a total of 186,946 were vaccinated with the third dose of CoronaVac, 2,019,260 with a BNT162b2 booster dose, and 1,921,340 with an AZD1222 booster dose.
The team also found a significant difference in the occurrence of COVID-19 infections with respect to the vaccinated status of the participants, gender, age, existing comorbidities, country of origin, residential area, and income. A total of 4,127,546 booster doses were administered after primary vaccination with two doses of the CoronaVac vaccine. Almost 95% of the booster doses received were heterologous, including 46.5% who received an AZD1222 booster dose and 48.9% who received a BNT162b2 booster dose, while 4.5% received a homologous booster dose with CoronaVac.
After primary vaccination with CoronaVac, the vaccine effectiveness of the CoronaVac booster dose was 63.8% against infection, 59.3% against related hospitalization, 71.2% against ICU admission, and 62.7% against death. The vaccine effectiveness of the BNT162b2 and AZD1222 booster doses were 93.5% and 88.4% against infection, 86.6% and 93.0% against related hospitalization, 84.1% and 95.9% against ICU admission, and 90.7% and 94.7% against death, respectively.
Overall, the study findings showed that a homologous or a heterologous COVID-19 booster vaccine dose in individuals vaccinated with the primary series of the CoronaVac vaccine induced a significant level of protection against COVID-19, related hospitalization, and death.