A fourth dose of the AstraZeneca Covid-19 vaccine was found to be around 75 percent effective against infection of the highly transmissible Omicron variant, according to a real-world evidence study conducted by the Chiang Mai University in Thailand.
The study was designed to "evaluate the vaccine effectiveness against Covid-19 infection of various heterologous vaccination schedules," particularly a three dose schedule during the Delta wave, and three and four dose schedules during the onslaught of Omicron.
The study was the first to assess the effectiveness of a heterologous four-dose Covid-19 schedule.
The findings, published in preprint research platform "Research Square," showed an estimated 75 percent vaccine effectiveness, with a 95 percent Confidence Interval.
The results say that the study is 95 percent confident that the vaccine effectiveness overall is between 71 to 80 percent.
A three dose heterologous schedule yielded 98 percent protection against severe Covid-19 infection or death, researchers found in their preliminary analysis of hospital data on vaccine effectiveness for patients during the Omicron wave in February and March of 2022.
Only a single person with comorbidities who received the fourth dose died, the study said.
“This study provides much-needed data showing a fourth dose of any Covid-19 vaccine can help prevent infection due to the highly contagious Omicron variant. Providing continuous protection with boosting is especially important for at-risk groups such as the elderly and those living with chronic health conditions," said Professor Suwat Chariyalertsak, MD, Dr.PH, the lead author Emeritus of the study, and the head of the Faculty of Public Health in the university.
"The data also support the effectiveness of heterologous, or ‘mix and match’, vaccination schedules which may help ongoing efforts to increase population coverage of booster doses," Chariyalertsak added.
The study focused on data from the Chiang Mai Province in Northern Thailand, collected through the Ministry of Public Health Immunization Center (MOPH IC) database.
It covered 27, 301 respondents from October 1 to December 31, 2021, wherein the Delta variant was widespread in the country. 2,130 of the participants tested positive for Covid-19, and 25,171 tested negative.
It also included 36,170 participants from February 1 to April 10, 2022 at the height of the Omicron variant. A total of 14,682 participants were Covid-19 positive and 21,488 negative.
The study is said to be "test-negative, and a case-controlled analysis of data."