JMIR Publications

ABSTRACT

Background:

Investigations into the durability of COVID-19 vaccine heterologous effectiveness have primarily been conducted in high-income countries. Limited evaluations of heterologous vaccine policies in LMICs are partly due to inadequate population databases. In this study, we analyzed Thai national databases to address this knowledge gap. Our findings are poised to offer valuable insights for LMICs that initially employed heterologous vaccinations with inactivated virus vaccines as part of their pandemic response efforts.

Objective:

This study aimed to assess the durability of vaccine effectiveness (VE) for heterologous vaccine sequences in mitigating serious outcomes, specifically severe and fatal COVID-19 cases.

Methods:

We formed a dynamic cohort by linking records of Thai citizens aged ≥18 years from citizen vital, COVID-19 vaccine, and COVID-19 cases registry databases between May 2021 and July 2022. The encrypted CID was used to merge data between databases. The study focused on eight common heterologous vaccine sequences: CoronaVac/ChAdOx1, ChAdOx1/BNT162b2, CoronaVac/CoronaVac/ChAdOx1, CoronaVac/ChAdOx1/ChAdOx1, BBIBP-CorV/BBIBP-CorV/BNT162b2, ChAdOx1/ChAdOx1/BNT162b2, and ChAdOx1/ChAdOx1/mRNA-1273, utilizing a group of unvaccinated individuals for comparison purposes. The cohort was stratified by vaccination status, age, sex, province location, and month of vaccination and outcomes. Data analysis employed the Mantel-Haenszel method and logistic regression to determine relative risk and VE, accounting for potential confounders and the durability over time.

Results:

The study involved 52,580,841 individuals, with approximately 32% and 29% having received two-dose and three-dose common heterologous vaccines (not mutually exclusive), respectively. Two-dose heterologous vaccinations offered around 50% VE against severe and fatal COVID-19 for 2 months; however, protection significantly declined over time. Three-dose heterologous vaccinations sustained over 50% VE against both outcomes for at least 8 months, as determined by logistic regression with durability time interaction modeling. The vaccine sequence consisting of CoronaVac/CoronaVac/ChAdOx1 demonstrated over 80% VE against both outcomes, with no observed evidence of VE waning. The final monthly measured VEs (95% CI) of CoronaVac/CoronaVac/ChAdOx1 against severe and fatal COVID-19 at seven months after the last dose were 82% (80.3%, 84.0%) and 86.3% (83.6%, 84.0%).

Conclusions:

The observed durability of VE in heterologous COVID-19 vaccination supports the adoption of heterologous vaccination strategies by LMICs to prevent similar pandemics within their settings.