Objective: To investigate the status of asymptomatic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in medical facilities.

Methods: A cross-sectional study survey was conducted among healthcare workers toward the end of the eighth wave of the coronavirus disease 2019 (COVID-19) epidemic in Japan. Participants completed a self-administered questionnaire providing details their COVID-19 and COVID-19 vaccination history and provided a blood sample for SARS-CoV-2 nucleocapsid (N) antibody testing as evidence of past SARS-CoV-2 infection. Participants were categorized into four groups, according to whether they had received 0–2, 3, 4, or 5 doses of vaccine. The prevalence of N antibodies was compared between groups.

Results: Of 494 participants, 288 reported no history of SARS-CoV-2 infection, of whom 185 accepted serological testing for past infection. Of the 185, 14 (7.6%) showed serological evidence of past infection, indicating asymptomatic infection. The risk of asymptomatic COVID-19 was inversely associated with the number of doses of COVID-19 vaccine received.

Conclusions: The cumulative effect of vaccination on preventing asymptomatic COVID-19, suggests that repeated COVID-19 vaccination of healthcare workers may serve as an infection control measure.

This study focused on the association between malnutrition, income, and living alone among residents of a large-scale housing complex in Japan to clarify the relationship between household food insecurity and social isolation. We adopted a cross-sectional research design and conducted a survey by means of a self-administered questionnaire. Subsequent logistic regression analysis showed that social isolation and subjective household financial hardship were statistically independent risk factors for food insecurity (males had respective ORs of 2.79 and 8.80; females respective ORs of 2.31 and 8.66). We showed that in addition to general financial support, support catering specifically to social isolation is needed to address food insecurity among residents of this housing complex.