The allocation of COVID-19 vaccines and antivirals against emerging SARS-CoV-2 variants of concern in East Asia and Pacific region: A modelling study

Abstract

Background: In view of emerging variants of concern (VOCs), we aimed to evaluate the impact of various allocation strategies of COVID-19 vaccines and antiviral such that the pandemic exit strategy could be tailored to risks and preferences of jurisdictions in the East Asia and Pacific region (EAP) to improve its efficiency and effectiveness.

Methods: Vaccine efficacies were estimated from the titre distributions of 50% plaque reduction neutralization test (PRNT50), assuming that PRNT50 titres of primary vaccination decreased by 2-10 folds due to antibody waning and emergence of VOCs, and an additional dose of vaccine would increase PRNT50 titres by 3- or 9-fold. We then used an existing SARS-CoV-2 transmission model to assess the outcomes of vaccine allocation strategies with and without the use of antivirals for symptomatic patients in Japan, Hong Kong, and Vietnam.

Findings: Increasing primary vaccination coverage was the most important contributing factor in reducing the total and peak number of COVID-19 hospitalisations, especially when population vaccine coverage or vaccine uptake among older adults was low. Providing antivirals to 50% of symptomatic infections only further reduced total and peak hospitalisations by 10-13%. The effectiveness of an additional dose of vaccine was highly dependent on the immune escape potential of VOCs and antibody waning, but less dependent on the boosting efficacy of the additional dose.

Interpretation: Increasing primary vaccination coverage should be prioritised in the design of allocation strategies of COVID-19 vaccines and antivirals against emerging VOCs, such as Omicron, in the EAP region. Heterologous vaccination with any available vaccine as the additional dose could be considered when planning pandemic exit strategies tailored to the circumstances of EAP jurisdictions.

Funding: Health and Medical Research Fund, General Research Fund, AIR@InnoHK.

Keywords: Allocation strategy; Antibody waning; Antiviral; Booster vaccination; COVID-19; Delta variant; Immune escape; Omicron variant; Pandemic exit strategy; SARS-CoV-2; VOC; Vaccination; Variant; Variant of concern.

Figure 1

Estimated vaccine efficacies of BNT162b2, AZD1222 and CoronaVac vaccines from PRNT50 titre distributions. The mean PRNT50 titres of vaccinees against the hypothetical variants of concern were assumed to decrease by 2, 4, 7, and 10 folds respectively compared with the PRNT50 titres against the original virus strain. The vaccine efficacies were estimated by bootstrapping the PRNT50 titres of 100 vaccinees by 1000 times. The error bars showed the 95% CI of the estimates with the uncertainty from PRNT50 titre distributions. (A) Vaccine efficacy in reducing susceptibility to infection (σ_m). (B) Vaccine efficacy in reducing infectiousness if infected (σ_t). (C) Vaccine efficacy in reducing symptomatic disease and hospital admission (σ_s).

Figure 2

Comparative effectiveness of allocation strategies of vaccines and antivirals in Japan assuming all vaccinees received the BNT162b2 vaccines. As of 1 August 2021, 39.7% of the population had taken at least first dose of vaccine and all of them received the BNT162b2 vaccines. We assumed that the effective reproductive number before the vaccination program (R_e) was 6 and that the effects of vaccination (characterized by σ_m, σ_t, and σ_s) were realized instantaneously after the target vaccine uptake was achieved. For Strategy 4, the additional dose would be given to vaccinees with the original PRNT50 titre lower than 74.1, which corresponded to the estimated thresholds of 70% protection against the original virus. The epidemics were seeded with one introduction event immediately after the target vaccine uptake was achieved. We assumed PRNT50 titres reduced by 4, 7 or 10 folds due to VOCs and one dose of booster vaccine increased the PRNT50 titres by 3 or 9 folds (i.e., b = 3 or 9). We assumed the target vaccination coverage (c) was 70%, 80% or 90%. (A) The total number of hospitalisations per thousand population. (B) The maximum daily number of hospitalisations per thousand population. (C) The number of hospitalisations averted per thousand vaccine doses, compared with the existing vaccine uptake as of 1 August 2021 (i.e., about 39.7% coverage).

Figure 3

Comparative effectiveness of allocation strategies of vaccines and antivirals in Hong Kong assuming 60% vaccinees received the BNT162b2 vaccines and 40% received CoronaVac vaccines. (A) The total number of hospitalisations per thousand population. (B) The maximum daily number of hospitalisations per thousand population. (C) The number of hospitalisations averted per thousand vaccine doses, compared with the existing vaccine uptake as of 1 August 2021 (i.e., about 44.5% coverage).

Figure 4

Comparative effectiveness of allocation strategies of vaccines and antivirals in Vietnam assuming 40%, 30% and 30% of vaccinees received BNT162b2, ADZ1222 and CoronaVac vaccines respectively. (A) The total number of hospitalisations per thousand population. (B) The maximum daily number of hospitalisations per thousand population. (C) The number of hospitalisations averted per thousand vaccine doses, compared with the existing vaccine uptake as of 1 August 2021 (i.e., about 6% coverage).