Objective: Monitoring the prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants is important due to concerns regarding infectivity, transmissibility, immune evasion and disease severity. We evaluated the temporal and regional replacement of previous SARS-CoV-2 variants by the emergent strains, Alpha and Delta. Methods: We obtained the results of polymerase chain reaction screening tests for variants conducted in multiple commercial laboratories. Assuming that all previous strains would be replaced by one variant, the new variant detection rate was estimated by fitting a logistic growth model. We estimated the transmission advantage of each new variant over the pre-existing virus strains. Results: The variant with the N501Y mutation was first identified in the Kinki region in early February 2021, and by early May, it had replaced more than 90% of the previous strains. The variant with the L452R mutation was first detected in the Kanto-Koshin region in mid-May, and by early August, it comprised more than 90% of the circulating strains. Compared with pre-existing strains, the variant with the N501Y mutation showed transmission advantages of 48.2% and 40.3% in the Kanto-Koshin and Kinki regions, respectively, while the variant with the L452R mutation showed transmission advantages of 60.1% and 71.9%, respectively. Discussion: In Japan, Alpha and Delta variants displayed regional differences in the replacement timing and their relative transmission advantages. Our method is efficient in monitoring and estimating changes in the proportion of variant strains in a timely manner in each region.

Background: Quantifying the impact on COVID-19 transmission from a single event has been difficult due to the virus transmission dynamics, such as lag from exposure to reported infection, non-linearity arising from the person-to-person transmission, and the modifying effects of non-pharmaceutical interventions over time. To address these issues, we aimed to estimate the COVID-19 transmission risk of social events focusing on the Japanese Coming-of-Age Day and Coming-of-Age ceremony in which "new adults" practice risky behavior on that particular day.Methods: Using national surveillance data in Japan in 2021 and 2022, we conducted difference-in-differences regression against COVID-19 incidences by setting "new adults" cases as the treatment group and the cases 1 year younger or older than these "new adults" as the control group. In addition, we employed a triple differences approach to estimate the risk of holding the Coming-Age ceremony by using a binary variable regarding the presence or absence of the ceremony in each municipality.Results: We estimated the relative risks (RRs) of the Coming-of-Age Day as 1.27 (95% confidence interval [CI] 1.02-1.57) in 2021 and 3.22 (95% CI 2.68-3.86) in 2022. The RR of the Coming-of-Age ceremony was also large, estimated as 2.83(1.81-4.43) in 2022.Conclusions: When planning large social events, it is important to be aware of the unique risks associated with these gatherings, along with effective public health messages to best communicate these risks.

Based on national surveillance data, we describe an unprecedented increase in syphilis case reports in Japan, with a surge in 2021–2022 reaching 10 141 cases in Week 42, 2022, a 1.7-fold increase over the same period in 2021. This already represented the highest annual case count in nearly half a century; by Week 52, 2022, the number reached 12 966, far surpassing the 7978 cases in 2021. Predominantly affecting heterosexual men and young women, the proportionate increase in primary and secondary syphilis cases suggests a true increase in incidence. The syphilis surge during the pandemic poses a serious public health concern and underscores the importance of adequate testing and preventive measures.