Abstract
We conducted a test negative case control study to assess the Vaccine Effectiveness (VE) of various vaccine regimens for preventing COVID-19 infection and its severity during the period when Omicron was the dominant causative virus in Thailand. All 8,600 individuals, aged ≥5 years, at risk for COVID-19 (Patient Under Investigation, PUI) who presented for nasopharyngeal real time polymerase chain reaction (RT-PCR) testing were prospectively enrolled at Thammasat University hospital during January to June 2022. There were 3,546 cases who were infected. For preventing omicron variant associated infections, the VE increased along with the increase in the number of vaccine dose and the four dose regimen (2 boosters) had the highest but only moderate VE. For preventing severity, the three and four dose regimens (1 and 2 boosters) had comparable high VE and their effectiveness was more durable than the two dose regimens. Significant protective factors to prevent infection were vaccine doses, female and healthcare personnel. Significant risk of infection included elderly, at least one chronic morbidity and household infection. Significant risk of severe COVID-19 was incomplete two dose vaccination, at least one chronic morbidity and elderly. Longcovid condition was prevalent after 6 months of COVID-19. It consisted of multi system symptoms. Weakness and neurological symptoms were the most common manifestations. Risk factors of longcovid condition were female, cough symptom at acute infection and severe COVID-19. Based on the study findings, current mass vaccination programs should focus on reducing COVID-19 severity and mandate at least one booster dose after primary series vaccines. The heterologous boosters with viral vector and mRNA vaccines were highly effective and can be used in individuals who had previously received the primary series of inactivated viral vaccine.
