The effectiveness of repeated vaccination for COVID-19 symptom resolution during the KP.3-dominant epidemic period in Japan: A retrospective multicenter study

Objective

To evaluate the effect of repeated vaccination on coronavirus disease (COVID-19) symptoms.

Methods

This retrospective, multicenter study was conducted across 15 Japanese medical institutions during the 11th wave of the COVID-19 epidemic in Japan. Patients diagnosed with COVID-19 via rapid antigen testing between July 1, 2024, and September 30, 2024, were enrolled. Data were collected using a questionnaire.

Results

The data of 408 patients were available for analysis; including 42.6 % were 40–59 years of age, 61.5 % female, and 35.8 % had a history of COVID-19. Seven doses of vaccine were associated with a significantly lower maximum body temperature and shorter fever duration (−0.79 °C, p < 0.001, and −1.50 days, p < 0.001, respectively). Past infection was associated with a significantly lower maximum body temperature and shorter fever duration (−0.35 °C, p < 0.001, and −0.75 days, p < 0.001, respectively). The symptoms in patients with past infection were significantly fewer at days 7, 14, and 30 than were those without past infection. For patients vaccinated seven times, symptoms were fewer after day 14. No significant difference was observed in the interval between onset and the last vaccination.

Conclusion

Repeated COVID-19 vaccination, particularly seven doses, was correlated with a lower maximum body temperature and shorter fever and symptom duration for patients during the 11th wave dominated by KP.3 variant. These findings highlight the importance of repeated vaccinations to mitigate the symptoms of COVID-19 with emerging variants.