Abstract
OBJECTIVE: To investigate the real-world effectiveness of COVID-19 vaccine boosters during China's Omicron wave. METHODS: In January 2023, we surveyed Shenzhen, China residents via online questionnaires to investigate their COVID-19 symptoms and vaccination history. The outcomes of interest included fever, other COVID-19-related symptoms, severity of symptoms, whether early onset (before December 23, 2022) and duration. Respondents were categorized as no booster, one booster 6mo ago, one booster within 6mo, or two boosters based on dose count and vaccination timing. We used multivariable logistic regressions and Tobit models to assess COVID-19 vaccine booster impacts. RESULTS: Compared to the no booster group, two booster recipients had a lower fever risk (OR = 0.35, 95 %CI = 0.16-0.76) but not lower risks of COVID-19-related symptoms (OR = 0.74, 95 %CI = 0.26-2.06) and self-reported severe symptoms (OR = 0.47, 95 %CI = 0.19-1.15). Nor did the two booster recipients had a shorter illness duration (marginal effect = -0.79 days, 95 %CI = -1.65-0.07) and a lower risk of symptom onset delay (OR = 0.48, 95 %CI = 0.19-1.23). Compared to the no booster group, both one booster within six months (OR = 2.17, 95 %CI = 1.34-3.52) and one booster six months ago (OR = 1.30, 95 %CI = 0.92-1.82) did not reduce the risks of fever and symptoms (one booster within six months: OR = 1.57, 95 %CI = 0.84-2.90; one booster six months ago: OR = 1.23, 95 %CI = 0.79-1.93). Regardless of timing, one booster did not reduce illness duration (within six months: marginal effect = 0.25 days, 95 %CI = -0.20-0.70; six months ago: marginal effect = 0.27 days, 95 %CI = -0.08-0.62). However, receiving one booster within six months delayed symptom onset (OR = 0.54, 95 %CI = 0.34-0.86), while one booster six months ago did not (OR = 1.03, 95 %CI = 0.74-1.44). CONCLUSIONS: Receiving two booster doses reduced the onset of fever during the Omicron outbreak in mainland China.