Changes in social contacts in England during the COVID-19 pandemic between March 2020 and March 2021 as measured by the CoMix survey: A repeated cross-sectional study

Abstract

Background: During the Coronavirus Disease 2019 (COVID-19) pandemic, the United Kingdom government imposed public health policies in England to reduce social contacts in hopes of curbing virus transmission. We conducted a repeated cross-sectional study to measure contact patterns weekly from March 2020 to March 2021 to estimate the impact of these policies, covering 3 national lockdowns interspersed by periods of less restrictive policies.

Methods and findings: The repeated cross-sectional survey data were collected using online surveys of representative samples of the UK population by age and gender. Survey participants were recruited by the online market research company Ipsos MORI through internet-based banner and social media ads and email campaigns. The participant data used for this analysis are restricted to those who reported living in England. We calculated the mean daily contacts reported using a (clustered) bootstrap and fitted a censored negative binomial model to estimate age-stratified contact matrices and estimate proportional changes to the basic reproduction number under controlled conditions using the change in contacts as a scaling factor. To put the findings in perspective, we discuss contact rates recorded throughout the year in terms of previously recorded rates from the POLYMOD study social contact study. The survey recorded 101,350 observations from 19,914 participants who reported 466,710 contacts over 53 weeks. We observed changes in social contact patterns in England over time and by participants' age, personal risk factors, and perception of risk. The mean reported contacts for adults 18 to 59 years old ranged between 2.39 (95% confidence interval [CI] 2.20 to 2.60) contacts and 4.93 (95% CI 4.65 to 5.19) contacts during the study period. The mean contacts for school-age children (5 to 17 years old) ranged from 3.07 (95% CI 2.89 to 3.27) to 15.11 (95% CI 13.87 to 16.41). This demonstrates a sustained decrease in social contacts compared to a mean of 11.08 (95% CI 10.54 to 11.57) contacts per participant in all age groups combined as measured by the POLYMOD social contact study in 2005 to 2006. Contacts measured during periods of lockdowns were lower than in periods of eased social restrictions. The use of face coverings outside the home has remained high since the government mandated use in some settings in July 2020. The main limitations of this analysis are the potential for selection bias, as participants are recruited through internet-based campaigns, and recall bias, in which participants may under- or overreport the number of contacts they have made.

Conclusions: In this study, we observed that recorded contacts reduced dramatically compared to prepandemic levels (as measured in the POLYMOD study), with changes in reported contacts correlated with government interventions throughout the pandemic. Despite easing of restrictions in the summer of 2020, the mean number of reported contacts only returned to about half of that observed prepandemic at its highest recorded level. The CoMix survey provides a unique repeated cross-sectional data set for a full year in England, from the first day of the first lockdown, for use in statistical analyses and mathematical modelling of COVID-19 and other diseases.

Fig 1. Mean contacts over time by age and by age and setting with timeline of survey participation with 95% CI of bootstrapped mean.

(A) Hospitalisations due to COVID-19 in England. (B) Mean contacts and 95% bootstrapped CIs in adults and children in age groups of 0 to 4, 5 to 17, 18 to 59, and 60 or more year. (C) Mean contacts and 95% bootstrapped CIs by age group and setting. (D) The number of participants and when they respond by panel over time. CI, confidence interval; COVID-19, Coronavirus Disease 2019; LD, lockdown.

Fig 2. Relative difference in mean contacts by study period and age group with 95% CIs.

Relative differences calculated using a GAM with Lockdown 1 as the reference period for each age group adjusted to the UK population by age and gender (when available) for the age groups 0 to 4, 5 to 17, 18 to 59, and over 60 years old. Note the facets have different scales on the y-axes. Table 1 provides corresponding dates for each study period. CI, confidence interval; GAM, generalised additive model.

Fig 3. Mean contacts by risk perception or risk category by adult age groups of 18 to 59 and 60 or more years with 95% CI of bootstrapped mean weighted by age, gender, and weekday.

Participants answered a series of questions about their risk perception with Likert scale response options. Answers of “Strongly agree” and “Somewhat agree” were combined into a category of “Agree, as were answers of “Strongly disagree” and “Somewhat disagree” to “Disagree.” (A) Answers to the statement “I am likely to catch coronavirus.” (B) Answers to the statement “I am worried I might spread coronavirus to someone who is vulnerable.” (C) Answers to the statement “Coronavirus would be a serious illness for me.” (D) Participant reported they were an individual at high risk for complications as defined in the questionnaire. CI, confidence interval; LD, lockdown.

Fig 4. Proportion of adult participants who report wearing a mask by age category with 95% CI of bootstrapped proportion.

Proportions plotted for all participants and for participants who reported any nonhousehold contacts, with the start date of face covering mandates in some settings indicated on July 24, 2020. CI, confidence interval; LD, lockdown.

Fig 5. Mean contacts by employment and income status.

Mean contacts of participants who worked on the previous day and their workplace was open on the previous day weighted by age, gender, and weekday. (A) By employment type: full time, part time, or self-employed. (B) By annual income level: less than £20k, £20k to £44.9k, and over £45k. LD, lockdown.

Fig 6. Contact matrices and their dominant eigenvalues for England in each period considered.

(A) Contact matrices for England across the 9 periods (1. Lockdown 1, 2. Lockdown 1 easing, 3. Relaxed restrictions, 4. School reopening, 5. Lockdown 2, 6. Lockdown 2 easing, 7. Christmas, 8. Lockdown 3, and 9. Lockdown 3 + schools). (B) Points show relative change in R_0 (compared to Lockdown 1) based on the dominant eigenvalues of effective contact matrices calculated for periods 1 to 9, with equal transmissibility in all age groups and age-stratified transmissibility based on Davies and colleagues for SARS-CoV-2. Coloured blocks show durations of each period as annotated. COVID, Coronavirus Disease; R_c, basic reproduction number under controlled conditions; SARS-CoV-2, Severe Acute Respiratory Syndrome Coronavirus 2.