Durability of the Effectiveness of Heterologous COVID-19 Vaccine Regimens in Thailand: Retrospective Cohort Study Using National Registration Data.

BACKGROUND: The durability of heterologous COVID-19 vaccine effectiveness (VE) has been primarily studied in high-income countries, while evaluation of heterologous vaccine policies in low- and middle-income countries remains limited. OBJECTIVE: We aimed to evaluate the duration during which the VE of heterologous COVID-19 vaccine regimens in mitigating serious outcomes, specifically severe COVID-19 and death following hospitalization with COVID-19, remains over 50%. 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. Encrypted citizen identification numbers were used to merge the data between the databases. This study focuses on 8 common heterologous vaccine sequences: CoronaVac/ChAdOx1, ChAdOx1/BNT162b2, CoronaVac/CoronaVac/ChAdOx1, CoronaVac/ChAdOx1/ChAdOx1, CoronaVac/ChAdOx1/BNT162b2, BBIBP-CorV/BBIBP-CorV/BNT162b2, ChAdOx1/ChAdOx1/BNT162b2, and ChAdOx1/ChAdOx1/mRNA-1273. Nonimmunized individuals were considered for comparisons. The cohort was stratified according to the vaccination status, age, sex, province location, month of vaccination, and outcome. Data analysis employed logistic regression to determine the VE, accounting for potential confounders and durability over time, with data observed over a follow-up period of 7 months. RESULTS: This study includes 52,580,841 individuals, with approximately 17,907,215 and 17,190,975 receiving 2- and 3-dose common heterologous vaccines (not mutually exclusive), respectively. The 2-dose heterologous vaccinations offered approximately 50% VE against severe COVID-19 and death following hospitalization with COVID-19 for 2 months; however, the protection significantly declined over time. The 3-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 >80% VE against both outcomes, with no evidence of VE waning. The final monthly measured VE of CoronaVac/CoronaVac/ChAdOx1 against severe COVID-19 and death following hospitalization at 7 months after the last dose was 82% (95% CI 80.3%-84%) and 86.3% (95% CI 83.6%-84%), respectively. CONCLUSIONS: In Thailand, within a 7-month observation period, the 2-dose regimens could not maintain a 50% VE against severe and fatal COVID-19 for over 2 months, but all of the 3-dose regimens did. The CoronaVac/CoronaVac/ChAdOx1 regimen showed the best protective effect against severe and fatal COVID-19. The estimated durability of 50% VE for at least 8 months across all 3-dose heterologous COVID-19 vaccine regimens supports the adoption of heterologous prime-boost vaccination strategies, with a primary series of inactivated virus vaccine and boosting with either a viral vector or an mRNA vaccine, to prevent similar pandemics in low- and middle-income countries.

Economic Evaluation of Evusheld for Preexposure Prevention of COVID-19 in High-Risk Populations: Early Evidence from Thailand.

BACKGROUND AND AIMS: The introduction of Coronavirus disease 2019 (COVID-19) vaccines urged all Thais to seek prevention of serious illness and death from COVID-19. However, immunocompromised individuals might not be able to achieve an efficient immune response from these vaccines. This study aimed to evaluate the cost-effectiveness and budget impact of introducing Evusheld (tixagevimab plus cilgavimab) for three patient groups-organ transplant, autoimmune disease, and dialysis patients, from the Thai government perspective. METHODS: A Markov decision model was developed to compare the use of Evusheld plus COVID-19 vaccines versus COVID-19 vaccines alone. The methodology followed the National HTA Guidelines of Thailand. Model input parameters were collected locally from retrospective data and from a literature review. RESULTS: Evusheld helped prevent COVID-19 infection, severe infection, and death in all three patient groups. Using the Thai threshold of 160,000 Thai Baht (THB) per quality-adjusted life year (QALY) gained, the only scenario found to be cost-effective was that of dialysis patients with inadequate immune response, with an incremental cost-effectiveness ratio (ICER) of 54,700 THB per QALY gained. To make a policy of Evusheld provision cost-effective in other groups, the price of Evusheld had to be lower (a reduction of 44-88% of its current price). The results of one-way sensitivity analysis indicated that the cost-effectiveness of Evusheld was sensitive to changes in the rate of infection, cost and efficacy of Evusheld, proportion of inadequate immune responses, and the probability of moving from a 'recovered' to 'susceptible' status. CONCLUSION: Among three COVID-19-vaccinated immunocompromised patient populations, this study concluded that Evusheld was cost-effective for dialysis patients with inadequate immune response to the COVID-19 vaccine.