Product application
NAS and normal saline solution (placebo) were produced, packaged, and labeled with a double-blind, randomized code by the Government Pharmaceutical Organization (GPO), Ministry of Public Health, Thailand. Each participant was randomly assigned to receive either NAS or a placebo. Site staff gave participants instructions on the study product application, storage, and return. In brief, a trained nurse administered the study products into the nostrils of all participants for the first application to ensure that the correct dosage was delivered and that the application technique was consistent across all participants allowing for direct observation and immediate feedback to ensure proper administration by all participants. The participants were then instructed to self-administer the study products by spraying two puffs into their nostrils (100 µl per puff) thrice daily at 8 am, 2 pm, and 8 pm for 7 days. The training ensures that participants understand the correct technique, dosage, and frequency of administration. The volume and frequency of the study product application were based on reports of other nasal spray products containing anti-SARS-CoV-2 antibodies