Waning vaccine response to severe COVID-19 outcomes during omicron predominance in Thailand.

BACKGROUND: The COVID-19 pandemic has evolved quickly, with different variants of concern resulting in the need to offer continued protection through booster vaccinations. The duration of enhanced protection with booster doses against severe COVID-19 is still unclear. Understanding this is critical to recommendations on the frequency of future booster doses. METHODS: Utilising a Hospital Information System for COVID-19 established in Chiang Mai, Thailand, we conducted a cohort study by linking patient-level data of laboratory-confirmed COVID-19 cases to the national immunization records, during the omicron predominant period (1 February- 31 July 2022). RESULTS: Out of 261,103 adults with COVID-19 included in the study, there were 333 (0.13%) severe COVID-19 cases and 190 (0.07%) deaths. Protection against severe COVID-19 was highest with boosters received >14-60 days prior to positive test (93%) and persisted at >60-120 days (91%) but started to wane at >120-180 days (77%) and further at >180 days (68%). The rate of waning differed with age. Those ≥70 years showed faster waning of booster vaccine responses as compared to those aged 18-49 years, who retained good responses up to 180 days. Equivalent risk reduction against severe COVID-19 was seen with all the vaccine types used as boosters in Thailand. CONCLUSIONS: Booster doses provided high levels of protection against severe COVID-19 with omicron, up to 4 months. Repeat boosters will be required to continue protection beyond 4 months, particularly in the elderly. mRNA and viral vector vaccines can be used flexibly to improve booster coverage.

Effectiveness of heterologous third and fourth dose COVID-19 vaccine schedules for SARS-CoV-2 infection during delta and omicron predominance in Thailand: A test-negative, case-control study.

Background: The Coronavirus disease 2019 (COVID-19) pandemic has evolved quickly, with numerous waves of different variants of concern resulting in the need for countries to offer continued protection through booster vaccination. To ensure adequate vaccination coverage, Thailand has proactively adopted heterologous vaccination schedules. While randomised controlled trials have assessed homologous schedules in detail, limited data has been reported for heterologous vaccine effectiveness (VE). Methods: Utilising a unique active surveillance network established in Chiang Mai, Northern Thailand, we conducted a test-negative case control study to assess the VE of heterologous third and fourth dose schedules against SARS-CoV-2 infection among suspect-cases during Oct 1-Dec 31, 2021 (delta-predominant) and Feb 1-Apr 10, 2022 (omicron-predominant) periods. Findings: After a third dose, effectiveness against delta infection was high (adjusted VE 97%, 95% CI 94-99%) in comparison to moderate protection against omicron (adjusted VE 31%, 95% CI 26-36%). Good protection was observed after a fourth dose (adjusted VE 75%, 95% CI 71-80%). VE was consistent across age groups for both delta and omicron infection. The VE of third or fourth doses against omicron infection were equivalent for the three main vaccines used for boosting in Thailand, suggesting coverage, rather than vaccine type is a much stronger predictor of protection. Interpretation: Appropriately timed booster doses have a high probability of preventing COVID-19 infection with both delta and omicron variants. Our evidence supports the need for ongoing national efforts to increase population coverage of booster doses. Funding: This research was supported by the National Research Council of Thailand (NRCT) under The Smart Emergency Care Services Integration (SECSI) project to Faculty of Public Health Chiang Mai University.

Reduction in severity and mortality in COVID-19 patients owing to heterologous third and fourth-dose vaccines during the periods of delta and omicron predominance in Thailand.

OBJECTIVES: The COVID-19 pandemic has evolved quickly, with different variants of concern resulting in the need for countries to offer booster vaccinations. Although studies have assessed homologous schedules in detail, the effectiveness of heterologous booster vaccine schedules against severity and mortality with newer variants remains to be explored fully. METHODS: Utilizing a Hospital Information System for COVID-19 established in Chiang Mai, Thailand, we conducted a cohort study by linking patient-level data on laboratory-confirmed COVID-19 cases to the national immunization records, during delta-predominant and omicron-predominant periods. RESULTS: Compared to omicron, COVID-19 cases during the delta period were 10 times more likely to have severe outcomes and in-hospital deaths. During omicron, a third vaccine dose had an 89% reduced risk of both severe COVID-19 and death. The third dose received 14-90 days before the date of the positive test showed the highest protection (93%). Severe outcomes were not observed with the third dose during delta, and the fourth dose during the omicron period. All the vaccine types used for boosting in Thailand offered similar protection against severe COVID-19. CONCLUSION: Booster doses provided a very high level of protection against severe COVID-19 outcomes and deaths. Booster campaigns should focus on improving coverage by utilizing all available vaccines to ensure optimal protection.