Context: The social isolation measures resulting from the COVID-19 outbreak changed work practices in various sectors, especially with the shift to working from home. However, it is still unclear the implications of the pandemic on the maintenance and evolution of open-source software (OSS). Objective: In this study, we analyze the effects of COVID-19 on the development activity of OSS and how social isolation changed the productivity and emotional state of OSS contributors. Method:To investigate this issue, we have conducted a multi-method study. We first quantitatively investigated how the COVID-19 outbreak impacted the development activity of open-source projects by mining the development history of 155 open-source projects. We then qualitatively investigated the perceptions of open-source core developers about the impact of COVID-19 on their activities, using a survey as an instrument. Results: Our results indicate that the pandemic led to some effect on the developmentactivity of OSS repositories, especially in the early days of the outbreak. For instance although we observe an increase in the number of pull requests accepted, our results also indicate a sudden increase in the turnover rate of core developers. However, a few months after the declaration of the current pandemic, our results suggest stabilization in these metrics. Other effects were more far-reaching, such as a decrease in newcomers throughout the observed period after the pandemic was declared. Some findings are supported by the results of our qualitative study, whose results indicate that most of the respondents of our survey consider that COVID-19 did not change their productivity substantially. Conclusions: Our findings can help practitioners and researchers to better understand the effect of COVID-19 on the software engineering field.