Effectiveness of third vaccine dose for coronavirus disease 2019 during the Omicron variant pandemic: a prospective observational study in Japan

Abstract

The administration of a third booster dose of messenger ribonucleic acid (mRNA) vaccines against coronavirus disease 2019 (COVID-19) has progressed worldwide. Since January 2022, Japan has faced a nationwide outbreak caused by the Omicron variant, which occurred simultaneously with the progression of mass vaccination with the third booster dose. Therefore, this study evaluated the effectiveness of the third dose of vaccine by reverse transcription-polymerase chain reaction (RT-PCR) test using nasopharyngeal swab samples from adults aged ≥ 18 years tested after having close contact with COVID-19 cases between January and May 2022. Participants who completed only one dose were excluded from the study. Among the 928 enrolled participants, 139 had never been vaccinated, 609 had completed two doses, 180 had completed three doses before the swab test, and the overall RT-PCR test positivity rate in each group was 48.9%, 46.0%, and 32.2%, respectively. The vaccine effectiveness of the third dose to prevent infection after close contact was approximately 40% (95% confidence interval: 20-60%), which was the highest at 10-70 days after receiving the third dose. In conclusion, the effectiveness of the three-dose mRNA COVID-19 vaccine after close contact during the Omicron outbreak is approximately 40%.

Figure 1

Flow diagram of the study design. Among the overall individuals tested by reverse transcription-polymerase chain reaction (RT-PCR) test using nasopharyngeal swab samples at a large screening test center in Japan between January and May 2022, (1) adults aged < 18 years, (2) those without a certain contact history, (3) those who had completed only one vaccine dose, and (4) those who were less than 7 days after the last vaccination were excluded. Consequently, 767 adults were eligible for subsequent analyses.

Figure 2

RT-PCR test-positive rate after close contact with COVID-19 cases by the time of the last vaccination. Line graphs for the RT-PCR test-positive rate after close contact with COVID-19 cases, according to the days of the last vaccination in those who completed two (a) or three (b) doses of mRNA COVID-19 vaccines are shown. The lines represent the rolling average of the test-positive rate within the nearby ± 5 days for each day relapsed from the last vaccination. The blue-filled areas above and below the line graphs represent the 95% confidence interval of the test-positive rate. COVID-19, coronavirus disease 2019; mRNA, messenger RNA; RT-PCR, reverse transcription-polymerase chain reaction.