SARS-CoV-2 Antibody Level after Homologous and Heterologous Booster Vaccines: an 18-month Longitudinal Observational Cohort Study in Indonesia

Recently, there have been reports of the rise of COVID-19 cases in several sites. The effectiveness of the COVID-19 vaccine was reported elsewhere. There are still questions on how the kinetics of antibody response during relatively long periods, the need for additional doses, and the effect of homologous and heterologous boosters. The study was conducted to analyze the kinetics of antibody response after the primary dose and the third dose of the ChAdOx1 vaccine in individuals previously receiving two doses of the ChAdOx1 (homologous) and CoronaVac (heterologous) COVID-19 vaccines.

The study population comprised 52 men and 98 women, divided into CoronaVac Recipients and ChAdOx1 Recipients for the first two doses according to the recommended schedule by the Ministry of Health of Indonesia (MoH).Six months after the second dose, the third dose of ChAdOx1 was administered as a homologous and heterologous booster. COVID-19 antibody levels were measured by the CMIA method before the first dose (time-point or TP1), two weeks after the first dose (TP2), before the second dose (TP3), 1 month after the second dose (TP4), 12 months after the second dose (TP5), and 18 months (TP6) after the second dose administration. Six months after the second dose, the third dose of ChAdOx1 was administered as a homologous and heterologous booster. Along with these, several epidemiological data were collected from subjects on TP1.

A total of 153 serum samples were collected from subjects who had received the third dose, and the antibody response was measured. On TP1, COVID-19 antibody reactivity (the level was >50 AU/mL) was detected on 100 (66,67%) of subjects, indicating a possible previous exposure to SARS-CoV-2. On TP2, the sharp increase in antibody level was documented in the ChAdOx1 group. However, in the following data during the cohort, the gap was narrowing, and on the TP6, the antibody levels showed no significant difference between groups (p>0.05). Likewise, no significant differences were shown between groups with or without a history of COVID-19 antibody reactivity on TP1 (p>0.05). Considering epidemiological characteristics, no significant differences were documented based on sex, age groups, and BMI level.

This study provides a deeper understanding of the kinetics of antibody levels longitudinally among those with and without previous history of SARS CoV-2 infection, among the recipients of different vaccines, and the recipients of homologous and heterologous boosters. It is necessary to elucidate further in the next study how the level of antibody reflects the neutralizing antibody level as an indicator of protection against the infection risk.