The current study draws on an active HIS network established in Chiang Mai, located in Northern Thailand, with a population of 1.6 million. Adult patients with confirmed COVID-19 during October – December 2021 and February– April 2022 time periods were included in the study. Molecular testing revealed 96.5% delta and 95.6% omicron lineage during 1st Oct 2021 – 31st Dec 2021 and 1st Feb 2022 – 30th Apr 2022 periods, respectively. Patients with tests done in Jan 2022 were excluded due to mixed delta-omicron lineage among samples (Omicron 75%, Delta 25%).
Non-Thai residents and migrants were excluded as the vaccination data and outcome capture for this group may be incomplete. Patients with missing age were also excluded. The patient selection flow is presented under Figure 1.
We have previously published the details on creating and implementing the information systems used in this study.25 In brief, all COVID-19 cases detected in Chiang Mai province are reported into the hospital information system of Chiang Mai Provincial Health Office (CMC-19 HIS), under the Communicable Disease Control Act (B.E. 2558) which mandates national reporting of all COVID-19 cases. CMC-19 HIS is a web-based reporting system which was launched since April 2021 and was created by Chiang Mai Provincial Health Office and Faculty of Public Health of Chiang Mai University. When a COVID-19 case is detected, the healthcare staff enter the patient details, including laboratory results into the CMC-19 HIS under a unique individual ID and this platform is also used for bed management in the overall province. The criteria for hospitalization were different during delta and omicron periods. Even mild cases who test positive for SARS-CoV-2 were admitted to hospital and remained for at least 7 days in hospital during delta period whereas asymptomatic or mild cases were treated as out-patient and isolated in designated places in community, community-isolation, or home-isolation since the beginning of omicron period. Data on severity and progression of the disease including requirement of ventilatory support and treatments are recorded in each hospital’s information system. Death cases reported to Chiang Mai Provincial Health Office are routinely updated in CMC-19 HIS.
All national vaccination records are available from the Ministry of Public Health Immunization Center (MOPH IC) database and deposited in the “Morprom” application, maintained by the Ministry of Public Health, Thailand.
The study was conducted on routine data collected as part of the national COVID-19 response under the Communicable Disease ACT (B.E. 2558) and was exempted from ethics review. Data were de-identified at source and analysed by Chiang Mai Provincial Health Office and Faculty of Public Health, Chiang Mai University.
We conducted a retrospective cohort study on Thai residents aged 18 years or older, with laboratory confirmed SARS-CoV-2 infection during 1st October – 31st December 2021 (Delta predominant) and 1st February– 30th April 2022 (Omicron predominant) time periods. Date of first positive SARS-CoV-2 test served as the index (baseline) date. If two positive SARS-CoV-2 tests were available for the same individual >90days apart, the second was considered a reinfection and the earlier episode was included in the analysis. Reinfections accounted for <0.01% of the total cohort.
Demographic data, baseline clinical characteristics were extracted from the CMC-19 hospital management platform. The types of COVID-19 vaccines, and dates of vaccinations were extracted from MOPH-IC immunization database.
Severe COVID-19 outcome was defined as requiring Invasive Mechanical Ventilation (IMV) during hospital admission or death during hospital admission. Records of all included subjects were followed till death, or up to 30 days from first positive SARS-CoV-2 test. The severe outcome capture for the study population is near complete as the clinical information of all hospitalised COVID-19 cases of the 26 public and 8 private hospitals in Chiang Mai province, including the only two tertiary care referral hospitals in Chiang Mai, are entered into a single CMC-19 HIS platform.
Descriptive statistics are reported separately for the subjects with and without severe COVID-19, stratified by delta and omicron predominance. A separate comparison was done comparing subjects during delta predominance as compared to omicron predominance to understand how the clinical characteristics and other risk factors differed between the periods. Continuous variables are summarized as mean and standard deviation (SD) or median and interquartile range (IQR) depending on the distribution. Categorical variables are summarized as frequency and percentages. Between group comparisons were done using Mann-Whitney-U test or t-test for continuous variables and Chi-squared test or Fisher’s exact test for categorical variables.
Cox proportional hazards regression was used to estimate hazard ratios (HRs) for severe COVID-19 and mortality outcomes, separately for omicron and delta predominance. Follow up period was taken from the first positive SARS-CoV-2 test date and censored at the earliest of: severe COVID-19 (either death or IMV), date of discharge for hospitalised or 30 days from first positive SARS-CoV-2 test date. If the outcome occurred on the first positive SARS-CoV-2 test date, the follow-up time was taken to be 0.5 days. Age, gender, calendar day of test (in weekly units), vaccination status and schedules, and time since last vaccine were added as factors in the regression model to estimate adjusted HRs (95% CI) for severe COVID-19 and mortality outcomes.
All statistical analyses were be conducted using stata (version 15.0 SE, College station, TX:StataCorp LP). Significance tests are 2 sided and a p-values <0.05 was considered statistically significant.