Heterologous vaccination as a strategy to minimize inequity in COVID-19 vaccine access: A modeling study in Thailand (preprint)

Background: Vaccinations are the best hope to control the COVID-19 pandemic and save lives. Due to the high demand and failure to share vaccines equitably, there were not enough vaccine supplies to cover the majority of people in low- and middle-income countries during the early stage of vaccination. To cope with this problem, Thailand, an upper-middle-income country, decided to employ a heterologous vaccination strategy as the primary COVID-19 vaccination regimen in the country. The CoronaVac (CV) vaccine was administered as the first dose, followed by the ChAdOx1 nCoV-19 (AZ) vaccine as the second dose. However, there is no study to assess the effectiveness of the heterologous vaccination employed in Thailand compared to the standard homologous vaccination. Methods: We delineated the course and timeline of COVID-19 vaccination in Thailand. An age-structured compartmental model for COVID-19 transmission and vaccination was constructed and employed to assess the effectiveness of the heterologous vaccination strategy. The impact of the vaccine prioritization strategies on COVID-19 mortality and infections was also investigated. Results: We found that the CV+AZ heterologous vaccination strategy outperforms the CV and AZ homologous vaccinations in reducing cumulative cases and deaths when combined with other non-pharmaceutical interventions. Furthermore, the results suggested that prioritizing vaccines for the elderly could be optimal in reducing COVID-19 mortality for a wide range of vaccination rates and transmission dynamics. Conclusions: Our modeling results suggested that to minimize inequity in COVID-19 vaccine access in low- and middle-income countries, those countries may use early accessible but maybe lower-efficacy vaccines as the first dose of heterologous vaccination in combination with higher-efficacy vaccines as the second dose.

Community vaccination can shorten the COVID-19 isolation period: an individual-based modeling approach (preprint)

Background: Isolation of infected individuals and quarantine of their contacts are usually employed to mitigate the transmission of SARS-CoV-2. While 14-day isolation of infected individuals could effectively reduce the risk of subsequence transmission, it also significantly impacts the patient's financial, psychological, and emotional well-being. It is, therefore, vital to investigate how the isolation duration could be shortened when effective vaccines are available and in what circumstances we can live with COVID-19 without isolation and quarantine. Methods: An individual-based modeling approach was employed to estimate the likelihood of secondary infections and the likelihood of an outbreak following the isolation of an index case for a range of isolation periods. Our individual-based model integrates the viral load and infectiousness profiles of vaccinated and unvaccinated infected individuals. The effects of waning vaccine-induced immunity against Delta and Omicron variant transmission were also investigated. Results: In the baseline scenario in which all individuals are unvaccinated, and no nonpharmaceutical interventions are employed, there is a chance of about 3% that an unvaccinated index case will make at least one secondary infection after being isolated for 14 days, and a sustained chain of transmission can occur with a chance of less than 1%. We found that at the outbreak risk equivalent to that of 14-day isolation in the baseline scenario, the isolation duration can be shortened to 7.33 days (95% CI 6.68-7.98) if 75% of people in the community are fully vaccinated during the last three months. In the best-case scenario in which all individuals in the community are fully vaccinated, isolation of infected individuals may no longer be necessary, at least during the first three months after being fully vaccinated, indicating that booster vaccination may be required after being fully vaccinated for three to four months. Finally, our simulations showed that the reduced vaccine effectiveness against Omicron variant transmission does not much affect the risk of an outbreak if the vaccine effectiveness against infection is maintained at a high level via booster vaccination. Conclusions: The isolation duration of a vaccine breakthrough infector could be safely shortened if a majority of people in the community are immune to SARS-CoV-2 infection. A booster vaccination may be necessary three months after full vaccination to keep the outbreak risk low.

Estimation of excess all-cause mortality due to COVID-19 in Thailand (preprint)

Thailand has experienced the most prominent COVID-19 outbreak, resulting in a new record for COVID-19 cases and deaths in 2021. To assess the influence of the COVID-19 outbreak on mortality, we estimated excess all-cause and pneumonia mortality in Thailand during the COVID-19 outbreak from April to October 2021. We used the previous five years’ mortality to estimate the baseline number of deaths using generalized linear mixed models (GLMMs). The models were adjusted for seasonality and demographics. We found that the estimated cumulative excess death was 14.3% (95% CI: 8.6%-18.8%) higher than the baseline. The results also showed that the excess deaths in males were higher than in females by approximately 26.3%. The excess deaths directly caused by the COVID-19 infections accounted for approximately 75.0% of the all-cause excess deaths. Furthermore, excess pneumonia deaths were also found to be 26.2% (95% CI: 4.8%-46.0%) above baseline. There was a significant rise in excess fatalities, especially in the older age groups. Therefore, the age and sex structure of the population are essential to assessing the mortality impact of COVID-19. Our modeling results could potentially provide insights into the COVID-19 outbreaks and provide a guide for outbreak control and intervention.

Reconstruction of the transmission dynamics of the first COVID-19 epidemic wave in Thailand (preprint)

Thailand was the first country reporting the first Coronavirus disease 2019 (COVID-19) infected individual outside mainland China. Here we delineated the course of the COVID-19 outbreak together with the timeline of the control measures and public health policies employed by the Thai government during the first wave of the COVID-19 outbreak in Thailand. Based on the comprehensive epidemiological data, we reconstructed the dynamics of COVID-19 transmission in Thailand using a stochastic modelling approach. Our stochastic model incorporated effects of individual heterogeneity in infectiousness on the disease transmission, which allows us to capture relevant features of superspreading events. We found that our model could accurately capture the transmission dynamics of the first COVID-19 epidemic wave in Thailand. The model predicted that at the end of the first wave, the number of cumulative confirmed cases was 3,091 (95%CI: 2,782 - 3,400). We also estimated the time-varying reproduction number (Rt) during the first epidemic wave. We found that after implementing the nationwide interventions, the Rt in Thailand decreased from the peak value of 5.67 to a value below one in less than one month, indicating that the control measures employed by the Thai government during the first COVID-19 epidemic wave were effective. Finally, effects of transmission heterogeneity and control measures on the likelihood of outbreak extinction were also investigated.