This online cross-sectional study was conducted during 10–23 January 2021, the period of the second outbreak of COVID-19 in China, when the people were under state-enforced strict lockdown. During this period, some cities such as Shijiazhuang in the Hebei province were locked down with traffic restrictions and the closure of work units. The details about the outbreak and the corresponding preventive measures have been described in past literature [20]. During the outbreak, individuals were invited to participate in the present web-based anonymous survey, which could be entered by scanning QR codes or clicking the linkage on Wechat. Self-reported questionnaires were administered to the participants through the Wenjuanxing platform (Changsha Haoxing Information Technology Co., Ltd., China). Finally, our electronic questionnaire was clicked 1,002 times. The inclusion criteria were being a Chinese citizen and being at least 18 years of age. We excluded participants aged <18 years and those who gave incomplete responses. Ultimately, the responses of 944 participants were analyzed in our study. The response rate was 94.2%. Among the included participants, 43.4% (n = 410) participants were from the Hebei province, 16.0% (n = 151) were from the Beijing city, and 18.4% (n = 174) were from the Liaoning province. No monetary compensation was provided to the participants. All participants provided their written informed consent for participation.

Quarantine duration was measured by two questions: 1) “Did you experience quarantine before the second outbreak” and 2) “How long have you been quarantined.” As the public health center recommended, people were required to have quarantined for 7 days without exposure and 14 days with close contact. Thus, the quarantine duration was categorized into three groups: 0 days, 1–7 days, and >7 days.

COVID-19 related psychological distress was measured as detailed elsewhere [21]. The following five items were inquired to reflect the participants’ psychological state during the COVID-19 pandemic: 1) I am nervous when I think about the current circumstances; 2) I am calm and relaxed; 3) I am worried about my health; 4) I am worried about the health of my family members; and 5) I feel stressed about leaving my house. The answers were coded on a 5-point scale, ranging from 1 (does not apply at all) to 5 (strongly applies), which indicated the extent to which each statement applied to the participant. The total scores of the psychological distress ranged from 5 to 25. The scale presented an acceptable Cronbach’s alpha score of 0.77.

The World Health Organization-Five Wellbeing Index (WHO-5) was applied in the current study to measure the stated of wellbeing of the participants [22]. The WHO-5 recommends five positive items: “I have felt cheerful in good spirits”; “I have felt calm and relaxed”; “I have felt active and vigorous”; “I woke up feeling fresh and rested”; and “My daily life has been filled with things that interest me.” The extent to which the positive feelings were experienced in the last 2 weeks was scored on a 6-point Likert scale ranging from 0 (at no time) to 5 (all the time). The total score ranged from 0 to 25. The scale showed good internal consistency reliability of α = 0.91 in this study.

Increasing social distancing as another preventive intervention was encouraged for people when the city was under lockdown. Social distancing was determined during the second outbreak based on three questions concerning the coronavirus situation [21]. These questions mentioned the frequency of someone staying at home, not attending social gatherings, and keeping a distance of at least 2 m from other people for the past week based on a 5-point scale, ranging from 1 (Never) to 5 (Always). The total scores were summed in the current study. The total scores ranged from 3 to 15. The scale had an acceptable Cronbach’s alpha score of 0.77.

The COVID-19 vaccination intention was measured using a 1-item question: “The vaccine against COVID-19 infection has been available in the market, would you take it?” The answer was scored on a 5-point scale from “Definitely not” to “Definitely”. The participants who answered with “Definitely” or “Probably” were regarded as having an intention to take the vaccine. However, the participants who answered “Possibly,” “Probably not,” or “Definitely not” were regarded to have no intention to take the vaccine.

The covariates covered the demographic characteristics and health status. Demographic variables included gender (male or female), age (in years), residence (rural or urban), marriage status (married or unmarried), education (secondary and below or tertiary), and monthly per-capita income (≤1,000 yuan, 1,001–3,000 yuan, 3,001–5,000 yuan, or >5,000 yuan). The health status was measured by using a self-reported health status (fair/poor, good, or very good) and based on whether the participants had chronic diseases. The participants who had cardiovascular diseases, diabetes, hepatitis B, chronic obstructive pulmonary disease, chronic kidney diseases, or cancer were defined as having chronic diseases.

