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

Abstract 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 and understanding this is critical to recommendations on 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 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 waning of booster vaccine response as early as >60 days as compared to those aged 18-49 years, who retained good response 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 level 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 elderly. mRNA and viral vector vaccines can be used flexibly to improve booster coverage.