Effects of vaccination and non-pharmaceutical interventions and their lag times on the COVID-19 pandemic: Comparison of eight countries

Many countries implemented measures to control the COVID-19 pandemic, but the effects of these measures have varied greatly. We evaluated the effects of different policies, the prevalence of dominant variants (e.g., Delta), and vaccination on the characteristics of the COVID-19 pandemic in eight countries. We quantified the lag times of different non-pharmaceutical interventions (NPIs) and vaccination using a distributed lag non-linear model (DLNM). We also tested whether these lag times were reasonable by analyzing changes in daily cases and the effective reproductive number (Rt)over time. Our results indicated that the response to vaccination in countries with continuous vaccination programs lagged by at least 40 days, and the lag time for a response to NPIs was at least 14 days. A rebound was most likely to occur during the 40 days after the first vaccine dose. We also found that the combination of school closure, workplace closure, restrictions on mass gatherings, and stay-at-home requirements were successful in containing the pandemic. Our results thus demonstrated that vaccination was effective, although some regions were adversely affected by new variants and low vaccination coverage. Importantly, relaxation of NPIs soon after implementation of a vaccination program may lead to a rebound.Author summary: In response to contain the COVID-19 pandemic, countries worldwide have implemented multiple non-pharmaceutical interventions (NPIs) that restrict movement, such as school closure and stay-at-home requirements, and introduced large nationwide COVID-19 vaccination campaigns. However, there were variations in population compliance and vaccine coverage worldwide. The virus is continuously spreading and new genetic variants are emerging. In this context, we chose eight countries as being representative of the worldwide ranges of population density, variant spread, vaccination, population compliance to countermeasures, and effectiveness of implemented measures. Our study identified the most common effective countermeasures and estimated the lag times for a response to NPIs and for the onset of a vaccination effect. We found a rebound was most likely to occur during the 40 days after the first vaccine dose and both NPIs and vaccine coverage are indispensable in the current context of Delta variant high prevalence. We hope that the results of this study could provide some guidance for controlling the pandemic.