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A 2-dose CoronaVac vaccine schedule elicits a good immune memory. A primary 3rd dose administered two months following the 2nd dose elicits slightly increased antibody titres when compared to the primary 2 doses.
In healthy adults aged eighteen years and older, primary vaccination with two doses of CoronaVac (an inactivated vaccine against coronavirus disease) was found to generate good immune memory. Although the neutralizing antibody titres dropped to near or below the lower limit of seropositivity six months following the 2nd dose, a 3rd dose administered eight months following the 2nd dose illustrated high effectiveness at recalling coronavirus-specific immune response, resulting in a substantial rebound in the levels of antibody, as elucidated from a study published in The Lancet Infectious Diseases.
Researchers undertook this study for exploring the immune persistence of a 2-dose schedule of CoronaVac, and the safety and immunogenicity of the 3rd dose of CoronaVac in healthy adult participants. In the first half of two single-centre, randomized, double-blind, placebo-controlled phase II clinical trials, participants aged 18–59 years were divided into 2 vaccination schedule cohorts: (i) Cohort 1: A day 0 and day 14 vaccination cohort, and (ii) Cohort 2: A day 0 and day 28 vaccination cohort.
Each of the cohorts was randomized to either placebo or 3 μg dose or 6 μg dose of vaccine. After amending the protocol, half of the subjects in each group were assigned to get an additional dose twenty-eight days (window period thirty days) following the 2nd dose. On the other hand, the other half were assigned to get a 3rd dose six months (window period sixty days) following the 2nd dose.
Considering the other phase II trial, subjects aged sixty years and more were segregated sequentially to get 3 injections of either placebo or 1·5 μg, 3 μg, or 6 μg of vaccine, given twenty-eight days apart for the initial 2 doses and six months (window period ninety days) apart for doses 2 and 3. The endpoints of the study included geometric mean increases (GMIs), geometric mean titres (GMTs), and seropositivity of neutralizing antibody to coronavirus, as evaluated in the per-protocol population (all the people who finished their assigned 3rd dose). The reporting is focused on the 3 μg groups (the licensed formulation).
Notably, 90% (540/600) of people (aged 18–59 years) exhibited eligibility to get a 3rd dose. Out of this, 50% (269/540) were given the primary 3rd dose two months following the 2nd dose (cohorts 1a-14d-2m and 2a-28d-2m) and 50% (271/540) were given a booster dose eight months following the 2nd dose (cohorts 1b-14d-8m and 2b-28d-8m). Considering the 3μg cohort, the neutralizing antibody titres elicited by the initial 2 doses dropped following six months to near or below the seropositive cutoff (GMT of 8) for cohort 2b-28d-8m (n=49; GMT 6·8) and for cohort 1b-14d-8m (n=53; GMT 3·9).
Following administration of a booster dose eight months after the 2nd dose, the GMTs evaluated 14 days later raised to 137·9 for cohort 1b-14d-8m and 143·1 twenty-eight days later for cohort 2b-28d-8m. A moderate rise in GMTs was noted following a primary 3rd dose, from 21·8 on day twenty-eight following the 2nd dose to 45·8 on day twenty-eight following the 3rd dose in cohort 1a-14d-2m (n=54), and from 38·1 to 49·7 in cohort 2a-28d-2m (n=53).
The GMTs were noted to decay to near the positive threshold by six months following the 3rd dose: GMT 10·0 in cohort 2a-28d-2m and GMT 9·2 in cohort 1a-14d-2m. Likewise, in people (age sixty years and above) who were given booster doses, (87% [303/350] people had eligibility to get a 3rd dose), the neutralising antibody titres dropped to near or below the seropositive threshold by six months following the primary two-dose series.
A 3rd dose that was administered eight months following the 2nd dose remarkably raised neutralizing antibody concentrations: GMTs elevated from 42·9 on day twenty-eight following the 2nd dose to 158·5 on day twenty-eight after the 3rd dose (n=29). All the side effects noted within twenty-eight days following a 3rd dose were of grade I or II severity in all the vaccination groups. Notably, 3 severe noxious reactions (2%) were noted by 150 subjects in group 1a-14d-2m, 4 (3%) by 150 subjects from group 1b-14d-8m, 24 (7%) by 349 subjects from group 3-28d-8m, and 1 (1%) by 150 subjects in each of the groups 2a-28d-2m and 2b-28d-8m.
Thus, the 3rd dose of
CoronaVac in people given eight months following the 2nd dose led to
a strong boost in immunity against coronavirus, that had considerably dropped
six months following 2 doses of vaccine, resulting in a significant rise in the
antibodies concentration. A homologous booster dose may offer longer-lasting
immunity and greater levels of protection when compared to the two-dose
schedule. However, additional study is required for monitoring neutralization
ability and efficacy against the variants.
The Lancet Infectious Diseases
Immunogenicity and safety of a third dose of CoronaVac, and immune persistence of a two-dose schedule, in healthy adults: interim results from two single-centre, double-blind, randomised, placebo-controlled phase 2 clinical trials
Gang Zeng et al.
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