Nagasaki University research group demonstrates efficacy of vaccine against Omicron variant

A research group led by Professor Konosuke Morimoto and Research Fellow Haruka Maeda of the Institute of Tropical Medicine at Nagasaki University, in cooperation with medical institutions nationwide, announced earlier this year that the efficacy of the COVID-19 vaccine against the Omicron variant in preventing outbreaks of the disease was estimated to be 42.8% after two doses of either the Pfizer or Moderna vaccine, and 68.7% after three doses following their analysis of information on 2,000 patients aged 16-64. The group found that when compared vaccine efficacy to the Delta variant, they saw a decrease in the efficacy due to replacement by the Omicron variant. However, there was an increase in efficacy in patients vaccinated three times, suggesting a certain level of effectiveness. The research group is continuing its evaluation.

The research group has been conducting research since July 2021 with the aim of evaluating the efficacy of COVID-19 vaccines in preventing the onset of the disease in Japan while monitoring the situation over the long term. The group has established a cooperative system with 14 medical institutions in 11 prefectures throughout Japan, and continuously evaluates the vaccines by receiving patient information and disclosing their findings on their website (https://covid-19-japan-epi.github.io/output/index.html).

Their analysis took patient information from 2,505 patients aged 16 years and older, who were symptomatic and tested for COVID-19 between January 1 and February 28, 2022, when the Omicron variant was considered to have become the dominant variant. The research group received information from 13 medical institutions and analyzed 2,000 people, excluding 39 people who were examined more than 15 days after the onset of symptoms, 38 identical patients, five people vaccinated with vaccines other than Pfizer or Moderna and 423 people between the ages of 65 and 105 years old. Of the 2,000, 758 (37.9%) tested positive and 1,242 (62.1%) tested negative. 36.7% of patients were aged 16-29, 42.2% were aged 30-49, and 21.1% were aged 50-64. The male to female ratio was 50.4% male and 49.6% female. Unvaccinated persons accounted for 13.4% of the total.

Patients were analyzed in eight categories based whether they tested positive or negative and their vaccination status. The results showed that vaccine efficacy was 42.8% (95% confidence interval: 23.6-57.1) in the group that completed two doses (14 days or more after vaccination), with unvaccinated being counted as receiving one dose, and 68.7% (95% confidence interval: 37.1-84.4) in the group that completed three doses (14 days or more after vaccination). Vaccine efficacy after two doses of either of the two vaccines was found to decrease over time: 41.4% within 90 days after completion of two doses, 43% between 91 and 180 days, and 31.7% at 181 days or more.

By vaccine, the efficacy of two Pfizer doses (at least 14 days after vaccination) was 41.8%, and the efficacy of two Moderna doses (at least 14 days after vaccination) was 46.4%. A more reliable assessment of efficacy should be possible as more people receive their third shot. Since there are data from medical institutions that were not included in this analysis, the research group plans to include these data in future analyses.