Frequency and mean were used to describe the characteristics of the study samples. Chi-square tests and F-test were used to compare the demographic differences among the participants with different quarantine duration. One-way analysis of variance (ANOVA) or Chi-square test was applied to explore the associations between quarantine duration and vaccination intention, social distancing, psychological distress as well as the stated of wellbeing. Logistic regression was applied to examine the association between quarantine duration and vaccination after adjusting for covariates. Odds ratio (OR) and 95% confidence intervals (95% CI) were computed. Multiple linear regression was performed to examine the association between quarantine duration and social distancing, psychological distress as well as the state of wellbeing after adjusting for covariates. To demonstrated the adjusted standardized difference of social distancing, psychological distress, and wellbeing among different quarantine durations, a forest plot was prepared based on multiple linear regression. The adjusted standardized score and 95% CI were calculated. Analyses were performed in the Stata version 15 (Stata Corp., College Station, TX, United States) and R version 3.4.3 (R Development Core Team, 2018). We considered a two-sided p value of <0.05 to be significant.

Of the 944 participants, 17.2% (n = 162) of the participants had been quarantined during the COVID-19 pandemic and 9.4% (n = 93) had been quarantined for at least 7 days. Table 1 shows that 72.9% of the participants were female 65.8% were based in urban areas, 53.8% were unmarried, and 89.0% of the participants had university degrees. The average age was 32.7 years. The health status of 28.7% of the participants was poor/fair and 8.8% had chronic diseases. In addition, the participants who had been quarantined were younger (p < 0.001) and unmarried (p = 0.001).

Table 2 shows the results of the univariate analyses of the association between quarantine duration and psychological distress, wellbeing, social distancing as well as vaccination intention. The results of one-way ANOVA analyses showed that quarantine duration affected psychological distress (p = 0.003), wellbeing (p = 0.016), and social distancing (p < 0.001). The least significant difference test results showed that participants who had been quarantined for >7 days had the largest psychological distress (17.2 ± 3.8) and social distancing (13.3 ± 1.9) as well as the poorest wellbeing (15.2 ± 5.4) compared with those without quarantine. Overall, 62.1% of the participants intended to take a COVID-19 vaccine, whereas 37.9% did not intend to take the vaccine. The chi-square test results showed that participants who had been quarantined during the pandemic had a higher vaccination intention than those who had not been quarantined (p = 0.005).

The multiple linear regression (Table 3) shows that quarantine for 1–7 days increased the social distancing by 2.61 (95% CI = 1.90–3.33), whereas quarantine for >7 days increased the social distancing and psychological distress by 3.00 (95% CI = 2.37–3.64) and 1.03 (95% CI = 0.22–1.86), respectively. However, quarantine for >7 days decreased wellbeing by 1.27 (95% CI = 0.29–2.26). Figure 1 shows the standardized differences in social distancing, psychological distress, and wellbeing. Quarantine duration shows a dose-response relationship (p trend <0.01), indicating that individuals with longer quarantine duration reported increased social distancing, increased psychological distress, and decreased wellbeing during the COVID-19 pandemic. The multiple logistic regression results showed that the participants, who had been quarantined for 1–7 days, were 2.16 times more favorable to take the vaccine than those without quarantine after adjusting for parameters such as gender, age, residence, marriage, education, income, health status, and chronic diseases (Table 3).

To conclude, this study explored the effects of the different lengths of quarantine on psychological outcomes and vaccination intention during the second outbreak of COVID-19 in China. Quarantine with a longer duration (>7 days) was associated with the increase in psychological distress and the decrease in wellbeing, whereas quarantine for 1–7 days was associated with high vaccination intention. Though quarantine is deemed necessary to limit COVID-19 spread, the duration of quarantine should be considered by policy-makers when strategizing immunization programs against COVID-19 and preventing psychological disorders during the COVID-19 pandemic.