Background Although vaccination against SARS-CoV-2 is considered the central strategy against the pandemic, uptake lags behind target rates. Method To explore whether this rate could be enhanced by a nudging strategy that exploits the status quo bias, we conducted a randomized controlled trial in northern Italy comparing vaccination acceptance among 2000 adults, ages 50 to 59 years, who were either invited to set an appointment (opt-in group) or assigned an individual appointment (opt-out group). Results Results indicate a difference of 3.2 percentage points, which represents a 32% relative increase in the vaccination rate for the opt-out group compared with the opt-in group. Conclusions A significant portion of those who remain unvaccinated may not hold strong beliefs against vaccination but rather tend to inaction and may therefore be nudged toward vaccination with a reduction of action required. Highlights Reluctant adults (50-59 years), who had not yet received vaccines against COVID-19, were sent letters announcing appointment availability In an RCT, the status quo option in the notices influenced the rate of vaccine acceptance Nudging via pre-scheduled appointments encouraged vaccine uptake more than invitations to schedule did Switching the default option yielded a 32% relative increase (13.1% vs. 9.9%) in vaccination

We analyzed 221 COVID-19 cases identified between June 2020 and January 2021 in 6,074 individuals screened for IgG antibodies in May 2020, representing 77% of residents of five Italian municipalities. The adjusted relative risk of developing symptomatic infection in SARS-COV-2 seropositive participants was 0.055 (95%CI: 0.014 - 0.220)

Although vaccination against SARS-CoV-2 is considered the central strategy against the pandemic, uptake lags behind target rates. To explore whether this rate could be enhanced by a nudging strategy that exploits the status quo bias, we conducted a randomized controlled study in northern Italy comparing vaccination acceptance among 2,000 adults aged 50-59 who were either invited to set an appointment (opt-in group) or assigned an individual appointment (opt-out group). Results indicate an increase in vaccination rate for the opt-out group compared to the opt-in group of 3.2 percentage points — a 32% relative increase. This suggests that a significant portion of those who remain unvaccinated may not hold strong beliefs against vaccination, but rather tend to inaction and may therefore be nudged toward vaccination with a reduction of action required.

To what extent infection with SARS-CoV-2 protects against subsequent reinfection or symptomatic reinfection is still unclear. In this cohort study, we analyzed surveillance records of COVID-19 cases identified between June 2020 and January 2021 in five Italian municipalities, where 77.7% of the entire population was screened for IgG antibodies in May 2020. We compared the risk of observing symptomatic infections in two mutually exclusive groups defined by the initial serological response. We estimated that the cumulative incidence of identified symptomatic infections in the IgG negative and positive cohorts was 2.67% (95%CI: 2.12%-3.37%) and 0.14% (95%CI: 0.04%-0.58%), respectively. The adjusted odd ratio of developing symptomatic infection in individuals previously exposed to SARS-CoV-2 was estimated at 0.054 (95%CI: 0.009-0.169). Quantifying protective immunity against COVID-19 disease elicited by natural infection with SARS-CoV-2 is essential to inform strategies for controlling the pandemic in the forthcoming months.